Why an HVAC system has to be sized to the house

Many older furnaces and central air conditioners are bigger than the house needs, because they were sized by a rule of thumb instead of a calculation. The right size depends on the house, since insulation, air sealing, better windows and tighter ducts all cut how much heating and cooling it needs. A house that has been insulated and air sealed since the last system went in may need a smaller one now.

What a load calculation is

The Air Conditioning Contractors of America publishes the procedures a trained HVAC contractor uses. Manual J estimates how much heat the house loses in winter and gains in summer. Those loads go into Manual S, which picks the equipment, and the ducts are sized and laid out under Manual D. Under Manual S, a furnace stays within 140% of the house's peak heating load, and cooling equipment no more than 15% over the cooling load.

Equipment comes in set sizes, so the closest match is sometimes a little larger than the load. When the next size down can't meet the peak load, Manual S allows the larger unit.

What goes wrong when it's too big

An oversized air conditioner cools the house quickly and then shuts off. It runs in short cycles, and it's least efficient in its first minutes, reaching peak efficiency after about 10 minutes. When each run grows from 5 to 9 minutes, efficiency improves by 17%.

Short cycles also leave moisture in the air. An air conditioner dehumidifies by running long enough for water to condense on its coil and drain away. With short cycles, less water drains and some evaporates back into the house. Air that isn't dried properly feels uncomfortable and helps mold and mildew grow indoors, which matters most where summers are humid.

Bursts of cold air from an oversized unit can make the thermostat shut the system off before the whole house is cool. Frequent starts add wear, and the unit's large output hides other problems, such as dirty filters, leaky ducts and the wrong refrigerant charge.

What goes wrong when it's too small

Equipment is sized so the peak load will be met, and a unit too small for the house may not meet it.

An annotated example

What to do next

Before any new heating or cooling system goes in, ask for a Manual J load calculation, and list the insulation and air sealing done since the last system. Note any rooms that stay clammy, drafty or uncomfortable, so the contractor can check the ducts and airflow to them as well.