Daikin Altherma vs Vaillant aroTHERM: the two units you’ll be quoted

Quick answer: For a radiator retrofit, the Vaillant aroTHERM plus. It runs R290 to 75°C flow where the Daikin Altherma 3 M stops at 65°C on R32, and it’s 7 to 9 dB(A) quieter on published sound power. The Altherma 3 M is cheaper per kilowatt and fine on underfloor heating.

We haven’t tested either unit. Everything here comes from Daikin’s Altherma 3 M technical data book, Vaillant’s installation and maintenance instructions, and UK trade listings, all checked on 28 July 2026.

A note on the ‘most installed’ claim

You’ll see these two described as Britain’s most-fitted heat pumps. MCS does not publish installation counts by manufacturer, so nobody can prove that from the certification data. What’s true is that these are the two units UK homeowners are quoted most often, and that Daikin and Mitsubishi Electric are usually placed at the top of UK sales share by commercial market research. Take the ranking loosely and the comparison seriously.

Specification against specification

The refrigerant decides the argument

R290 is propane. Its global warming potential is 3, against 675 for R32, and it holds pressure well enough to push flow temperatures to 75°C. R32 tops out lower. That single physical fact is why the aroTHERM plus can heat a Victorian terrace on its original radiators and the Altherma 3 M usually can’t.

It also means the aroTHERM plus needs a flammable refrigerant exclusion zone around the unit, so siting is more constrained. Your installer has to keep drains, light wells, windows and electrical ignition sources outside a defined area. It’s manageable. It isn’t nothing, especially on a narrow terraced side passage.

Daikin’s answer is the Altherma 4, its first residential R290 machine, with 4, 6 and 7kW single-phase classes announced on 15 January 2026 and a leaving water temperature up to 70°C. If you’re being quoted a Daikin for a radiator retrofit in 2026, ask why it isn’t an Altherma 4.

Noise

Vaillant publishes 51 dB(A) sound power for the 3.5 and 5kW and 53 dB(A) for the 7kW at A7/W35, rising to 54 and 55 dB(A) when the flow goes to 55°C. Daikin’s Altherma 3 M runs 58, 60 and 62 dB(A) across its 4, 6 and 8kW.

Nine decibels is a lot. It’s roughly halving the perceived loudness, and in a terraced garden it can be the difference between passing the MCS 020 sound calculation at your preferred position and having to move the unit somewhere inconvenient. Note that several UK retailers list the aroTHERM plus at 44 dB(A) on the strength of its Quiet Mark accreditation. That number doesn’t appear in Vaillant’s own EN 12102 table. Use the manual figure.

Efficiency, honestly

Daikin’s 4.61 at 35°C beats Vaillant’s smaller units on paper. But Daikin also publishes the 55°C figure, 3.35 on the 8kW, and Vaillant doesn’t put one in the installation manual we checked. That’s a transparency point against Vaillant, and it matters because the 35°C number describes a house with underfloor heating throughout, which is not most people.

What we can say from the physics: both machines lose roughly a quarter of their efficiency between 35 and 55°C flow, in line with every other air source unit. Every 1°C you take off your design flow temperature is worth about 2 to 2.5% on running cost, so bigger radiators pay you back every year. The arithmetic is in COP and SCOP explained.

Warranty and price

Daikin gives 2 years as standard. You get to 5 free through a Sustainable Home Installer registered on Stand By Me within two years, then 7 only as a paid extension via a Sustainable Home Expert. Vaillant also starts at 2 years, moves to 5 via a myREWARDS Gold, Platinum or Renewable Partner installer, and reaches 7 only when a Vaillant hot water cylinder goes in alongside, fitted by a Platinum member or Renewable Partner. Both require annual servicing.

On price, the Vaillant 7kW listed at £3,492.74 ex VAT at The Heat Pump Warehouse on 28 July 2026, and the 5kW at £3,193.56. We couldn’t find a like-for-like single-unit UK listing for the Daikin EBLA08EV3 the same day, so we won’t invent a number for it. Supply-only retail sits well above trade price either way. Compare against the £11,014 modelled installed average on our costing page.

The verdict

aroTHERM plus if you’re keeping your radiators, if the unit needs to be quiet, or if you want propane. That’s most British retrofits. It’s why this machine became the default, and why we’d still put it first.

Altherma 3 M if you have underfloor heating throughout, where the 65°C ceiling never bites and the 55°C SCOP gap disappears. Or if the nearest Vaillant Renewable Partner is two counties away and your local Daikin installer is genuinely good, because a competent fitter on the second-best box will beat a poor one on the best.

Wrong choice for both: a large detached house with a 12kW-plus heat loss needing high flow temperatures. Look at the wider field in our best heat pump brands guide, read the individual reviews of the aroTHERM plus and the Altherma 3, then price the job with the calculator.

FAQs

Is R290 dangerous?

Propane is flammable, which is why installations need a defined exclusion zone around the outdoor unit and why installers need the right qualification. The refrigerant charge in a domestic monobloc is under two kilograms and sealed in the outdoor unit. Look at the table above: propane is now the mainstream choice rather than the exception.

Can I get the full 7-year guarantee on either?

Only through specific installer tiers. Vaillant’s 7 years needs a Platinum or Renewable Partner installer plus a Vaillant cylinder. Daikin’s 7 years is a paid extension through a Sustainable Home Expert. Ask your installer to state the guarantee length on the quote.

Which one do more installers fit?

Nobody publishes brand-level MCS installation data, so any answer is an estimate. Both have wide UK installer networks. The more useful question is which brand the specific company quoting you is accredited with, because that decides the warranty you’ll get.