Correcting Common Problems in Your Air Conditioning Ductwork
Most residential air conditioning systems are fairly straightforward and should follow a handful of guidelines without being designed according to industry standards. The best known of these procedures is the Air Conditioning Contractors of America (ACCA) Manual D.
Best Practices for Ductwork Runs
• Changes in airflow direction should be accomplished as infrequently and as smoothly as possible.
• The inner liner of flex duct should be pulled to its full extent to avoid the spiral “accordion” effect which causes a lot of flow resistance, or use smooth metal duct.
• Start with the size of the fan outlet/inlet connection. Use that size if the duct length is fairly short (less than 10 ft.) Increase the duct size 1 inch if the duct run is not long (less than 25 ft.) and there are few fittings (less than 3). Go up 2 inches in duct size if the duct run is long or there are many fittings. Size the wall cap or roof jack to match the final duct size.
• Seal all joints with long-lasting sealant material such as duct mastic. Some UL 181 listed tapes will work well on clean surfaces, but do not use cloth-backed tape.
The outlet connection for standard bathroom exhaust fans is usually 3 inch diameter. The better fans are 4 inch. Remote fans usually have 4, 6, or 8 inch diameter inlet or outlet connections. Smooth transition fittings should be used to change duct sizes. Odd-sized diameter duct is less common, but they are available and can make achieving the right air flow much easier.
If you are designing the duct system, when laying out duct runs and sizes, plan for air velocity of:
Supply ducts should be sized for 500 ft/min or less. To determine this velocity you need to know the amount of air supplied by your unit. It may be stamped on the label near the model and serial number or you can look it up in the owner’s manual. If all else fails you can research it online through the manufacturers website. Divide the amount of air (cubic feet per minute, CFM) by the area of the main supply trunk in square feet. Measure the metal duct, not the insulation around it; for fiberboard ductwork, which is usually an inch thick, subtract 2” from the height and width of the ductwork. Example: Air handler the supplies 1,600 CFM through a metal duct that measures 30” x 16”
1,600 CFM / ((30” x 16”)/144) = 480 ft/min < 500 ft/min; velocity is within recommended range.
(To convert the duct dimensions from square inches to square feet divide the area of the duct 144 square inches per square foot)
Return ducts should be sized for 350 ft/min.
By keeping the velocity below 500 ft/min will help keep static pressure and noise down, while keeping throw and efficiency up. Throw and efficiency have to do with how well the air is injected into the room so that it mixes well with room air. It will also keep energy cost down by keeping the motor from working too hard and maintaining the proper temperature.
General conventions for branching round ducts are:
• One 5” duct branches to two 4” ducts
• One 6” duct branches to two 5” ducts
• One 8” duct branches to two 6” ducts
Exhaust fans in the same dwelling can share a common discharge duct, but each fan must have a back-draft damper to prevent movement of air from one fan back through another.
Stamped metal grilles are inexpensive, but they are not adjustable and are inefficient. Aluminum grilles should be used where moisture will be present instead of standard painted steel. Grilles with adjustable curved blades that can direct the airflow to suit the room layout are the best choice. Grilles with a means to adjust the volume of air flow can help with balancing, but cannot limit the flow as well as volume dampers mounted in the duct branch. They can also be noisy if the are less than 50% open.
Transfer Grilles or Jump Ducts
Jump ducts are a popular method for improving the energy efficiency of homes with forced-air heating and cooling systems. They address the critical issues of equalizing air pressure in various parts of the home and of handling return air. When doors are closed, as with the case of bedrooms, it can be difficult to return enough air back to the air handling unit and create negative pressure inside the house. This would cause unconditioned air to enter the home through cracks and leaking windows or doors.
Inside the room, if more air is supplied than is returned, the air pressure in the room increases and exceeds the pressure outside it. Without balanced air pressure, energy efficiency decreases as conditioned air escapes through the building envelope, through uncontrolled routes such as windows or around electric outlets.
When return ductwork or plenums are installed, the cavities between studs and floor joist are used by attaching a piece of sheet metal to one side and using the floor joists and subfloor to complete the return. Inside walls, two studs and the layers of drywall are used and a hole is cut in the bottom plate of the wall. It a 2x4 framed wall, the area is about 0.35 square feet. If more than 125 CFM of air is supplied to the room, the return would be undersized. Return grilles have about 80% free area, so they should be larger than 3 ½ x 14 ½ or about 6 x 15. Larger rooms have holes cut between two stud cavities to increase the increase the area.
The problem with using wall and floor cavities is that they leak and can introduce humidity into the structure during the cooling season. The holes may not be cleanly cut and can compromise the wall or floor structure.
It is a fairly easy process to install a jump duct. A grille on the ceiling of the room is connected through ductwork to a grill in the hallway's ceiling. Air exits the room through the jump duct into the hallway, and then moves through the hallway to the main return duct. A transfer grille can also be installed near the ceiling to accomplish the same thing. A baffle should be installed to prevent light from leaking through. If noise and privacy are concerns, a length of six to eight feet of flexible duct will help prevent light pollution, as well as dampen noise moving between a room and the hallway. Low-cost stamped-metal grilles are typically used to provide a finished look on the ceiling. Doors should still be undercut by at least 1” as well.
Typical Jump Duct
Transfer Grille (Top View)
If you are experiencing problems related to return air, such as extreme inefficiency, dampness or other comfort problems, jump ducts are worth considering. It is relatively straightforward to gain access to the space above the bedrooms and hall, and a central return can be accommodated in the main space of the house. In a few homes I inspected, the wall cavity used for a return was open to the attic and was pulling in unconditioned air year-round. Insulation and dust was also pulled into the system and reduced efficiency and reduce indoor air quality.
Most homeowners and many home builders give little thought to the air conditioning condensate line or trap. The primary purpose of a condensate trap is to prevent air from moving in or out of the air handler during operation. Traps must be installed in a manner that will stop the air from passing through, but still allow the condensate to drain from the condensate pan.
