Daikin Altherma 3 Review

The Daikin Altherma 3 doesn't shout about itself. It sits quietly at the premium end of the market, matches the Vaillant Arotherm Plus on headline COP, and does something most other heat pumps struggle with: it keeps performing when the temperature drops hard. If you're in a colder part of the UK (northern England, Scotland, anywhere that regularly sees sub-zero winters), this deserves serious attention.

The flash injection advantage

Most heat pumps see efficiency drop significantly as outdoor temperatures fall below zero. The physics makes this inevitable: there's less heat energy in cold air to extract. Daikin's flash injection technology partially addresses this by injecting refrigerant vapour into the mid-point of the compression cycle, maintaining higher capacity and efficiency at low ambient temperatures.

In practical terms: at -7°C, a standard heat pump might be working hard to keep up. The Altherma 3 handles it more comfortably. For most of England and Wales this rarely matters. UK winters are milder than the test conditions suggest. For Scotland, northern England, or elevated locations, it's a genuine advantage.

The specs

An SCOP of up to 5.0 at 35°C flow temperature puts the Altherma 3 among the most efficient air source heat pumps available. The capacity range of 4–16kW is also one of the widest available, which is useful for both smaller well-insulated homes and larger properties with higher heat demand.

The outdoor unit is notably compact for its output range. If space is tight (a narrow side passage, a small courtyard, a busy utility area), the Altherma 3's footprint works in its favour.

Cold weather performance in numbers

Daikin publishes performance data at ambient temperatures down to -25°C. At A-7/W35 (outdoor temperature -7°C, flow temperature 35°C), the Altherma 3 maintains a COP comfortably above 3. Some competitors fall to 2.5 or below at that temperature. When you're running the system hardest (cold days, highest heat demand), the efficiency advantage is most meaningful.

The honest downsides

The base warranty is only 2 years, shorter than you'd expect for a premium unit. You can extend to 5 years free with Daikin's Stand By Me registration, or 7 years through a Daikin Sustainable Home Expert installer. Make sure you register.

The Altherma 3 is also less familiar to UK consumers than the Mitsubishi Ecodan. It's well-known among installers, but if you're doing your own research and asking around, you'll find less peer-to-peer feedback than you will for Mitsubishi. That's changing as the installer base grows, but it's worth knowing.

R32 refrigerant: same caveat as the Ecodan. It's serviceable and widely available now, but it's not where the industry is heading long term. Daikin has launched the Altherma 4 with R290 refrigerant, which is the natural successor. If you're buying new, ask your installer about the Altherma 4 availability.

Who should buy this

• Anyone in Scotland, northern England, or exposed locations where cold-weather performance matters.
• Homes needing a wide capacity range: the 4–16kW span covers a lot of ground.
• Space-constrained installations where the compact outdoor unit is a practical benefit.
• Anyone who wants Vaillant-level efficiency without the Vaillant price: the performance numbers are comparable, the price slightly lower.

Who should look elsewhere

• If long warranty coverage matters to you, the Vaillant's 10-year option is hard to ignore.
• If installer familiarity in your area is a priority, Mitsubishi's larger UK network is an advantage.
• If you want natural refrigerant for future-proofing, you need the Vaillant Arotherm Plus.

The Altherma 3 is a quiet achiever in the best sense. If you're choosing on pure performance and cold-weather capability, it belongs at the top of your shortlist alongside the Arotherm Plus. For the full brand-by-brand comparison, see our best heat pumps UK guide. For costs after the £7,500 BUS grant, see our heat pump costs guide.