Case study · heat pump comparison
Both are premium R290 monobloc heat pumps designed to replace gas and oil boilers. The Vaillant aroTHERM plus reaches flow temperatures up to 75°C, comes in 3.5 to 12kW and carries Quiet Mark on most sizes. The Viessmann Vitocal 250-A reaches up to 70°C, is built for very quiet running and came top in a 2023 German consumer test. For most homes, UK availability and your installer’s experience with the brand decide it.
The Vaillant aroTHERM plus and the Viessmann Vitocal 250-A are two of the leading premium monobloc heat pumps from German manufacturers. Both use the natural refrigerant R290, both keep the refrigerant sealed in the outdoor unit, and both reach the high flow temperatures that help when replacing a boiler in an older home. Here we compare them on temperature, output, noise and hot water. See our Vaillant and Viessmann brand pages for the wider ranges, or the heat pump hub for an overview of every brand we cover.
Vaillant aroTHERM plus at a glance
The aroTHERM plus is Vaillant’s monobloc air source heat pump, using monobloc technology with a hermetically sealed refrigerant circuit. It runs on R290, delivers flow temperatures up to 75°C and is available in 3.5, 5, 7, 10 and 12kW outputs. Vaillant says it operates down to -25°C outside.
The 3.5, 5, 7 and 12kW models are Quiet Mark certified; Vaillant’s own footnote excludes the 10kW model. Vaillant also confirms the aroTHERM plus is eligible for the Boiler Upgrade Scheme grant in England and Wales.
Viessmann Vitocal 250-A at a glance
The Vitocal 250-A is Viessmann’s premium monobloc air-to-water heat pump. The complete refrigerant circuit sits in the outdoor unit, so the refrigerant stays outside the home. It uses R290 and reaches flow temperatures up to 70°C at outdoor temperatures down to -10°C. Viessmann launched it with outputs from 2.6 to 13.4kW and has since extended the range to larger sizes in Europe.
Viessmann reports that Germany’s consumer testing body, Stiftung Warentest, named the 250-A the clear winner in its 2023 test of air-to-water heat pumps, with an overall grade of GOOD (2.1). There is also a floor-standing version, the Vitocal 252-A, with an integral 190-litre hot water cylinder. One caution: Viessmann UK’s recent announcements focus on the Vitocal 150-A and 151-A, the new Vitocal 200-A ie and the larger Vitocal 250-A Pro for commercial buildings, so check with your installer that the domestic 250-A is available for your project and which sizes they can supply.
Flow temperature and radiators
On paper, the aroTHERM plus has the higher maximum at 75°C against 70°C, which gives a little more headroom where existing radiators stay and for hot water. In practice, both heat pumps run most efficiently far below their maximum, and a good design sizes the radiators so the heat pump rarely needs its top temperature. A room-by-room heat loss calculation matters more than five degrees on a datasheet. Our guide to heat pump retrofit vs full system upgrade explains how installers decide which radiators can stay.
Noise
Both are designed for quiet operation. The aroTHERM plus has independent Quiet Mark certification on four of its five sizes. Viessmann promotes the 250-A as very quiet, with a noise-reduced operating mode. Manufacturers measure and quote sound in different ways, so ask your installer for a noise assessment of the exact spot where the unit will go.
Hot water and indoor space
With the aroTHERM plus, your installer pairs the outdoor unit with a separate hot water cylinder. Vaillant ties its extended guarantee to registration with a Vaillant cylinder through approved installers, so ask what is included in your quote. The Viessmann range offers the floor-standing Vitocal 252-A with a built-in 190-litre cylinder, which can simplify the layout in a utility room or garage. Both brands also use separate cylinders where that suits the home better.
aroTHERM plus vs Vitocal 250-A compared
Which should you choose?
Choose the aroTHERM plus if:
- You want the higher maximum flow temperature, up to 75°C
- You want Quiet Mark certification on the 3.5, 5, 7 or 12kW model
- Your home is small and well insulated and suits a 3.5kW unit
- Your installer is experienced with Vaillant
Choose the Vitocal 250-A if:
- Your installer can supply it and has Viessmann experience
- You’d like a floor-standing unit with a built-in cylinder (252-A)
- An independent consumer test result matters to you
- Very quiet running is a top priority
See how the aroTHERM plus compares with Worcester Bosch Compress 5800i, or how the Vitocal 250-A compares with its sibling in Viessmann Vitocal 150-A vs 250-A. If you’re unsure which design suits your home, read monobloc vs split heat pumps.
Frequently asked questions
Which is quieter, the aroTHERM plus or the Vitocal 250-A?
Both are designed to be quiet. The aroTHERM plus holds Quiet Mark certification on its 3.5, 5, 7 and 12kW models, and Viessmann markets the 250-A as very quiet with a noise-reduced mode. Because sound figures are quoted differently, ask your installer for a noise check on your site.
Can either work with my existing radiators?
Both reach high flow temperatures, 75°C for the aroTHERM plus and 70°C for the 250-A, which helps where radiators stay. The heat loss calculation decides whether any need upgrading so the system can run efficiently.
Can my installer supply the Vitocal 250-A?
Viessmann UK’s recent news focuses on the Vitocal 150-A and 151-A, the Vitocal 200-A ie and the commercial Vitocal 250-A Pro. Ask your installer whether they can supply the domestic 250-A before you set your heart on it.
Do both qualify for the Boiler Upgrade Scheme grant?
Vaillant confirms the aroTHERM plus is eligible. For any model, your installer checks eligibility and applies for the grant on your behalf: £7,500 in England and Wales, or £9,000 for off-gas-grid homes replacing oil or LPG until March 2027. See our grant guide.
Our recommended installer for your area can tell you which models they can supply and design the system around your home’s heat loss.
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