When the trap is omitted, efficiency is lowered as air is lost through the condensate drain in blow-through systems. Failure to install a trap on a blow-through system is the same as drilling a hole in the ducts for each drain connection. The pressure around the pan on a blow-through system guarantees the pan will drain whether it is trapped or not.
Trapping is a major issue on draw-through systems. Untreated air can be drawn into the airstream while the system is running. If the coil is located in an attic or other warm space, there is even greater reason for concern. As on a blow-through system, a drain on a draw-through system that is not trapped is an efficiency issue. But more importantly, the air being sucked through the drainpipe can prevent the pan from draining, causing it to run over.
Without proper trapping, air pulled back into the equipment can lift the water up from the condensate pan, like sucking water up into a straw. This can cause the insulating liner material and many of the components located nearby to become wet. Microbes in the water can become a problem if they grow on the insulation. As the microbes multiply they can release spores into the airstream, raising the probability of health risks.
When a trap seal is too shallow, it dries out during each cooling cycle startup. Running traps are prone to fail in this manner. Some manufacturers specifically recommend against the use of running traps.
If the seal is too deep, it actually can cause condensate to be held in the pan. A trap with an excessively deep seal also is prone to clogging. Because fan speed, duct size, coil condition or other issues are specific to unit configurations, the trap design should be specific to the unit as well. Rarely, if ever, do static pressure considerations come into play when condensate traps are installed.
Equipment manufacturers often recommend a drop from the condensate pan at a minimum of 2 inches and then a minimum of a 2-inch trap seal. If your trap is outside these parameters, you should look more closely for signs of moisture concerns in or around the equipment.
Cap open cleanouts
There are a number of piping mistakes made when installing a condensate trap. An open cleanout between the trap and coil is one of the most common mistakes. It could be that installers think leaving the pipe open will help the system drain, working much as a vent does on the house plumbing. As we now know, an open cleanout at this location allows air to bypass the trap altogether. This mistake is easily corrected by placing a cap over the cleanout pipe and only removing it for cleaning purposes. It should be easy to remove and does not need to be glued. If you are concerned about a clogged trap, you can glue a threaded adapter on the end of the pipe and screw in a plug.
Connecting multiple units to one trap often allows air to be pulled through one or more of the units, bypassing the trap as well. Each unit should have its own individual trap, and some equipment makers recommend against installing the trap more than 4 feet away from the coil.
Many manufacturers and trap makers recommend using removable caps on tees and crosses for cleaning purposes. Not necessarily a mistake, you should install a cleanout at the end of your cooling season. There are products on the market now which have clear fittings so you can see if it starts to clog and clean it out before you have problems.
EZ Trap® Kit with clear trap
Air conditioning (AC) equipment is a complex mix of electrical, mechanical, and plumbing components that must work together to provide sufficient cooling and air flow in an HVAC system. Small problems can result in higher energy costs, lower comfort levels, and damage to the equipment which can cost thousands of dollars to replace. A common problem seen with air conditioning and refrigeration equipment are frozen evaporator coils. Refrigerators and freezers typically have a built in thaw cycle to limit ice build up, but standard AC equipment usually is not equipped with this feature, but there are a few simple causes that you should be aware of that will help prevent freezing coils.
The two causes for frozen coils is either a lack of airflow across the evaporator coil or a problem within the refrigeration system.
Most homeowners don’t realize there is a problem until they see that the unit runs continuously, the temperature is too warm, or there is reduced airflow. When people here the hum of the blower and compressor running, they assume it is working. If they happen to go outside and see ice on the line coming out of the home, they might become concerned. The real cause for the reduced capacity is that the evaporator coil, the one inside the air handler, is either partially or completely frozen preventing any air from getting through. To see this you would have to remove an access panel on your unit to inspect the coil. If you go to the air handler you may notice the area where the coil is located may be sweating due to the ice inside coming in contact with the metal casing of the unit or plenum. The condensate drain may also be sweating from the colder than normal temperatures. In extreme instances, ice will begin to form on the outside of the unit.
Ice forming on exterior refrigerant line around filter drier
Typical Air Conditioning System
The best thing to do once you notice your air conditioner is frozen is to turn it off. If it is summer and you have a frozen heat pump system you can turn it to the heating mode and it will pump hot refrigerant through the coil to accelerate the defrosting process. If an excessive amount of ice is on the evaporator coil, defrosting the frozen coil may overflow the condensate drain and you may have some water damage. This may happen on any type of system if the air handler is located in a hot attic. For standard AC systems, go to the thermostat and turn the air conditioner to the off position and turn the fan switch to on or manual at the selector switch on your thermostat. This will help defrost the frozen evaporator coil and by blowing warm room air over the coil.
On manual thermostats the selector switch is on the bottom or side to place the fan to ON
Also note that a heat pump will form frost or ice on the outside condenser coils in the winter which is common and most heat pumps are equipped with a method to defrost the condenser coils.
A-coil completely frozen
A-coil partially frozen
While the coil is defrosting, check to see if your filters are dirty and need to be changed. The reduced airflow from dirty filters is the most common cause for frozen coils. If you have new filters, throw away the dirty ones and replace with the new set. If you have a washable filter, rinse out the mesh screen in a laundry sink or outside with a hose. Do not use soap to wash the filter and only use water unless the manufacturer recommends some cleaning agent. Replace your filters at least every 60 days during the cooling season, and at least every 90 days during the rest of the year. If you have flat filters instead of a pleated style, you may want to replace them once a month.
