Would you buy a pair of pants twice your size so you have that extra capacity just in case you might need it? No? Did you know that most new homes have a similarly ridiculous case of oversizing? It's true. Look no further than the HVAC system to find it.
That's because HVAC load calculations require time and attention to detail, and most HVAC contractors rely on rules of thumb to determine the sizes of the cooling systems they install. Usually it's based on square footage of conditioned floor area, and contractors in many areas generally use about 500 or 600 square feet per ton as their rule. But every house is different. Even the same house rotated ninety degrees could vary in cooling load by 25% or more.
You may think that having a bigger system is better, but the Texan way doesn't always prevail. In the case of air conditioning, oversized cooling systems result in:
A clammy house because they don't run long enough to dehumidify the air
Shorter lifetime for the system because it turns on and off frequently (also called short cycling). Since it's so big, it doesn't take long to cool off the house.
More expensive to install. That extra size isn't free.
Another reason to installed right-sized HVAC systems is that the ENERGY STAR new homes program requires no more than 15% oversizing of cooling systems.
The correct way to size an air conditioning system is with Manual J, a protocol developed by the Air Conditioning Contractors of America (ACCA). Manual J HVAC load calculations determine how much cooling a house actually needs. It used to be done by engineers with pen, paper, and a slide rule, so you can see why contractors developed rules of thumb. Now it's almost always done with computer programs.
Here at Economy Plumbing Supply we have a team ready to help size your project for the correct sized BTU and load.