Vaillant aroTHERM Plus Review

The Vaillant Arotherm Plus is the one to beat in the UK right now. If you want the best-performing air source heat pump on the market and you can stretch the budget, this is it. Full stop.

Why R290 matters

Most heat pumps still run on R32 or R410A refrigerants. Both are synthetic HFCs with high global warming potential, and both are being progressively phased down under F-gas regulations. The Arotherm Plus uses R290 (propane), a natural refrigerant with a GWP of just 3. Compared to R410A's GWP of 2,088, that's not a marginal improvement. It's a different league entirely.

This isn't just an environmental talking point. It's a genuine future-proofing argument. When synthetic refrigerants become harder and more expensive to service in the years ahead, R290 systems won't have that problem. You're buying ahead of the curve.

The performance numbers, and what Vaillant actually claims

Vaillant does publish a COP of up to 5.4 for the R290 aroTHERM plus, but read the small print, or rather notice that there isn't any. Vaillant states no test condition alongside it, and writes it as "a COP of up to 5.4 kW", which is a unit error: COP is a ratio and doesn't have units. There is no SCOP anywhere on vaillant.co.uk, and the product pages themselves fall back to "up to A+++" rather than repeating the 5.4.

So treat 5.4 as a best-case laboratory point, not a planning figure. What you pay for is the seasonal average, and well-installed UK air source systems average 3.87 across the 252 machines reporting to heatpumpmonitor.org. A good aroTHERM plus install sits at the upper end of that. Plan on 3.5, treat anything better as a bonus, and if an installer builds a running-cost estimate on 5.4, ask them which condition it was measured at.

Vaillant's own ErP product data sheets do give proper numbers, and they're worth asking for. They quote COPd at +7°C for the 55°C application, not 35°C, at 4.28 to 5.26 depending on model, with seasonal efficiency of 123–142% at 55°C. That's a far more realistic picture of a retrofit on existing radiators than the headline is.

Noise: the number Vaillant advertises isn't the number that matters

Vaillant advertises 44 dB(A) as "our quietest air-to-water heat pump", and the footnote says that's based on the 3.5kW model. It doesn't say whether it's sound power or sound pressure. The older aroTHERM plus page manages to say "sound power as low as 54 dB(A) in a 3 metre distance", which is self-contradictory, because sound power doesn't have a distance.

Ignore all of that and go to the ErP data sheets, where the row is unambiguously labelled "Sound power level, outdoor, LWA outdoor, dB(A)". Across the aroTHERM plus range that's 51 to 60 dB(A). The 60 dB(A) model is the one to watch if your unit is going anywhere near a boundary.

Get the sound power level (LwA) for the exact model on your quote. That's the figure the MCS 020 a) noise assessment starts from, and the assessment has to show no more than 37.0 dB LAeq,5mins at the assessment position, 1 metre outside the centre of a door or window to a habitable room next door. It's also the only noise number that's comparable between brands. It genuinely matters if the unit is going near a boundary, a bedroom window, or the corner of the garden where you'd like to sit in July.

The capacity range works for most homes

Vaillant lists five outputs on its own product page: 3.5, 5, 7, 10 and 12kW. That covers everything from a well-insulated modern flat to a large detached house. The 3.5kW unit in particular is useful for smaller, better-insulated homes where a lot of heat pumps are oversized. Oversizing kills efficiency, so having a genuinely small unit available is a real advantage, and it's one the Mitsubishi and Grant ranges can't quite match at the bottom end.

Build quality and warranty

Vaillant is a German manufacturer with over 140 years in heating. The build quality reflects that. Components are well-engineered, tolerances are tight, and the units are built to last.

The warranty needs reading properly, though. Vaillant's own page says the aroTHERM plus comes with a 2-year standard guarantee, extendable up to 7 years when it's installed with a Vaillant cylinder by a myREWARDS installer and registered. The footnote goes further: the 7 years is limited to Platinum and Renewable Partner tier installers, and "upgrades may incur charges if the installer is a lower tier or if a cylinder has not been installed".

That's three conditions, not one. If your quote pairs a Vaillant heat pump with someone else's cylinder, or your installer is a lower myREWARDS tier, you are buying a two-year warranty on a ten-thousand-pound machine. Ask before you sign, get the tier and the term in writing, and check the registration actually went through afterwards.

The honest downsides

This is the most expensive option here. At £10,000–£15,000 installed, it's £1,000–£3,000 more than a comparable Mitsubishi or Daikin. For some households that's meaningful money.

Installer availability is also worth thinking about. R290 requires specific F-gas handling qualifications, different to those needed for R32 systems. There are fewer trained installers in the UK right now. That's changing fast as the market moves, but if you're in a rural area, you may have to search a bit harder for someone qualified.

Who should buy this

• Anyone who wants the best-performing unit and has the budget to match.
• Long-term thinkers who want to own a system built around the refrigerant direction the industry is heading.
• Anyone with a noisy-unit concern: neighbours, garden, bedroom proximity.

Who should look elsewhere

• If you're budget-constrained, the Samsung EHS Mono gives you solid performance at significantly lower cost.
• If you're in a rural area with limited installer access, the Mitsubishi Ecodan's wider installer network may be more practical.

The Arotherm Plus is the benchmark. Everything else on this list is measured against it. See how it compares in our best heat pumps UK roundup, or check our heat pump costs guide for the full financial picture including the £7,500 BUS grant.