Adding Air Conditioning To Your Existing Furnace

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Steps required to retrofit air conditioning to your existing furnace.

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:

Evaporator Coil

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

Condenser 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.

Refrigeration Lines

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.

Electrical Connections

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

Condensate Drain

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.

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Posted on Apr 16, 2011