Best heat pump for a large detached house: high-output picks that cope

Quick answer: For a large detached house with a heat loss of 12 to 16kW, the Daikin Altherma 3 H HT is our pick: up to 18kW of output and 70°C flow from the compressor alone, so existing radiators often stay. Above 16kW of loss, a Mitsubishi Ecodan cascade is the proven route. Budget from around £11,000 to £17,100 before the £7,500 grant.

The problem with big houses

Most heat pumps sold in the UK top out around 12kW. A 250m² Edwardian detached with solid walls can need 15kW or more on a cold day. That leaves you three options: a single high-output monobloc, two units cascaded together, or cutting the heat loss with fabric work until a smaller machine fits. The third option is boring and correct: every kW you insulate away is capital you don’t spend on hardware. Start with our guide to heat pump sizing by property type before you shortlist anything.

We haven’t tested these units — this is based on published specifications and warranty terms, checked July 2026.

The picks

Best single unit: Daikin Altherma 3 H HT (14–18kW)

The H HT is built for exactly this job. It delivers 70°C flow temperature from the compressor alone, keeps working down to -28°C, and comes in 14, 16 and 18kW classes (daikin.co.uk, checked July 2026). That 70°C figure is the headline: it means period radiators sized for a gas boiler can often stay where they are, which on a big house saves thousands in emitter swaps. Daikin quotes outdoor sound pressure of 43 to 48dB(A) across the range (daikin.co.uk, checked July 2026). The trade-off is efficiency: run any heat pump at 65°C flow and the SCOP falls well below what the brochure quotes at 35°C. So treat the high-temperature headroom as a safety margin, not a design target.

Best above 16kW: Mitsubishi Ecodan 14kW cascade

Mitsubishi’s 14kW Ecodan monoblocs can run in cascade with identical units, multiplying output well past anything a single monobloc manages (Mitsubishi Electric, checked July 2026), and the Zubadan versions are rated to keep working down to -28°C. For a very large house a pair of 11.2 or 14kW units running in cascade beats one maxed-out machine: the controller runs one unit at efficient part load in mild weather and only wakes the second when it’s genuinely cold. You also get redundancy. If one unit faults in January, half your heating survives. Warranties run up to 7 years through accredited installers.

Best value at the top end: Samsung EHS Gen 7 R290 16kW

Samsung’s Gen 7 R290 range runs to 16kW single phase, certifies 100% of rated output at -10°C and reaches 75°C flow (Samsung Climate Solutions, checked July 2026). That no-derating guarantee is worth reading twice: a unit that quietly sheds a quarter of its capacity at -7°C will leave a big house cold on the exact nights you need it. Samsung installs usually price under Daikin and Mitsubishi equivalents. Our full Samsung EHS Gen 7 review covers the range in detail, and our brand comparison puts all three head to head.

The Vaillant aroTHERM plus 12kW deserves its row in that table. If your fabric is good enough to get the loss under 12kW, it’s the efficiency pick. It shares the smaller aroTHERM models’ R290 refrigerant and Quiet Mark accreditation. A large modern detached with decent insulation frequently lands here, and it’s a simpler, cheaper install than anything above it.

Costs and the grant

Our modelled estimate for a heat pump installation is around £11,014 with a range up to about £17,100, and big-house installs sit at the top of that range once you add buffer tanks, larger cylinders and any radiator work (see how we calculate costs). The Boiler Upgrade Scheme pays £7,500 once per property, not per unit, and cascade systems can still qualify, so confirm the setup with your MCS installer before contracts are signed. VAT is 0% until 31 March 2027. As a running-cost illustration at the current price cap (26.32p/kWh electricity, Ofgem, October to December 2026): 20,000kWh of heat a year at a seasonal COP of 3.5 is about 5,714kWh of electricity, roughly £1,504. These are estimates, and our calculator will localise them.

FAQs

Do I need three-phase power?

Usually not. All the units above run on single phase, and most UK homes are single phase. Very large cascades or a heat pump plus two EV chargers can push you towards a three-phase upgrade, which your DNO prices case by case.

Is one big unit or a cascade better?

Below 16kW of heat loss, one unit is simpler and cheaper to install and maintain. Above that, a cascade usually wins on part-load efficiency and gives you redundancy, at the cost of more outdoor hardware and a more complex commissioning job.

Will a heat pump actually heat a poorly insulated period house?

Physically, yes, if it’s sized to the real heat loss. Financially, the case is weaker: a leaky house makes any heating expensive. Spend on the fabric first, then size the heat pump to the improved figure.