How is ASHP capacity determined from heat loss?

The heat pump must be sized to match the total heat loss at the design outside temperature (typically -2C to -5C in the UK depending on region). This includes fabric heat loss through walls, roof, floor, and windows, plus ventilation losses, plus domestic hot water demand. MCS guidelines limit oversizing to 125% of heat loss.

What is defrost derating?

When outside temperatures are near or below 0C, frost forms on the outdoor coil. The heat pump periodically reverses to defrost, reducing effective heating output by approximately 10%. This must be factored into sizing calculations to ensure adequate capacity in cold weather.

What is the backup/hybrid threshold?

The balance point is the outdoor temperature below which the heat pump alone cannot meet the full heating demand. Below this temperature, supplementary heating (electric element or gas boiler in hybrid systems) is needed. Well-sized systems typically have a balance point at or below the design temperature.

Why does COP change with outdoor temperature?

COP (Coefficient of Performance) measures heating efficiency. As outdoor temperature drops, the heat pump must work harder to extract heat from colder air, reducing COP. At 7C outside with 35C flow, COP might be 4.0+; at -5C outside with 45C flow, it might drop to 2.2-2.5.

Should I oversize my ASHP for extra capacity?

No. Oversizing causes short-cycling (frequent on/off), which reduces efficiency and increases wear. MCS recommends sizing within 100-125% of design heat loss. Variable-speed (inverter) heat pumps can modulate output to match demand at part loads.

TradeCalculator

ASHP Sizing Calculator

Calculate the correct air source heat pump capacity from whole-house heat loss, with defrost derating and backup threshold assessment.

Floor Area

Leave blank to use typical for property type

Property Type

Used for default area if not specified

Insulation Level

Overall fabric insulation standard

Region

Sets the design outside temperature

How We Calculate This

This calculator determines the correct ASHP capacity based on your property's whole-house heat loss at the regional design temperature, following MCS MIS 3005 guidelines.

Calculation method

Heat loss: Floor area x heat loss factor (W/m2) adjusted for ceiling height and insulation level
DHW addition: Domestic hot water demand typically adds 2-3 kW
Defrost derating: 10% output reduction applied when design temp is below 0C
Backup threshold: Temperature below which supplementary heating is needed

COP estimation

COP is estimated from the required flow temperature (45C assumed for existing radiators) and the design outside temperature. Lower flow temperatures and milder climates give better seasonal COP.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.