Another common cause for reduced air flow is a dirty blower. The fan located in the air handler is commonly referred to as a blower and it is usually located after the filters and either before or after the evaporator coil. Even if you change your filters regularly, the blower cage can still get dirt deposits on the fins and reduce airflow since some air can bypass the filters around the edges. You may want to check to see if the foam gasket is in place around the edge of the filter frame. Also make sure that your filter is the proper size by consulting your operation manual for the unit. Most evaporator coils are designed to have at least 400 cubic feet per minute (CFM) of airflow across the evaporator coil per ton of air conditioning. If you have a 3 ton air conditioning unit or heat pump you need 1200 CFM of airflow across the evaporator or the air conditioner will freeze.
Blower Motor and Cage
A few other causes for reduced airflow are:
• Collapsed ductwork
• Bad blower motor
• Obstructions to return grilles – such as furniture placed in front of wall returns
• A dirty Evaporator coil – see my article on cleaning coils
Turn off the power to the system prior to checking the blower or removing panels and covers.
After the coil has thawed completely, replace all access panels and covers and turn the system back to cooling mode and the fan back to AUTO. Check the system every hour to ensure that there is proper airflow and that there is no ice forming.
Caution: Never use any sharp objects to try to chip away the ice on the coil, while you may not damage the refrigerant lines, you will definitely bend the aluminum fins and reduce the airflow. The only recommended way to speed the defrosting process is to place a fan by the coil to increase air circulation. If you leave the access panel off of the unit and turn only the fan on, the blower will pull in enough air defrost the ice quickly. So people may want to use a heat gun or hairdryer to help melt the ice, but this is not usually necessary unless you need the unit running as soon as possible.
A frozen air conditioning system can also be caused by a refrigeration problem. The most common problem is a refrigerant leak, often referred to as Freon®. (Freon® is a trademark name of DuPont Corporation). This means that the air conditioning is low on refrigerant. This leak needs to be repaired and then the system recharged by a professional HVAC technician. A frozen air conditioning system can also be caused by a refrigeration problem when pressures drop in the evaporator. A pressure drop in the evaporator coil means that the refrigerant will be colder than the dew point. When the evaporator coil in your air conditioner operates below the dew point the moisture or humidity in the air will freeze to the coil. Frost will appear on the air conditioner evaporator coil and start to restrict airflow and then accelerate the freezing process. This will eventually form a solid block of ice and restrict the air flow of the air conditioning system. Only a trained HVAC professional has the tools to fix this type of problem.
Another refrigeration problem is a clogged filter/drier, which is a component installed in the copper tubing near the condenser unit outside to remove water and dirt from the inside of the system. They are usually installed after a repair if the system is opened to the atmosphere. This will lower the pressure inside the evaporator coil and cause it to freeze. If your system has an expansion valve installed before the evaporator coil, this may be defective and require replacement.
Low return air temperatures will cause the same results as the above conditions. The air entering the evaporator coil is too low and may be from supply air recirculating or the thermostat set too low. It is important to check these temperatures prior to troubleshooting.
If the outside temperature drops below 60 and there are no condenser fan controls, the condensing pressures drop below their normal operating levels. This causes a pressure drop in the refrigerant entering the evaporator coil and this will cause the same symptoms as when the unit is low on refrigerant. Operating a condenser in lower temperatures can be accomplished by installing a low-ambient kit which keeps the condenser fan off until the temperature/pressure inside the coil reaches a certain level.
Remember, when you see ice on your air conditioner pipes shut the system down and allow it to defrost. Check for proper airflow or call in an HVAC professional to find and repair the problem.
You’ve been outside when your air conditioner is running, right? You’ve heard that refrigerator-on-steroids buzzing sound, and felt the hot air rushing out the top of it. Wait. What? It’s not rushing out from the top?
What if the fan is sucking air down from the top, through the unit and out the sides? Turn it off immediately and call an electrician.
The air flow, and fan direction, in an exterior AC unit should be such that it is pulling cool air in through the louvers on the side of the case and ejecting hot air up through the top of the unit. This is because hot air rises, and it's simple physics—we wouldn’t want to engage in a brawl against the way nature has been organized. If your AC unit is attempting to suck air in through the top and eject it down and out through the louvers, you've got problems.
Additionally, the force of the gust of air coming out from the top ought to be strong enough to, say, move a beach ball or flap your gums (decidedly NOT recommended, but a funny word picture nevertheless). In other words, if the column of air being produced by the fan in the AC unit is weak, perhaps it’s not getting enough juice. These units run on 220 AC, like an electric stove. If it has been incorrectly wired to the service panel, the unit will not run correctly, possibly delivering a weak column of air or maybe even running in reverse, which, from your query sounds like a distinct possibility. Again, call an electrician or an HVAC service specialist and have the system looked at. Forcing a unit to run on less voltage than it was designed for will severely damage it.
Depending on the age of the installation, there could be any number of problems. It does pay dividends in the long run to have your system looked over by a qualified technician every once in a while. For newer systems that are younger, it’s perhaps overkill to do this every single year. But for older systems, about fifteen to twenty years old or more, you should be having regular checkups. It’s also a good idea to have your ducts cleaned at least every couple of years, and change the system filter twice a year at a minimum. And again, some of the older units may eject hot air out the front and intake cool air from the back. It just depends--but it's always money well spent to have a technician perform a service call.
Hopefully this tidbit helps explain the situation a bit. There’s no harm in calling in qualified help; if I were you I would have a technician look at it. Until then, if you need to cool down you can always go for a stroll at your friendly neighborhood Wal Mart—it’s like a freezer in there!
Most people assume that if their home is cool during the summer, everything is working properly and no action is needed. On the contrary, you air conditioning system may be working overtime to compensate for system deficiencies that will cause the compressor to fail prematurely. By being aware of your air conditioning run times you may be able to prevent a catastrophic failure of your compressor that may cost up to $1000 to fix.
