BTU per square foot by climate zone

Between about 12 and 30 BTU/h per square foot for cooling, depending on where you are — and the honest answer is that it is a range within each zone too, because two houses of the same size in the same city can differ by a third on insulation and glass alone.

So a 1,500 sq ft house in zone 4 needs about 32.5k BTU/h of cooling — 2.7 tons — and about 52.5k BTU/h of heating; a 2,000 sq ft house in zone 5, about 39k and 80k. For one room, the same figures apply to its floor area, plus about 600 BTU/h for each person beyond two and 1,200 for a kitchen.

What these figures are

Screening numbers. They are compiled from trade sizing practice by climate zone and they are published as ranges on purpose, because the underlying quantity genuinely is one.

They are not a Manual J. A real load calculation works room by room from the actual construction, orientation, window specification and air leakage of your house, and it routinely lands twenty to thirty per cent away from any per-square-foot figure. Use these to judge a quote or set a budget; get a Manual J before anyone installs equipment.

Where the two loads cross

Read down the table and something useful appears: in zone 1 the cooling load is more than double the heating one, and by zone 6 it is the other way round. They pass each other around zone 4.

That crossover is the technical argument for a heat pump. A single machine that heats and cools makes most sense where the two loads are similar, because it is sized once and used hard in both directions. It is also why the southern half of the country adopted them long before the north did.

The trap in the tonnage table

The second table is the one that gets misused. It is an average house — 8 ft ceilings, ordinary insulation, ordinary glass. A 1970s house with original walls needs roughly a quarter more; a well-sealed modern one needs a third less. That spread is wider than the difference between two adjacent climate zones, which is exactly why square footage alone is not a sizing method.

Using it

The BTU calculator applies these figures with adjustments for insulation, glazing, sun, ceiling height and occupancy, and then picks the smallest manufactured equipment size that covers the load — rather than the next one up, which is the habit that makes houses cold and clammy.