Size Check

Is the equipment on your quotethe right size at all?

Nine questions about your house, worked into a rough heat-loss figure. Nothing you enter leaves your browser.

Answer nine questions about your house and this works out roughly how much heat it loses on the coldest day of a normal winter — then says whether the equipment size on your quote is in that neighborhood. Nothing you enter leaves your browser; the arithmetic runs on this page.

What this is not: a Manual J. A real load calculation measures your house room by room — orientation, each window, duct runs, air leakage. This one estimates from a handful of typical assumptions, so it answers a coarse question: is the quoted size in the right neighborhood, or far enough off to be worth asking about? It is a reason to ask a question, never evidence that a contractor is wrong. And it covers heating only — cooling loads depend on sun, glass, and orientation in ways this can't guess.

1 The house

Heated floor area, square feetAll heated floors added together. Leave out an unheated basement or garage.Stories above groundCeiling height, feetSloped or cathedral ceiling? Use the average height, not the peak — a room running 8 ft at the walls and 18 ft at the ridge is about 13.StateSets the outdoor design temperature. One value per state, so your town can differ.

2 What it's made of

When it was built (or last re-sided with insulation added)Attic or ceiling insulation

The attic has been re-insulated at some point (this is the most common upgrade, and it overrides the answer above).The walls are solid brick or block, with no wood framing behind them. Rare outside older city rowhouses — tick it only if you're sure.

WindowsHow many windows

Under the lowest heated floorHow drafty it feels

Where the ducts run

3 The size on the quote

Optional. Without it you get the load estimate on its own; with it you also get whether the quoted size sits inside that range.

Size on the quoteStated inWhich number is thatEfficiency (AFUE %)Only used to turn an input rating into delivered heat.

4 Run it

What the arithmetic actually isOne formula and a table of assumptions, all of it shown. Nothing is estimated by a model.

Heat loss is the sum of what leaks through the surfaces plus what leaves with the air, times how cold it is outside:

Load (BTU/h) = [ Σ (U × A) + 0.018 × ACH × Volume ] × ΔT × duct factor

U × A is each surface's area times how fast heat moves through it — walls, ceiling, windows, floor.ACH is how many times an hour the house's air is replaced by outside air. ΔT is 70°F indoors minus the outdoor design temperature for your state — the temperature only about 1% of winter hours fall below, which is what equipment is sized against, not the record low. Theduct factor adds 15% when the ducts run through an attic or vented crawlspace.

The answers you pick map to standard U-values and air-change rates for that kind of construction. They are typical values, not your house, which is why the result is reported as a band of ±25% rather than a number.

Deliberately ignored: sun and internal heat gains (leaving them out is the conservative direction for heating), orientation, basement wall losses, rooms over garages, cathedral ceilings, additions built differently from the rest, wind exposure, and altitude. Every one of those is a reason a real load calculation would land somewhere else.

The full assumption table — every U-value, air-change rate, and state design temperature — lives in the repository asdata/size-check-assumptions.md.

Got the proposal itself in front of you? TheQuote X-Ray reads the whole document — scope, equipment, what it never mentions — and places its price.