A residential thermostat controls the operation of the air handler and condenser unit by sensing the indoor air temperature. This corresponds to about 3 cycles per hour at 50% system load. For example, during a moderate summer day in which heat load is 50% of the design heat load, a residential air conditioning system is on about 50% of time, and off about 50% of time. If cycle duration is 20 minutes, then it is turned off after about 10 min, stays off 10 minutes and is turned on again. For a different summer day, the average capacity and run times may be longer or shorter.
Compressors are more efficient the longer they run. The figure below illustrates that if the on-time of an air conditioner is only 5 minutes the efficiency (EER) is 6.0. If the air conditioner ran for 9 minutes to provide the same amount of cooling the efficiency would rise to 7.0. This represents a savings of 17%.
Most of the cooling season the cooling loads are well below the capacity of properly sized air conditioners, and for oversized units the short cycling is a substantial problem. Improper unit sizing is one of the main reasons for short cycling.
Along with compressor damage, shorter run times also limit the amount of moisture the evaporator coil can remove from the air. The coil needs to be cold for at least 5 minutes to allow the condensate to begin running down the coil and into the condensate drain. If the compressor shuts off prematurely, the condensate will remain on the coil and evaporate back into the air stream. This would reduce the comfort level of the home and may promote mold growth inside the ductwork and the home.
Some digital thermostats have a built in time delay to prevent the unit from operating on cycles shorter than 5 minutes. Heat pumps often have time delays built in. If you notice that your unit stays off for 5 minutes every time it may be waiting for this delay to expire and then comes on immediately. If the unit only runs for 5 to 7 minutes and then shuts off, it is short cycling.
Your problem may be with the thermostat, such as its location is too close to a supply register, there may be a draft coming through the hole beneath the thermostat where the wire comes out of the wall, or the sensor inside the thermostat is bad.
Most compressors are equipped with a low pressure control for compressor protection. This will shut off the compressor when the evaporator pressure falls below the cutout setpoint. When the compressor is off the pressure will rise if the case temperature is warm and restart the compressor. The compressor then runs and drops the pressure down below the setpoint and shuts it off again. This is often a indicator of a low refrigerant charge, but can also be caused by a clogged filter. If it is the filter that is clogged, there will be a temperature drop across the filter. If the temperature drops more than 2 degrees F from the filter inlet and outlet this indicates a restricted filter-drier.
See my related article on locating refrigerant leaks
If your unit has a sight glass installed in the liquid line, which is not typical, you would see a continuous stream of bubbles if the refrigerant charge is low. When the charge is correct, the sight glass will look clear.
You may notice that the compressor is very hot. You should not touch the compressor, but if you place your hand close to it of one of the refrigerant lines, you may be able to feel the heat coming off of the unit. In extreme cases you may be able to smell the paint on the compressor or a slight burning odor. Shut the unit off immediately and call an HVAC technician for service.
It is unfortunate that the a common cause for equipment failure is due to improper sizing when the unit is installed. Larger air conditioning systems are not better, they have shorter run times, reduced moisture removal, and have higher initial costs. They will also cost more to operate over the life on the unit. It would be better to slightly undersize the unit and have it run twice as long than to oversize the unit and have it have 10-minute cycles.
Being able to notice slight changes in the operation of your air conditioner can potentially save you hundreds of dollars in repair or replacement costs and make your home more comfortable during the heat of the summer months.
For more information, look at the Energy Star publication.
In other articles I have gone over some fairly technical repairs on heating, ventilation, and air conditioning (HVAC) systems, this article will discuss how to replace the drive belt on the blower of your furnace or air handling unit. Replacing the belt on the blower is easy and can prevent other issues from cropping up such as frozen coils or reduced airflow. Drive belts can last for years, but the more you use you air handler, the faster the belt will deteriorate and need replacement. Replacement belts can be purchased at automotive parts stores, home centers, hardware stores, or online.
Related Article: Fixing Frozen Air Conditioning Coils
While the belt replacement is simple, there are a few items to make note of as you can damage your blower motor or bearing if you are not careful.
When to Replace your Drive Belt
Most people wait until the drive belt breaks before they run out and buy a new belt and replace it. If you haven’t replaced your belt in 3 years, buy two so you have a spare after you replace the current belt inside your unit.
If you use your air handler for heating and cooling you should replace the belt at least every three years. Check the belt in the spring and fall and look for any abnormal wear on the edges or fraying and cracking. There may be some black powder or dust around the blower and inside the unit. If it appears to be excessive, you should replace the belt. If you take the belt off and twist it gently, it should not be brittle or cracked on the surfaces touching the pulley. It may be shiny which is called glazing.
Types of Belt Failure
Before you Get Started
Make sure that your air handler or furnace is belt-driven and not a direct drive motor. Direct drive units do not use belts, so if you here an unusual noise or the airflow is reduced you may need to replace the motor.
Take a look at the belt to see what size it is. Most residential units will be v-belts, but you may also come across cog belts. Sometimes cog belts are used when v-belts are not on hand, but some contractors like to use them because they last longer and perform better depending on the size of the pulley and motor. If you do not have the owner’s manual on hand for your furnace or air handler, replace the belt with the same one that you find inside.
All belts are marked on the outside with the width and circumference. The old numbering system used the letters A, B, C and D for the width, an X after the first letter means that the belt is a cog belt; and the circumference is a 2-digit number. The newer numbering system uses 3L, 3V, 3VX, 4L, 5L, 5V, 5VX, etc with a 3-digit number which is the circumference in millimeters. For example a belt labeled “AX-36” is a cogged belt with a circumference of 36 inches. If you can’t find the number on the belt, bring it with you to a location that sells drive belts so they can determine the size.
Tools and Materials:
Grease or oil
1. Shut the power off to the unit
Locate circuit breaker or disconnect for the furnace of air handler and shut it off. There is usually a switch on the side of the furnace with a red cover that should be shut off as well. Set the fan selector switch on the thermostat should be set to the ON position to ensure that the unit does not start.
