What you need to know
The right size is the one supported by the design calculation
An undersized system may rely too heavily on backup heat or fail to maintain the intended indoor temperature. A substantially oversized system can introduce different control and cycling problems. The aim is a defensible match between the building demand, selected equipment and heating distribution.
Ask to see the assumptions behind the answer. A single unexplained kilowatt figure is less useful than room losses, design temperatures and emitter requirements that can be checked before installation.
Request a £0 heat-loss survey and written whole-system quote
Tell FLO about the property and current heating so the right survey and design questions can be identified before equipment is selected.
What a heat-loss calculation is doing
The calculation estimates heat escaping through walls, windows, doors, roofs and floors, then adds an allowance for ventilation or air leakage. It is performed for a defined difference between the intended room temperature and the external design temperature.
Breaking the result down room by room matters because the total building load does not show whether an individual bedroom, extension or corner room has enough emitter output.
- Measured room dimensions
- Construction and insulation assumptions
- Window and door sizes and types
- Exposed surfaces and neighbouring spaces
- Indoor targets and the relevant external design temperature
Why boiler size is not the answer
Boilers are often installed with substantial spare capacity and may also produce hot water differently. Copying the boiler’s rating can therefore oversize the heat pump. Historic fuel consumption can be a valuable reasonableness check, but it needs to be interpreted alongside efficiency, weather and household behaviour.
Similarly, watts-per-square-metre rules can help with an early sense check but do not capture extensions, mixed construction or room-level emitter needs.
Check performance at the conditions that matter
A model name or headline output does not by itself establish winter capacity. Equipment performance changes with outdoor temperature and the heating-water temperature the system is asked to produce.
The designer should compare the selected unit’s documented capacity at the relevant design point with the calculated demand and explain the intended approach to supplementary heat, defrost and hot-water operation.
- Capacity at the local design condition
- Required heating flow temperature
- Expected operating range rather than one test point
- Backup or immersion strategy
- Electrical supply and starting requirements
Sizing is connected to radiators and controls
A correctly sized outdoor unit cannot compensate for emitters that cannot release enough heat at the design temperature. The heat-loss schedule should therefore connect each room’s requirement to its radiator or underfloor-heating output.
Controls also need to let the system operate steadily. The final design should explain weather compensation, zones, thermostats and any buffer or hydraulic arrangement rather than treating them as accessories.
Keep in mind: A transparent design should let you trace the route from room heat loss to emitter output, system flow temperature and equipment selection.
What to read next
Related guides and services
Questions people ask
Can a heat pump be sized from floor area?+
Floor area can support an early estimate, but final sizing should use property-specific heat loss and room-level emitter checks.
Should the heat pump match my boiler output?+
Not automatically. Boiler ratings often include spare capacity and different hot-water duties. The building heat loss is the more relevant starting point.
Why does the outside design temperature matter?+
The system needs to maintain the intended indoor conditions during the cold conditions used for design. That temperature affects calculated demand and equipment capacity.
Should I receive a copy of the calculation?+
Ask what design information will be supplied with the proposal and handover. The assumptions and room requirements should be clear enough to discuss before installation.
Independent sources
Scheme rules and guidance can change. Check these sources before making a financial or planning decision.