Answer first
The calculation estimates how much heat each room loses at agreed indoor and outdoor design temperatures. Emitters and water temperatures are then selected to replace that loss.
It also exposes uncertain fabric assumptions before equipment is ordered. A small error in one difficult room can matter even when the whole-house total looks reasonable.
Heat-loss design must resolve local loads, not just one house total
View sourceCapacity is checked at an agreed cold outdoor condition
View sourceField work found radiators and other constraints through detailed surveys
View sourceGet the room-by-room answer for your property
Tell FLO about the property and current heating. A survey turns the general guidance into a room-by-room design and written quote for the complete system.
The building loses heat in different places
A north-facing corner room with two external walls does not behave like an internal hallway. Glazing, roof, floor, air leakage and target temperature change the load.
Measurements and construction evidence are entered room by room. Unknown layers should be documented rather than replaced with optimistic guesses.
Emitter design starts from the room load
Radiator and underfloor output is checked at the water temperatures the heat pump is expected to provide. This identifies rooms needing larger emitters or fabric attention.
Without this step, the installer may compensate for one undersized radiator by running the whole system hotter, raising electricity use everywhere.
The total informs equipment capacity
Adding the room loads gives the building design heat loss. The selected heat pump is then checked against output and efficiency at relevant conditions.
Oversizing is not a harmless safety margin: it can affect modulation and cycling. Undersizing needs a clearly designed supplementary strategy.
Use it to check future changes
The calculation can show the effect of confirmed insulation or extension plans. It also creates a baseline for later troubleshooting.
Ask for a copy or summary with the quote, including design temperatures and fabric assumptions, so proposals can be compared on the same basis.
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Questions people also ask
Can floor area size a heat pump?+
It can support a rough early estimate, but not a final design.
Is an EPC heat demand enough?+
No. An EPC has a different purpose and does not replace room-by-room design.
Should planned insulation be included?+
Only with clear current and post-work cases and a firm plan for sequencing.
Evidence used for this answer
Figures are explained in context and links go to the independent source. Prices, tariffs, scheme rules and standards can change, so check the source again before making a financial or technical decision.
- MCS heat-pump design standardTechnical requirements for heat loss, equipment selection, domestic hot water and system design.
- MCS domestic heat-pump guideConsumer guidance covering design, installation, controls, handover and maintenance.
- Energy Systems Catapult: home surveys and installation reportEvidence from 742 installations, including radiators, cylinders, survey constraints and typical installation duration.