2. Access the Blower
Most blowers are located near the bottom of the unit in vertical configurations, or just after the filter rack.
There is usually an access panel on the side or front of the unit. On some furnaces the panel lifts off or is interlocked with another panel that needs to be removed first. Sometimes there are screws that hold an inside panel to cover the blower. You will need a screwdriver to remove this panel if present.
3. Remove the old Belt
One of the biggest mistakes one can make when replacing a belt is to just pull it off. Even though the old belt is worn and stretched out, it can still damage the shaft of the motor or blower if it is stretched around the pulley. With a marker, mark the location of the motor mount so it can be set back to its original position.
Use an adjustable wrench or a set of box wrenches to carefully loosen the mounting bolts of the motor. Just a few turns is enough as the bolts only need to be loose enough to slide the motor toward the blower housing. Once there is enough slack in the belt, pull the belt off around the smaller pulley of the motor and then off the larger pulley of the blower.
Clean off any dirt or debris from the pulleys and blower housing. You may want to vacuum the inside of the blower and squirrel cage also.
Dirty Squirrel Cage
If the motor and blower have oil ports or grease fittings you should lubricate them. Most motors will have an oil port covered by a set screw that can be removed with an Allen wrench. Add a few drops of oil into the hole; be careful not to overfill or get any oil onto the copper motor windings inside.
Adding oil to the blower motor
4. Replacing the Belt
Install the new belt onto the pulleys of the motor and blower, starting with the smaller pulley of the motor first. Slide the motor mount back to the mark you made previously. The belt should be taut, but it should have about 1/2 inch of give when pushed down in the middle of the span with your fingers. Tighten the bolts when the proper tension is achieved. Reattach the access panel and turn the circuit breaker back on.
Check for proper belt tension
Set the thermostat back to the normal settings and the fan selector switch to auto. Check to verify that the fan is running quietly and supplying adequate airflow throughout the house.
For various reasons your home may not have central air conditioning. Your home may have been built prior to air conditioning being considered standard, you may have a hot water heating system (hydronic), you may have used window air conditioners to cool only the rooms you use most often, or it may just not be economically feasible for you at this time. Whatever the reason there are several things you need to investigate before you are able to have air conditioning installed in your home and while some of them are fairly obvious, there are other significant details that you may not be aware of that will add considerably to the costs.
Installing a central air conditioning system is not something a do-it-yourselfer can do, but you should be aware of what is needed so you can compare bids from prospective contractors. You will also need to apply for a permit and have the unit inspected. This not only protects you and your home, it also keeps the contractor honest. Stay away from anyone who says that you don’t need to apply for a permit for installing a central air conditioner.
Furnace and Ductwork
The easiest component for a homeowner to verify is the existence of a furnace and ductwork. Air conditioning can't be installed on a boiler, radiant heat systems or space heaters. Furnace is the name for central units back when they were heating only, today they are referred to as air handlers since they move either warm or cool air.
What may not be apparent to the homeowner is that ductwork that carries warn air for heating is not large enough for cooler air required for air conditioning. Cool air is heavier and denser than hot air and it is harder to move and small ductwork increases the static pressure which causes the blower motor to work harder and possible fail prematurely.
The Air Conditioning Contractors of America (ACCA) have a design tool called Manual D which is used for sizing residential ductwork for the appropriate amount of airflow. For each ton of air conditioning you will need about 400 cubic feet per minute (CFM) of airflow. If the ductwork is too small it will restrict the amount of airflow crossing the air conditioner evaporator coil and not provide the required cooling and may damage the compressor. If the ductwork is too big the system may have a problem generating enough static pressure to maintain designed airflow across the coil. An informative guide is available here to see why ductwork calculations are important.
Remember that you not only need supply ductwork, you also need return ductwork. Usually the wall cavities can be used as return plenums in rooms and then a hole is cut through the subfloor and a sheet metal connection is installed in the hole to make a transition to the return trunk line back to the unit. In older homes it may be very difficult to install all of the ductwork required, you may decide on an open return to draw air into the unit if you have a smaller home or a single-story home. Otherwise you will need to use a closet or a corner of a room to run the ductwork to get to all of the rooms. Another option would be to install a high-velocity system which uses smaller ductwork. See article: https://knoji.com/installing-a-high-velocity-air-conditioning-system/
An evaporator coil must be added to the furnace. The coil may be an upflow A-coil for a vertical furnace and a slant coil for a horizontal configuration, which is less common. There must be sufficient height in the basement to set the coil on top of the unit. Most coils are 22 to 30 inches in height depending on the capacity. An A-coil may be around 24 inches high for a 10 SEER unit and over 30 inches high for a 12 to 14 SEER unit. If you have a horizontal furnace as would be found in a crawl space, the coil installs at the end of the furnace and again, there must be sufficient room.
A-Coil and Flat Coil
Most people are aware that they need an outdoor unit, commonly referred to as a condenser. This is where the heat from the inside of the home is rejected to the outside by way of a fan that moves outdoor air over the condenser coil and cools the refrigerant to a point where it can be condensed back to a liquid by the compressor. The condenser needs to be placed on a level surface, but it can be mounted on bracket above grade if building codes in your area allow this. If the condenser is placed on grade it should be on a lightweight concrete pad on a few inches of gravel.
The evaporator coil installed in the furnace needs to be connected to the outside condenser coil by two (2) copper lines, a suction line and a liquid line.
The distance from the evaporator to the condenser is usually not a problem, but if the lines are over 80 feet in length they may need to be oversized. Liquid and suction lines are usually 3/8” and 3/4” in diameter respectively and a good reference can be found at the Amana website.
Once the condenser is installed, an electrician needs to run main electrical power to it. The condensing unit typically operates using 230 volts. You main electrical panel must have space for a 2-pole breaker and also the main box you have must have the capacity needed electrically. Typical breaker sizes are 30A for a 4-ton unit. If you have an older home you may need to install a sub panel to supply the electricity to the condenser or you need to upgrade the main panel which can be a considerable cost. If you need to upgrade your panel you need to apply for an electrical permit and you may be required to upgrade other components in your home such as wiring, switches, and receptacles.
