The joke is that Americans are known for a “bigger-is-better” philosophy, but does that apply to home heating and cooling equipment?
As it turns out, the answer is no, for three good reasons:
- An oversized air conditioner will be less effective at humidity removal because of short-cycling. Because the unit has excess capacity for heat removal, it satisfies the setting on the thermostat more quickly, so less air is moved through the system, and therefore less humidity is removed. In extreme cases of over-sizing, this can lead to mold and mildew growth in ductwork and even on walls and ceilings, not to mention the poor comfort quality of cold, clammy air.
- Short-cycling caused by an over-sized system leads to reduced life of the equipment. On/off cycles cause most of the wear and tear on motors and compressors, and also more energy consumption than when they just run at a continuous speed.
- An over-sized heating and air system costs more than a right-sized system. (Who knew?)
So, what does “right-sized” mean? Licensed HVAC contractors (Heating, Ventilation and Air Conditioning) are trained (and should) use something called Manual J for doing load calculations on homes. Nowadays, this is a software package—back in the day, there was a lot of hand-ciphering to do a Manual J. A load calculation factors in: house dimensions and orientation; insulation values for walls, ceilings and floors; window areas, locations and specifications; building air leakage rates; HVAC duct location and leakage rates; internal gains from lighting and appliances, etc. The software spits out separate design loads for heating and for cooling, so the appropriate equipment can be matched to just barely meet those peak loads, with a minimal amount of over-sizing.
Over the years, some rules of thumb have been developed based on the experience of HVAC installers. Rules of thumb can be handy, but in this case they have not kept up with improvements in house construction. Tighter building envelopes, better insulation and other efficiency improvements required by modern energy codes or voluntary certification programs result in heating and cooling design loads that may be much less than called for by rules of thumb.
Manual J load calculations will be erroneous if incorrect inputs are used. This occurs quite often, intentionally or not, when actual building conditions are not properly represented. If a HERS rating was done for a new home, this gives an independent heating and cooling design load calculation that is equivalent to a Manual J.
So, if you’re in the market for a new home, inquire about HVAC sizing—did the contractor do a Manual J? Was the house HERS rated? If so, what did the HERS rating call for? Ask what tonnage of AC was installed; if it’s around one ton per 500 square feet, it sounds a lot like rule of thumb sizing. If the house is billed as energy efficient, you may have reason for concern. If you’re a builder, are you confident your HVAC systems are right-sized? Wouldn’t you rather put that extra cash in your pocket, all while delivering a better product to your customers?
By: Gary Kahanak, HERS Rater