Key takeaways
- An oversized system cycles on and off and leaves rooms clammy.
- An undersized system runs constantly and never catches up.
- A load calculation measures what your house actually needs.
- Ducts must be sized to match the equipment.
What sizing means
Air conditioning capacity is measured in tons, and furnace capacity in BTUs. Choosing the right size means matching capacity to what the house loses or gains on the hottest and coldest days.
Square footage alone is a rough guide at best.
What a load calculation measures
A load calculation considers floor area, ceiling height, insulation, windows, orientation, air leakage and the number of occupants. It gives a room-by-room number for heating and cooling.
The results guide equipment size and duct design.
Problems with oversizing
An oversized air conditioner cools the air quickly, shuts off before it can remove humidity and cycles on and off. The result is a house that feels cold and damp, uneven temperatures and more wear on the equipment.
An oversized furnace short cycles and can create hot and cold swings.
Problems with undersizing
An undersized system runs for very long periods on extreme days and may never reach the set temperature. It wears faster and can leave you uncomfortable in summer afternoons or winter nights.
A well-chosen size runs steadily and quietly.
Quick answers
Can I just match my old system?
It is not a good idea. The house or the old size may have been wrong.
Does a bigger system cool faster?
It cools the air faster but handles humidity worse. Comfort is usually better with the right size.
