Best heat pump for small homes UK: what fits a 3 to 5kW heat loss

Quick answer: For a small home with a 3 to 5kW heat loss, the Grant Aerona 290 4kW is the pick: SCOP 5.00 at 35°C, 3.66 at 55°C and 48 dB(A) sound power. If noise is the binding constraint, Worcester Bosch’s Compress 5800i AW 4kW publishes 40 dB(A), the lowest figure on the UK market.

We haven’t tested any of these units. This is based on published specifications and warranty terms, checked on 28 July 2026.

Small home does not mean small heat pump

Start here, because it’s the mistake that costs people money. A two-bed terrace with solid walls and single glazing can have a higher heat loss than a four-bed detached new build. Floor area tells you almost nothing. What decides your heat pump size is the calculated heat loss at design outdoor temperature, and only a room-by-room survey to MCS standards gives you that.

So the real question is which units modulate low enough to run properly on a small load. Badge size comes a distant second. Our guide to heat pump sizing walks through the survey, and what happens when a unit is too big explains why this matters more than any brand choice.

What’s available below 5kW

The specification to ask for is minimum modulated output

Here’s a detail buried in Worcester Bosch’s specifier guide. The CS5800i 4, 5 and 7kW all quote the same nominal output at 7°C air and 35°C flow: 2.84kW. The three sizes differ at the top end, not the bottom.

That tells you something important. Buying the smallest badge in a range doesn’t automatically get you a machine that turns down further. If your design heat loss is 3kW and the smallest unit in the range still won’t modulate below 2.8kW, you’ll get cycling in mild weather whichever size you pick. Ask the installer for the minimum modulated output, in kilowatts, at your design flow temperature. Most will have to look it up. That’s fine. It’s still the right question.

Hot water is the other constraint

A 4kW heat pump can heat a small house comfortably and still take a long time to reheat a 210 litre cylinder. Reheat is where small units feel small.

Two practical answers. Either accept a slower reheat and schedule it overnight on a cheap tariff, which is what most owners of 4kW units do, or fit a smaller cylinder sized to your actual usage. A couple in a two-bed flat rarely needs 210 litres. Don’t let an installer specify a large cylinder by habit and then upsize the heat pump to fill it, because you’ll end up with an oversized machine cycling all winter to solve a problem you didn’t have.

Space, weight and where it goes

Small homes tend to have small outside space, and the units are not small. Grant’s 4kW Aerona 290 is 99kg empty and 1,158mm wide. Worcester’s CS5800i is 143kg and 1,100mm wide. Vaillant’s 3.5kW aroTHERM plus is 114kg ready for operation and can go on a wall bracket, which is sometimes the only option on a narrow terrace.

R290 units also need a flammable refrigerant exclusion zone, which rules out positions near drains, light wells, opening windows and electrical sockets. On a narrow terraced side passage that can be genuinely difficult. Get the installer to confirm siting before you order anything. And if you’re in a flat, read our guide to heat pumps in flats first, because the answer is often no.

Our picks

The Grant Aerona 290 4kW is the buy for most small homes. It has the best efficiency at both flow temperatures of anything this size, 48 dB(A) sound power, and up to 7 years guarantee through a Grant G1 installer. The 9kW listed at £2,689.40 ex VAT in July 2026, so the 4kW sits below that.

Go for Worcester Bosch’s CS5800i 4 OR-S instead if noise decides it. At 40 dB(A) published sound power it gives you siting options nothing else does, which on a terrace with a neighbour’s bedroom window three metres away is worth more than a 0.5 difference in SCOP. You pay for that in a lower 55°C SCOP of 3.03, so this one wants a low design flow temperature and decent radiators.

Skip NIBE at this size. Its smallest unit is 8kW, and on a 4kW house that’s a machine looking for work it can’t find. Compare the full field in our brand comparison, or start with the cost calculator and heat pump product guide.

FAQs

What size heat pump does a two-bed terrace need?

Anywhere from 3kW to 9kW depending on insulation, glazing and wall construction. A well-insulated modern terrace might need 3.5kW. An uninsulated Victorian one can need three times that. Only a room-by-room heat loss calculation answers it.

Will a 4kW heat pump keep a small house warm in January?

Yes, if it was sized against your calculated heat loss at design outdoor temperature, typically around -2°C in the North West. The design point already includes the coldest normal weather. What it doesn’t cover is a badly insulated house where the real heat loss was underestimated.

Do small heat pumps still get the £7,500 grant?

Yes. The Boiler Upgrade Scheme grant is a flat £7,500 for air source heat pumps in England and Wales regardless of unit size, with no income test, running to March 2030. On a small installation it can cover most of the bill.