If two installers propose very different heat pump sizes for the same house, the issue is usually assumptions, not magic equipment. Good sizing is an engineering process that starts with heat loss at design conditions.
This guide gives you a homeowner workflow: collect room data, run a rough estimate, then challenge quote assumptions with clear questions before you sign anything.
All figures in this article are broad estimates. Energy prices, fuel quality, installer design, weather, grants, and household habits can change the result, so use the numbers as a planning guide rather than a guarantee.
Quick answer
Use a room-by-room heat-loss calculation at local design temperature, then check whether emitters can meet each room load at the planned flow temperature.
Floor-area rules are only a rough first pass. Boiler size is an even weaker shortcut because many boilers were oversized.
Worked homeowner scenario (example)
Example only: A 120 m2 semi-detached home gets two quotes: 6 kW and 10 kW. The 10 kW quote was based mostly on old boiler output. The 6 kW quote used room-by-room losses and emitter outputs.
The room-by-room check shows design loss around 6.4 kW at local design temperature, with one north-facing bedroom short on emitter output. Upgrading that room radiator resolves the gap without upsizing the whole heat pump.
This is why one weak room should usually trigger an emitter fix, not automatic whole-system oversizing. Label all such examples as planning illustrations, not guaranteed outcomes.
Decision tree: is the proposed size credible?
If the quote includes room-by-room losses, design outdoor temperature, and flow-temperature assumptions, move to emitter checks.
If the quote only references floor area or old boiler size, request a full heat-loss method before comparing price.
If one or two rooms fail emitter output at target flow temperature, compare radiator upgrades versus slightly higher flow temperature before changing heat pump capacity.
If many rooms fail emitter output, revisit fabric and emitter strategy first, then re-run the sizing model.
Red flags in installer sizing quotes
No design outdoor temperature is stated.
No room-by-room table is provided, only a total kW claim.
Emitter outputs are quoted at boiler-like temperatures with no lower-flow correction.
The installer cannot explain how hot-water load assumptions were handled.
Quote comparison table
Use this to compare quality of evidence, not just equipment price.
| Quote feature | Good sign | Needs caution |
|---|---|---|
| Heat-loss method | Room-by-room with design temperature | Floor area only |
| Emitter evidence | Per-room output at target flow | Generic radiator statement |
| Method transparency | Assumptions documented | Headline kW with no method |
Related calculators and guides
Run a rough size using the sizing calculator, then compare with quote documents line by line.
When any calculation appears in a quote discussion, keep a methodology link in the same email thread so assumptions stay auditable.
Key sources
SEAI and UK retrofit guidance both emphasize heat-loss-led design and commissioning detail rather than rule-of-thumb sizing.
Use official guidance for framework, then rely on your project-specific survey for the final decision.
Common mistakes to avoid
- Choosing size from old boiler output.
- Ignoring room-level emitter deficits.
- Accepting a quote with no documented heat-loss assumptions.
Conclusion
A credible size comes from transparent heat-loss and emitter evidence, not from a headline kW guess.
Use the decision tree and red-flag list to challenge quote quality before you compare final prices.