What Size Air Conditioner Do I Need?

Every homeowner and commercial building owner should keep a copy on hand of the calculations that determine the size air conditioner needed to cool and heat the structure.  This engineering information is commonly called heat gain, heat loss calculations, load calculations or energy audit.  Most competent air conditioning contractors or service companies will provide this information at no charge.  If they don’t, watch out.  The consumer can also learn to gather the data and calculate the size unit or units needed themselves.  This knowledge is essential when selecting insulation, windows, roof color, window coverings, landscape features and many more decisions made from construction through the life of the building.

For many years energy was cheap, houses were build fast and loose (they leak air) and AC units were sized based on the square footage of the structure.  An example was 500 square feet per ton for cooling.

The price of energy has gone up, builders have learned to build a tight house and I’ve seen a 3000 square foot home in Texas that only needed one ton of cooling.  3000 square feet per ton.

What size air conditioner do you need?  You need somewhere between one ton for each 300 to 3000 square feet.  Skipping the engineering step when sizing a unit is air conditioning malpractice, but don’t worry.  See the online calculator at the end of the article that will do most of the math.

Heat Gain Calculations

Building Envelope.  The proper method to determine the size unit needed is to measure or calculate how much heat gets into the structure or building envelope.  Heat gets in through the ceiling, walls, windows doors and sometimes the floor.  If you measure or calculate the heat gain through each of these building envelope components, you can determine how many BTUH’s per hour you need to offset the heat gain.  If the heat gain characteristics are different, these calculations can be done for each room or zone.

If you had a 3000 square foot ceiling with R-38 insulation under a ventilated attic in Dallas Texas, and you want the inside of the house 30 degrees cooler than the outside of the house on a 100 degree day, you need 3000 X 1.8 Btuh per square foot of ceiling.  The heat gain is 5,400 Btuh.  (12000 Btuh per ton) about a half ton.  If you had R-19 ceiling insulation you need 3000 x 3.4 or 10,200 Btuh.  You can determine this value for all building envelope components.  It’s good information to know if you’re deciding to add insulation and downsize your AC unit.

Internal Heat Gain.  In addition to heat gain through the structure heat is generated inside the building envelope from lights, appliances, people, etc.  (10) 100 watt light bulbs produce 1000 watts x 3.413 Btuh for 3413 Btuh.  If you replace the 100 watt lamps with 19 watt CFL’s, this is good data.  You can reduce the size of the AC Unit more.

Infiltration and Ventilation.  Air gets into your structure through leaky windows, doors, dryer exhausts, kitchen exhaust etc.  The air you have already cooled gets out and is replaced with hot humid air from outside.  The humidity is called latent heat.  The AC must use additional capacity to eliminate this humidity.  The capacity that is used to remove the humidity is not available to reduce the temperature of the air.  This humidity or latent heat must be measured or calculated and Btu’s from the AC Unit must be available to remove it. 

A blower door test can be run in an existing building to measure the amount of outside air entering the house.  The decision can then be made to put in a bigger AC to take care of it, or to seal up the source of the leaks.  In a normal house 30% of the AC Units capacity is used for dehumidification or removing latent heat.  You need to get this calculation right so you don’t live in a cool humid cave.

One last calculation is to measure or calculate the amount of leakage and heat gain into your ductwork if your ductwork is in an unconditioned attic.  This is normally 15% of the unit size or more.  It reality it can be a lot more.  You might want that ductwork under the floor, in conditioned drywall fur- downs or you might want to insulate your roof and not your ceiling.

You can invest some time and learn to calculate your heat gain and or you can require your HVAC contractor to do it.  See the online heat gain calculator at Consumer Reports.