The most common misconception that homeowners have about HVAC equipment is the fallacy that “bigger is better.” The rationale is that it doesn’t have to run very long to satisfy the thermostat. In reality, the longer the run cycle with less BTU’s results in a much more comfortable home. An oversized piece of equipment will short cycle, which can cause premature failure and makes for a very uncomfortable home. With furnaces, short cycling will destroy the heat exchanger and with cooling it results in a cold, clammy home.
Rules of Thumb for Sizing HVAC Equipment?
In the last 40 years, I have seen many “rules of thumb” for sizing equipment. Unfortunately, this resulted with the vast majority of equipment being oversized. The manufacturers responded by designing multistage units, and introducing variable speed motors. This resolved some problems, but not all. The only proven method for sizing equipment is to do a “Manual J” load calculation.
These load calculations vary based on geographic locations. Our area has completely different design criteria as opposed to Arizona. Within the last few years, ACCA (Air Conditioning Conditioning Contractors of America) changed the design criteria in many regions, including ours. The current heating standard is to be able to heat a home to 70 degrees F with an outdoor temperature of 2 degrees F. For cooling, it is 75 degrees F inside with an 89 degree F temperature outside. The crux of the matter is to make sure when replacing your old HVAC equipment, to only use N.A.T.E. certified companies that adhere to strict factory standards.