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Whole-house heat loss at the design outdoor temperature.
From a room-by-room heat loss calculation, or as a screening figure from the load calculator. Sizing a heat pump to the old boiler's output is the most common and most expensive error — boilers were routinely oversized by a factor of two.
The external design condition for your location.
A temperature the outdoor air stays above for all but a small share of the year's hours — not the coldest night on record. A US design commonly takes the 99% heating figure; a UK heat pump design under MCS takes the colder 99.6% figure from MIS 3005-D Table 2, which is London's −3.0 °C (26.6 °F), or the 99% one. Sizing to a record low guarantees an oversized, short-cycling machine for the rest of the winter.
The heat pump's nameplate output at the standard rating point.
Nameplate capacity is quoted at a mild outdoor temperature — 7 °C in Europe, 47 °F in North America. Real output at the design condition is materially lower, which is the whole point of this calculation.
How fast output falls as it gets colder, per °C.
Around 2% per °C is typical for an air-source machine, though it varies by refrigerant and compressor. Use the manufacturer's capacity table where you have it — this default is a planning figure, not a specification. This field wants the PER °C figure even when the design temperature above is showing Fahrenheit: a rate read per °F is 5/9 of it, so 2% per °C is 1.11% per °F, and typing 1.11 here would be accepted without complaint and would under-derate the machine by 44%. Multiply a per-°F rate by 1.8 before entering it.
Capacity required at design temperature
8 kW at design
The derate is a linear planning approximation. Use the manufacturer's published capacity table for the actual machine before committing.
- Nameplate capacity at 7 °C
- 8 kW
- Capacity at the design temperature
- 6.4 kW
- Shortfall needing supplementary heat
- 1.6 kW
- Balance point
- 32 °F
- Capacity retained at design
- 80 %
They open the calculator with your figures already in it
Heat Pump Sizing Calculator: 8 kW at design — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Heat pump capacity falls with outdoor temperature; manufacturers publish capacity tables at 47/17/5 °F (8/−8/−15 °C)
- ACCA Manual S — equipment selection against the calculated load, including balance-point analysis for heat pumps
- MCS 3005 (UK) requires sizing to the calculated design heat loss rather than to the existing boiler output
Inputs used
- Design heat loss
- 27297.13 BTU/hr
- Design outdoor temperature
- 26.6 °F
- Rated capacity at 7 °C
- 27297.13 BTU/hr
- Capacity loss per °C below rating point (%)
- 2
Intermediate steps
- Nameplate capacity at 7 °C
- 8 kW
- Capacity at the design temperature
- 6.4 kW
- Shortfall needing supplementary heat
- 1.6 kW
- Balance point
- 32 °F
- Capacity retained at design
- 80 %
Confidence note: The derate is a linear planning approximation. Use the manufacturer's published capacity table for the actual machine before committing.
What this calculation does not cover
- Capacity derate is linear here; real machines follow a curve that varies with refrigerant, compressor type and defrost behaviour.
- Defrost cycles reduce delivered output in damp near-freezing conditions and are not modelled.
- Says nothing about whether the emitters — radiators or underfloor — can deliver the heat at the lower flow temperature a heat pump uses. That is usually the harder problem in a retrofit.
Add the equipment this sizes
This result is a specification — 8 kW at design — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-03 · in the site-wide review of 2026-09-06 · v1.1.1
Regulatory standards & verification citations3
- Heat pump capacity falls with outdoor temperature; manufacturers publish capacity tables at 47/17/5 °F (8/−8/−15 °C)
- ACCA Manual S — equipment selection against the calculated load, including balance-point analysis for heat pumps
- MCS 3005 (UK) requires sizing to the calculated design heat loss rather than to the existing boiler output
Which documents these citations point at
Standards referenced: ACCA Manual S (Air Conditioning Contractors of America, United States).
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