A service disconnect will also need to be installed near the condensing unit outside. An electrical disconnect must be located in sight and not directly behind the condenser so it’s easily accessible. Depending on the manufacturer, enclosures have a disconnect handle located on the side of the enclosure or under a weather-proof cover.
Electrical Disconnect mounted to the condenser
The thermostat will need to be changed to a Heat/Cool thermostat if you have a heat only model. Also the wiring from your furnace to your thermostat needs to have at least 4 conductors to be able to utilize the new condenser. If you have only 2 conductors which is typical with most heat only systems, a new 4-wire thermostat wire needs to be run from the thermostat location to the furnace.
Thermostats can be simple manual types(shown) or digital programmable devices
As the new air conditioning system cools your home it will also remove moisture from the air. This moisture is collected in a condensate pan under the evaporator coil and then drained away by a PVC drain line. The condensate can be dumped into an existing sump pit or if you do not have any drain lines available, you will have to install a condensate pump and run the discharge line to the outside or to a utility sink. You cannot connect a condensate drain line to a waste line since the air handler can dry out the trap and pull in sewer gases.
Condensate drain line connected to a condensate pump
The best way to save money with your air conditioner is to have it sized properly. Rules of thumb are only estimates that are not sufficient to size the capacity of the air conditioner compressor and coil. Even if your neighbor has the same house as you have there may be slight differences which can affect the capacity of the unit. Number of windows, window coverings, the orientation of the home, color of the roof, and shade from trees all have an impact on the calculation of the cooling load. Properly sizing the unit will not only save money on cooling costs, it can also lengthen the life of the unit so it does not short cycle.
Have a qualified contractor perform a cooling load calculation using Manual J.
Hopefully this article sheds some light on this expensive addition to your home.
If you have ever had the experience where one room in your house is either too hot or too cold, you may need to adjust your balancing dampers in the branch ductwork in your home’s HVAC system. You may find that you don’t have any balancing dampers installed, which is all too common.
Many contractors will simply use the floor or wall registers to adjust the amount of supply air entering a room. This is not a good practice for several reasons;
1) the dampers on floor and wall registers can completely seal off any airflow which can lead to the branch duct being pressurized and splitting that will lead to energy being wasted,
2) the dampers on floor and wall registers can be very noisy which can be annoying since they are in the room they serve,
3) the dampers can be easily moved by children or when cleaning and change the airflow rate.
Balancing dampers are designed to have a gap around the edges to prevent pressure building up in the ductwork.
In forced air system the cooling load drives the size of the ductwork since the temperature difference is lower. If the outside temperature is 95 DF and the supply air temperature is 55 F, the temperature difference is 40 degrees F. In the winter, the outside temperature may be 25 F and the supply air temperature can be 110 F for a temperature difference of 85 F!
You can see that you would need less than half the amount of air to heat a space as you would to cool it, but since the difference between the indoor and outdoor temperature is also greater in the winter, you need more warm air because the house loses heat faster. It also depends on the type of construction, insulation, and number or windows, but still, the cooling load is greater than the heating load, unless you live in Northern climates.
CFM per Room – Estimates
Bedroom – 70 CFM – 5” round duct
Large Bedroom – 125 CFM – 7” round duct
Living Room - 150 CFM – 8” round duct
Family Room – 180 CFM – 8” round duct
To check the size of your ductwork, you can use this online calculator to see what size ductwork should be for various airflow rates. http://efficientcomfort.net/jsp/ResDuct_Web.jsp
Many times contractors will use the same size flexible duct for all their branch runs, usually 6 or 7 inches in diameter. This can cause some zones to be overcooled which is another reason why balancing dampers should be installed.
Installing a Balancing Damper
Balancing or volume dampers can be purchased at some home centers and also online. If you have rectangular sheet metal ductwork for your branch lines, you may need to go to an HVAC supply house or order them online.
Balancing dampers for round ducts are simple sheet metal disks that can be turned with a small handle and locked in place with a wing nut. They can be hard to find on flexible duct work since the insulation jacket is often left long and pulled over the handle. The damper should be located near the takeoff of the main duct trunk. You can buy balancing dampers inside a metal sleeve that can be connected to the branch line and then to the flex run.
With fiber board or sheet metal ductwork, there should be a round takeoff coming off of the main trunk. Flexible ductwork is then attached to the end of a rigid sheet metal duct. To install a balancing damper, cut the metal or plastic duct band that holds the flex duct to the branch line. You can attach a balancing damper and collar directly to the end of the metal duct and then reattach the flex duct to the other end of the collar. Use self-tapping sheet metal screws to secure the balancing collar to the metal duct and then a new plastic band for securing the flex. The plastic duct bands look like large wire ties.
Balancing Damper shown with Collar and Handle (Ruskin, Inc.)
For installing a new collar into fiber board ductwork, you will need to install a fiber board duct collar and then screw the balancing damper into that.
Rectangular Balancing Damper
The balancing damper should be at least 3 duct diameters away from the connection to the main trunk. For example: if the branch duct is 7 inches in diameter, the damper should be no closer than 21 inches from the takeoff connection on the main trunk. This reduces noise and provides better control of the airflow.
Fiberboard Takeoff Connection shown with Damper
If you are working with sheet metal ducts and are unable to remove a section of ductwork you can cut a slit in the side of the branch line with aviation shears and then drill a hole in the other side of the duct for the pin to set in place. Then drop the damper in place into the hole and secure with a washer and nut. For the side with the handle, cut a piece of sheet metal to cover the slit, drill a hole for the other pin and secure the cover with sheet metal screws. Attach the handle with the included wing nut.
