Sizing: the decision everything else depends on

Learn why a contractor should consider the home's actual heating and cooling loads rather than relying only on rules of thumb.

Too big is a problem, not a bonus

• Oversized cooling short-cycles: it hits the thermostat fast, then shuts off before removing humidity.
• Oversized heating can be noisy, uneven and harder on equipment.
• Both cost more to buy and to run.

Too small is just as bad

• The system runs constantly on the hottest and coldest days and still falls behind.
• Backup heat (on a heat pump) runs more, raising operating cost.

What proper sizing looks like

A load calculation — Manual J — accounts for your home's size, insulation, windows, orientation and local climate, and produces the capacity the home actually needs. Equipment is then selected (Manual S) to match that load. If a proposal lists a size with no basis, ask for the calculation. See Manual J, S & D in plain English.

Reference materials

• ENERGY STAR: 10 tips for hiring a heating and cooling contractor
• ACCA: Manual S residential equipment selection
• U.S. Department of Energy: heat pump systems
• Washington L&I: verify a contractor