What a load calculation looks at
- The size and layout of each room
- Insulation in the walls, attic and floors
- The number, size and direction of windows
- How airtight the house is
- The local climate
- The condition and layout of the ductwork
Why bigger is not better
An oversized air conditioner cools the air quickly and shuts off before it has run long enough to pull humidity out, leaving the house cool but clammy. It also starts and stops more often, which wears parts faster and can make rooms feel uneven.
An undersized system runs constantly on the hottest or coldest days and may not keep up.
Why the old system is not the answer
The system being replaced may have been sized by rule of thumb, and the house may have changed since — new windows, added insulation, an addition or a finished basement. Matching the old size can repeat an old mistake.
What to ask
Whether a load calculation will be done for the house, and whether the ductwork will be checked to make sure it can carry the air the new system needs. A system sized correctly but fed by undersized or leaking ducts still underperforms.