"Oversizing is the worst thing you can do," says Richard. An air conditioner that's too big cools quickly with a big blast and then shuts off, without removing as much moisture as a longer cycle does. The result is a cold, clammy house that's prone to mildew. A heater that's too big pumps out heat and then also shuts down quickly. "It's the most inefficient way to drive a combustion engine—stop and go, stop and go," Trethewey says. "You want a long on and a long off."
Good HVAC contractors electronically analyze many variables to determine correct equipment size. "The problem is the homeowner," Trethewey says. "If a contractor says a house needs a furnace that can pump out 100,000 BTUs, most homeowners ask, 'How much extra would it cost to get 150,000 BTUs?' It's like putting a V-12 engine in a VW Bug."