BTU SizerWhat size heat pump does this house need?
Heat pump size calculator
60,000 BTU/h≈ 17.6 kW
The room works out at 75,600 BTU/h — 60,000 is the nearest size actually sold.
- Floor area
- 1,800 sq ft
- Working rate
- 42 BTU/h per sq ft
Heat pumps are rated at 47 °F. At 17 °F expect roughly 60% of that, and at 5 °F about half — check the manufacturer’s capacity table at your design temperature.
One comfort check a month
MonthlyOne thing a month worth measuring, cleaning or switching over before the weather turns — the maintenance side of sizing, in two sentences. No product pitches.
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What size heat pump do you need?
Size it on the heating load, not the cooling load, in any climate with a real winter. An 1,800 sq ft house in zone 4 needs about 75,600 BTU/h of heat, so the ladder lands on the 4-ton, 48,000 BTU/h class — and that is where the interesting part starts.
A heat pump’s headline capacity is measured at 47 °F outdoors. As it gets colder the capacity falls while the house’s demand rises, and the temperature where the two lines cross is the balance point. Below it, backup heat runs. A cold-climate inverter model holds much more of its output — often 75–100% at 17 °F — than an older single-stage unit, which is the whole reason those models exist.
Sizing on cooling instead is the classic mistake in mixed climates: the house needs 3 tons of cooling and 4 tons of heat, the 3-ton is fitted, and resistance backup carries every January evening at four times the running cost.
Two numbers decide whether a heat pump installation is a success, and only one of them is on the box.
| Outdoor temperature | Conventional single-stage | Cold-climate inverter |
|---|---|---|
| 47 °F (rating point) | 100% | 100% |
| 35 °F | ~85% | ~100% |
| 17 °F | ~60% | ~85% |
| 5 °F | ~45% | ~75% |
| −5 °F | little or none | ~60% |
The percentages are typical, not universal — every model publishes its own table, and the gap between two units with the same nameplate can be enormous down the left column. Ask for that table before signing, and check it against the coldest design temperature for your area rather than against the average winter day.
Frequently asked questions
Should a heat pump be sized for heating or cooling?
For heating in zones 4 and above, for cooling in zones 1 to 3 where the heating load is small. In mixed climates the two disagree by a size or more. Sizing for cooling and covering the shortfall with electric backup works, but backup resistance heat runs at a COP of 1 while the heat pump runs at 2.5 to 3.5, so every hour on backup costs roughly three times as much.
What is a balance point?
The outdoor temperature at which the heat pump’s declining capacity exactly meets the building’s rising demand. Above it the heat pump carries the house alone; below it the backup helps. A well-matched system in a moderate climate has a balance point around 25 to 30 °F; a cold-climate model in a well-insulated house can be down near 5 °F.
Do heat pumps really work in cold weather?
Modern ones do. Inverter-driven cold-climate models are rated to produce useful heat at −13 °F and hold most of their capacity at 17 °F, which is a different machine from the units that gave heat pumps their reputation in the 1980s. What has not changed is that capacity always falls as it gets colder, so the manufacturer’s capacity table at your design temperature is the number that matters, not the headline.
Can a heat pump be oversized?
Yes, and there is a specific trap: sizing generously so it copes with the coldest night leaves it far too big for cooling, so it short-cycles all summer and never dehumidifies. The usual answer is a properly sized inverter unit plus sensible backup, rather than a single-stage unit large enough for January.
How many tons of heat pump per square foot?
In heating terms, roughly one ton per 400 sq ft in zone 3, per 300 sq ft in zone 5 and per 250 sq ft in zone 7 for an average house. Those are much tighter ratios than the cooling rule of thumb of one ton per 500 to 600 sq ft — which is exactly why heating and cooling sizing disagree.
Does a ducted system need a different size to a ductless one?
The building’s load is the same, but ducts running through an unconditioned attic or crawl space lose heat, and that loss is real — commonly 10 to 25% in older installations. A ductless system delivers heat directly into the room and avoids it. Sealing and insulating ducts often does more than a larger unit would.
How we calculate this
Worked example
With Floor area (sq ft) 1800, Ceiling height (ft) 8, Climate zone Zone 4 — Baltimore, Seattle, Insulation Average — typical home, this page works out 60,000 BTU/h ≈ 17.6 kW. The room works out at 75,600 BTU/h — 60,000 is the nearest size actually sold.
- Floor area1,800 sq ft
- Working rate42 BTU/h per sq ft
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