Why Bigger Is Not Better: HVAC Sizing In Washington Homes
The fastest way to get a new system wrong is to read the old nameplate and order the same size. It is also, unfortunately, the most common way it gets done.
What oversizing actually does
An oversized air conditioner cools the air quickly, satisfies the thermostat, and shuts off before it has run long enough to pull moisture out. You get a house that is cold and clammy instead of comfortable. It also short cycles, and every start is the hardest moment in a compressor's life.
An oversized furnace behaves similarly. It blasts, satisfies, and stops, leaving big temperature swings and cold spots at the far end of the duct run.
Why the old size is not a guide
Houses change. Washington homes built before 1990 have frequently been re-insulated, had windows replaced, had attics air sealed or had additions built. Each of those changes the heat load substantially. A furnace sized correctly for a 1978 house with single-pane windows can be two sizes too large for the same house today.
What a load calculation looks at
- Square footage, ceiling heights and volume
- Insulation levels in walls, attic and floors
- Window area, orientation and glazing type
- Air leakage and infiltration
- Local design temperatures, which differ a lot between Puget Sound and the Columbia Basin
- Duct location, whether ducts run through unconditioned crawlspace or attic
Ductwork is half the system
New equipment on undersized or leaky ducts will underperform no matter what the efficiency rating says. Common problems we find in Washington homes: return air that is far too small, flex duct crushed under crawlspace insulation, and boots disconnected from registers so conditioned air dumps under the floor.
What to ask for in a quote
Ask whether a load calculation was performed and ask to see the result. Ask what the static pressure measured. Ask whether the return air is adequate for the new blower. A contractor who measured will happily answer. See how we handle furnace installation and AC installation. You can book in any Washington city — Seattle, Spokane, Tacoma, Vancouver or Yakima — from our service areas page.
Need this fixed rather than explained? Call (888) 380-4427 any hour, or find your city on our service areas page.