What Size Air Conditioning Do You Need?

Most contractors will check the nameplate on a unit and select the new unit based upon the old credentials. Sometimes, they will even use a rule of thumb. They may do this for a quote, but for actual installation, they need to use a sizing system. The sizing systems known as Manual J and Manual D take these variable into account:

• The local climate
• Size, shape, and orientation of the house
• Insulation levels
• Window area, location, and type
• Air infiltration rates
• The number and ages of occupants
• Occupant comfort preferences
• The types and efficiencies of lights and major home appliances (which give off heat).

There is no way for a HVAC contractor to be able to ball park all of these variable without doing some calculations. So before you sign a contract, make sure the contractor uses a correct sizing calculation. It shouldn’t cost any additional fees because it is also offered by gas and electric utilities, and major heating equipment manufacturers. When designing your new heating and air conditioning system, the contractor you choose should provide:

• A copy of their calculations, assumptions, and the computer printout or finished worksheet
• A written contract listing the main points of your installation and includes the results of the heating and cooling load calculation
• A written warranty on equipment and workmanship
• Delay the final payment until you are satisfied with the new system.

Bigger Is Not Better

Homeowners have oversized A/C units because homes used to leak most of the air. Since most of the cooling/heating was lost, it was not uncommon to install furnaces and air conditioners that had two to four times the necessary capacity. Since many people have added high efficiency windows, caulking, weather-stripping, and insulation to their homes, going by the nameplate is likely to result in an oversized system. Home improvements and adding IceCOLD to reduce oil fouling in your A/C unit will allow you to install a smaller system while still being comfortable, as well as saving large amounts of energy.