Heat pumps are excellent—but only when they’re sized for the actual load of your home. Oversize it and you’ll spend more upfront, cycle inefficiently, and risk noisy short bursts of heat. Undersize it and February feels like an apology. The cure is a heat loss and system sizing study grounded in your building’s measured performance.
Why “rules of thumb” fail in winter Contractors often default to nameplate square-meter rules or copy the old boiler size. That ignores airtightness, insulation quality, window upgrades, and occupancy patterns. A blower door result or recent envelope upgrade can change the load by 20–40%.
What a proper sizing study includes • Room-by-room heat loss at a defined outdoor design temperature (e.g., −15°C), accounting for ventilation and infiltration. • Supply temperature targets for emitters (radiators, underfloor). Lower temps (35–45°C) maximize heat pump efficiency (COP). • Flow rates & delta-T to ensure quiet, stable operation. • Domestic hot water strategy and defrost considerations for your climate. • Electrical checks: breaker capacity, start-up current, and backup heat logic.
Envelope first = smaller, cheaper, better A short package of air sealing + attic top-up can drop your design load enough to choose a smaller outdoor unit and quieter indoor equipment. That’s better comfort for less money.
Radiators: keep, tweak, or swap? Many homes can keep existing radiators by: • Running longer cycles at lower temps. • Adding one or two larger panels in the coldest rooms. • Improving balancing to distribute heat evenly. When that’s not enough, selective emitter upgrades beat whole-house replacement.
Control strategy makes (or breaks) efficiency Heat pumps like steady operation. Use weather compensation curves, avoid frequent on/off scheduling, and let indoor temps float gently overnight. Pair with balanced mechanical ventilation to keep air fresh without forcing the heat pump to chase drafts.
Noise, placement, and neighbors Specify sound power levels, plan clearances, and pick a location that shields bedrooms and neighbors. A good study includes a simple site plan with noise notes—saves headaches and permit delays.
Commissioning checklist you should demand • Proof of refrigerant charge and leak test. • Flow verification across emitters, with delta-T logged. • Defrost cycle check in sub-zero conditions. • Handover of controller settings (photographed or printed). • First-month support window for curve tweaks.
Takeaway Don’t buy tonnage—buy fit. A heat loss and system sizing study ensures the right heat pump, the right flow temps, and the right comfort when it’s −10°C and dark by four.