Damper Hardware (Ventfabrics, Inc.)
Unless you have special measuring equipment you won’t know exactly how much air is flowing through the duct. The airflow is not linear, so closing the damper halfway will not cut the airflow in half. It will be a process of trial and air, sorry error, until you get the desired effect. The damper should always be open at least a third of the way.
Remember that without accurately measuring the airflow in your system’s ductwork you will be subjectively controlling the airflow in a room. You may have to readjust the position in the winter and summer, so it is a good idea to make a mark on the collar to so the correct position.
Ventfabrics, Inc. (Ventlok)
Most homes with central air conditioning and forced air heating systems have a single fan in the unit to distribute air throughout the home. In some instances, you may need to add a booster fan, also known as an inductor fan, in a branch duct to increase the airflow to a particular room. Long duct runs increase friction and reduce airflow as well as old leaky ductwork. A typical 6 inch booster fan uses less than 50 watts of power when running and can be connected to a switch or wired directly into the controls of your air handler so that it comes on when the unit is on. Increasing the airflow in a room will help mix the air and even out the temperature improving comfort and indoor air quality.
See my article on Duct Booster Fan Reviews.
Prior to installing a fan, you should inspect your system for any leaks in the ductwork and ensure that the fan and belt are functioning properly. See the end of the article to review proper belt tension and fan motor maintenance. You may also want to see my article about duct cleaning.
Purchasing the Proper Booster Fan
When considering a booster fan you must know a few things about your system. You need to know the duct size of the branch you are installing the unit in, the material your duct is made of, and if your ducts are connected by screws or by tape. Metal ductwork uses screws and fiberboard ductwork uses tape. Most branch ducts are round flexible ductwork, usually 6 inches in diameter, but can be 5” to 10”. Have all of this information on hand when you go to purchase your fan.
6 inch In-line Booster Fan
Locating the Fan
When picking a location for the booster fan you should consider how close it is to available electrical connections, whether the unit is hard-wired or plug-in, ease of access for replacement or cleaning, and keep the fan at least 5 equivalent duct diameters away from any connections or the room register. An equivalent duct diameter is the size of the duct the fan is located in multiplied by 5. So a booster fan in a 6 inch diameter duct should be 30 inches away from the connection from the main trunk of the duct, usually a rectangular duct, and 30 inches away from the register in the room. This will help keep the noise down in the room and deliver the stated amount of air. Turbulence occurs after elbows and takeoffs in ductwork and the farther the fan is from them, the better it will work.
Installing the Fan in the Duct
Follow the installation instructions included with the fan. For round flexible duct you will probably only need two quick-ties included in some kits and duct tape to seal the insulation around the connections. You can cut flexible duct with a utility knife or scissors and a pair of pliers to cut the spiral metal wire that gives the duct its shape. For installation in metal round duct, you will need to cut the ductwork with a reciprocating saw or sheet-metal shears. You should remove the length of duct and make your cuts on a workbench or similar area. The duct can then be slid over the connectors on the fan and screwed together with self-tapping sheet metal screws and then sealed with foil duct tape. If you have rectangular ductwork going to the register, you will need to install a transition on either end of the fan. This changes the fitting from a rectangle to a circle. When purchasing the fan and transitions, the diameter of the fan should be the same as the smallest dimension of the rectangular duct. Example: a 6 by 9 inch duct would have a 6 inch booster fan.
The fan will have an arrow on the side showing the direction of the airflow. Make sure you install the fan with the arrow pointing towards the room register.
Spin the fan blades of to make sure that there are no obstructions and they move freely. You may want to test the fan prior to completing the installation, if it is a plug-in type, simply plug it into a receptacle keeping your hands clear of the spinning blades. If it is a hard-wired type, you may want to temporarily wire a plug to the wires and plug it in. Follow the directions on the plug you are using; the black wire connects to the brass screw and the white wire is connected to the lighter aluminum screw.
Take the wires of your air duct fan from the side opening of the unit and then attach them to the duct with the use of tape or loose quick-ties. Your booster fan will have specific instructions when it comes to connecting your unit to a power source. But if you want to use a switch to turn on the fan, splice the fan wire and attach it to a plug which you can then plug into a circuit breaker that has a switch or run the wires to a junction box and then to a light switch.
Belt and Fan Maintenance
Most homeowners forget about them until they break, but the belt that connects the fan pulley to the motor stretches and cracks before eventually breaking. Most of the time they will break as soon as the fan starts since the tension will be greatest to overcome the inertia of the blower and air in the ductwork. Before breaking the belt becomes looser and more brittle, it slips on the pulley reducing the amount of airflow through the system. This causes the unit to remain on longer and could cause the evaporator coil to partially freeze, thus reducing the airflow even more.
A quick visual inspection can determine if the belt needs to be replaced. Turn the power off to the unit with the electrical disconnect or breaker, remove the access panel and press down on the belt at the midpoint between the 2 pulleys. It should not depress more than an inch, but if the belt is not frayed or cracked, you can loosen the motor mount bolts and slide the motor back slightly to take up the slack. New belts can be ordered on line, or purchased at most auto parts stores. Smaller belts wear out sooner than larger ones since they heat up faster.
Your fan might also have grease fittings or oil ports for lubricating the bearings on the fan shaft and motor. Inexpensive grease guns and oil cans can be purchased at home improvement centers. Don’t use spray lubricants as they can get on the surface of the pulley and belt and cause it to slip and deteriorate more quickly. If oil or dirt enter the windings of the motor they can create hot spots that could cause the motor to burn out prematurely. Newer units will most likely have sealed bearings that don’t require lubrication.
Depressing the belt more than 1" could mean that it's time to be replaced.
Ripped or damaged air conditioning flex ducts reduce the HVAC systems energy efficiency. This can add several dollars to each month's electricity bill. Calling an air conditioning duct mechanic to fix rips can be expensive. This type of service call is almost all labor and profit. Even in the worst cases damaged air conditioning flex duct can be repaired by a homeowner for far less money.
