Ask three contractors to quote a heat pump for the same house and you can get three different tonnages. That is not a difference of opinion. It usually means at least two of them never measured anything.
A load calculation is the measurement step. It is boring, it takes 20 to 40 minutes on site, and it is the single biggest difference between a system that runs quietly for 18 years and one that short-cycles, ices up, and gets replaced early. Here is what it is, what it includes, what it should cost you, and how to tell from a piece of paper whether anyone actually did it.
What a load calculation actually is
The industry standard is ACCA Manual J. It is a heat-transfer calculation, run room by room, that answers two questions:
- On the coldest design day, how much heat does this house lose per hour?
- On the hottest design day, how much heat does it gain?
The answers come out in BTUs per hour. Heating and cooling equipment is rated in the same units — 12,000 BTU/hr equals one ton — so once you know the load, you know the equipment. Manual S then matches specific equipment to that load, and Manual D sizes the ductwork to move the air.
That is the whole logic. No rules of thumb, no "400 square feet per ton," no copying the label off the 1998 furnace in the garage and ordering the same number. If you want to see real installed pricing alongside this, our heat pump cost guide for Washington breaks the ranges down, and a proper ducted heat pump install always starts from the load, not the price.
What goes into the calculation
Square footage is one input out of roughly a dozen, and it is not the most important one.
The inputs that matter most
- Insulation values — attic, walls, floor over crawlspace. A 1955 rambler with R-11 walls and a 2018 build with R-21 are different houses at the same size.
- Window area, glazing and orientation. Single-pane versus double-pane changes the number materially. So does which direction the glass faces — west-facing glass drives the cooling load in a Puget Sound August.
- Air leakage. Older homes leak. Leakage is heat loss you cannot insulate away.
- Ceiling height and volume. Vaulted ceilings add volume that square footage hides.
- Room-by-room layout. This is why it is room-by-room: a bonus room over a garage and a north-facing bedroom have wildly different loads even inside the same house.
- Duct location and condition. Ducts in an unconditioned crawlspace or attic lose heat before it reaches the room. If yours are compromised, that becomes a ductwork conversation, not a bigger-equipment conversation.
- Internal gains. People, appliances, and lighting all add heat on the cooling side.
- Your city's winter outdoor design temperature.
That last one is regional, specific, and constantly ignored.
Why your city's design temperature changes the answer
Washington's state energy code (WSEC Table C-1, WAC 51-11C-80100) assigns a winter outdoor design temperature to each area. The load calculation is run against that number. Across Varsity's service area it swings 12 degrees — which means the same floor plan needs different capacity depending on where it sits.
| Winter design temp | Cities |
|---|---|
| Winter design temp17°F | CitiesOlympia, Lacey, Tumwater, DuPont, Yelm |
| Winter design temp19°F | CitiesPuyallup, South Hill, Sumner, Bonney Lake, Graham, Orting, Spanaway |
| Winter design temp21°F | CitiesKent, Renton, Covington, Maple Valley |
| Winter design temp24°F | CitiesSeattle, Bellevue, Federal Way, Burien, Des Moines, SeaTac, Tukwila |
| Winter design temp25°F | CitiesAuburn, Algona, Pacific, Enumclaw, Black Diamond |
| Winter design temp29°F | CitiesTacoma, University Place, Fircrest, Lakewood, Gig Harbor, Port Orchard, Steilacoom |
Olympia and Tacoma are 40 minutes apart and twelve degrees apart on this table. Any sizing shortcut imported from a national spec sheet — or from a contractor who works one end of the region and quotes the other — is starting from the wrong outdoor temperature.
What happens when nobody runs one
Oversized — the common failure
Contractors oversize because it feels safe. Nobody calls back to complain the house got warm too fast. The costs show up later:
- Short cycling. The system satisfies the thermostat in minutes, shuts off, restarts. Every start is the hardest moment in a compressor's life.
- Poor dehumidification. In cooling, an oversized unit cools the air before it has run long enough to pull moisture out. You get a cold, clammy house.
- Temperature swings and noise. Big blasts of air instead of long, quiet, steady runs.
- More money up front for equipment you did not need, and shorter service life for it.
Undersized — the less common failure
Undersizing is rarer but ugly: the system runs continuously on the coldest nights, leans on auxiliary electric heat, and your bill spikes at exactly the moment you were promised savings. On a heat pump this is the difference between a 30–50% cut versus electric resistance heat and no savings at all.
What a load calculation should cost you
Nothing, on a residential replacement. It is part of our free in-home estimate — we do not quote equipment we have not sized. Paid standalone calculations exist and have a legitimate market, mostly new construction and owner-builders who need a stamped document for permitting. If you are replacing a system in an existing Puget Sound home, you should not be buying one separately, and you should be skeptical of any bid that treats it as an upsell.
Want a realistic budget number before anyone visits? Our instant estimate tool gives you Good, Better and Best starting prices in about 60 seconds. It is a price range, not a size — the size still comes from the in-home measurement, and we say that plainly rather than pretending otherwise.
How to tell whether a quote included one
Ask for the output. A real load calculation produces a document with numbers on it: design heating load, design cooling load, the outdoor design temperature used, and the assumptions for insulation and windows. Three questions that sort bids fast:
- "What heating and cooling load did you calculate, in BTUs?" A specific number with a room breakdown is a good sign. "About four tons for a house this size" is not an answer.
- "What outdoor design temperature did you use?" It should match the table above for your city.
- "Did you measure the return air?" Plenty of homes here have adequate supply ducts and starved returns. A bigger air handler on an undersized return just makes noise.
If the answers are vague, the tonnage on the proposal came off a habit, not a house.
What we do on every estimate
A technician walks the house, measures rooms and window openings, checks insulation where it is visible, inspects the duct condition in the crawlspace or attic, and runs the load against your city's design temperature. You get a written proposal with Good, Better and Best tiers, sized to what we measured. Where a local utility rebate is in play, the amount depends on what is being replaced and the equipment installed — we read the utility off your bill, confirm what it currently pays, and file the paperwork. Programs change, so we verify before quoting rather than after.
An estimate visit is a sales appointment; we are not going to pretend it is a free physics lecture. But it is also the only way anyone can tell you the right size, and every install carries a 10-year parts and labor warranty because we sized it ourselves.
Book a free in-home estimate or call (253) 785-4622 — tap to dial. Mon–Fri, 8–6.