The materials you need include HVAC duct mastic, paint brush, duct tape, duct mesh, knife, wire cutters, and panduit straps. Panduit straps are the large zip ties that hold the flex in place. Slice connections will also require a metal splice collar. Sections of flex duct that has been completely crushed will need that section replaced with the same size flex and two collars. Flex has its size printed on the outside covering.
Inspect the area where each flex run connects to a mixing box or a vent boot. If the flex connection has completely fallen off the collar then you must remove the old panduit straps and duct tape. There should be a strap on the inner layer and another around the outer layer.
Pull back the outer layer and the insulation. Reveal 8 to 12 inches of the inner layer. Slide the inner layer back onto the collar. Leave 1 inch of collar showing. Attach the inner liner in place with duct tape. Use a pair of pliers to install a new panduit strap over the inner liner. Pull the outer layer over the connection and install a panduit strap. Use a brush to apply duct sealing mastic to the connection.
Connections that are still connected but have small air leaks need the outer layer pulled back for inspection. If the inner is still attached simply seal the connection with mastic. Push the insulation back over the wet mastic and reattach with a panduit strap. Unattached inner layers will need to be completely redone. While you are at the connections make sure that the hanging straps are supporting the weight of the duct. Every place that there is a collar needs a hanging strap near-by.
This is the most common flex duct repair. During installation tiny rips go unnoticed. Through time these small tears become larger. The outer layer over the insulation acts as a vapor barrier. This keeps the cold air in and the hot air out. Close each rip with a piece of duct tape. place a section of duct sealing mesh over the tape. Cover the mesh with duct mastic. Large rips that require more than one layer of mesh should have a hanging strap close.
Crushed Flex Duct
There is not good way to repair crushed flex. All methods of rebending the wires will fail. The crushed section must be replaced. This does not mean that the whole duct run is bad. A new piece of flex can be spliced into place.
Cut the outer liner and insulation about six inches past the crushed section. Roll the outer liner over the insulation. Cut the inner liner 4 inches away from the rolled insulation. Wire cutters and a knife will be needed. Do this on both sides of the crush. Cut a section of new flex 1 foot longer than the piece that you removed. Roll both ends of the insulation. Install collars on both sides. To install the collars you will need to duct tape and strap the inner liner to both the original and the splice pieces of flex. Then pull the rolled ends together. Duct tape the seams. Wrap the seam with flex mesh and paint with mastic. A hanger must be installed close to each collar.
Repairing damaged flex duct will help lower your electric bill. You can save even more by doing the work yourself. The best part of this project is the money you invest now will come back to you each month from now on.
A common defect found during home inspections is either missing or deteriorated insulation on the air conditioner refrigerant lines, in particular the suction line.
On a split system air conditioner or heat pump the refrigerant line-set runs between the evaporator coil inside the air handler or furnace inside the home and the condensing unit that contains the compressor and condenser coil on the exterior of the house. The refrigerant moves between these two units by way of two lengths of copper tubing; the liquid and vapor lines.
Liquid Line – The smaller diameter copper tube that carries liquid refrigerant between the two units. This line gets very hot when operating as the refrigerant is pressurized. In most cases this line should not be insulated. There are circumstances where the line should be insulated when the surrounding temperatures are high to reduce heat gain, as in the case of attic installations. Another situation where the liquid line can be insulated is when the suction and liquid lines are clamped together so as to prevent heat transfer from the liquid to the suction line.
Vapor or Suction Line – The larger diameter tube is the suction line that carries refrigerant vapor between the two units, this line is very cold, around 20°F when the unit is in operation as the vaporized refrigerant cools as it expands. This line should be insulated for the entire length to prevent the vapor from gaining heat from the home or the outdoor temperature. If this line is exposed to ambient conditions, the efficiency of the air conditioning system will be reduced.
Condensation on the exterior section of the suction line is usually harmless if unit sits on the ground, but as stated previously, system will reduced efficiency and will have to run longer to maintain the room temperature setting on the thermostat. Missing or damaged insulation can also cause premature compressor failure due to the extended run times.
Refrigerant lines insulation should be also installed from the air handler or furnace penetration to the condenser unit and even inside the condenser to the point where the line enters the compressor. Verify that the suction line is insulated inside your air handler unit as well since uninsulated lines will sweat and the condensation may damage internal components such as the heat exchanger.
Missing damaged or improperly installed insulation on the refrigerant line may cause corrosion, discoloration, could compromise internal components of the air conditioning and heating system, and significantly shorten its life span of damage the unit which will result in costly repairs. Missing or inadequate insulation can cause ice to form on the suction lines, especially near the compressor or evaporator coil, and reduce system efficiency.
Ice forming where foam insulation is damaged or missing
Missing insulation on the suction line running inside walls, ceiling cavities, or through the attic will also result in condensation which could result in damage to interior finishes and can cause mold growth.
Missing refrigerant line insulation in an attic space
If the insulation on your air conditioners suction line is deteriorated or missing you can replace with foam insulation that can be purchased at your local home center or hardware store. Foam is resistant to deterioration caused by any condensation that may form on the surface of the copper tubing. Also, fiberglass insulation can breakdown outside if it is exposed to sunlight.
- Use electrical tape to secure the insulation if it does not have an adhesive strip. You can also purchase special UV resistant line set insulation tape.
- Be careful not to compress the insulation as you will reduce the thermal properties and ice may form at this point.
- Protect any insulation from damage where it can rub against hard surfaces or be damaged outside from lawn equipment or pests.
- When using slit insulation, place the slit side down so any condensation that does form can drain away from the tubing. If it is inside an enclosed or finished space, install the slit side up.
- For small repairs you can apply adhesive foam or cork insulating wrap around the missing section.