A high temperature heat pump delivers 65–80 °C flow water instead of the 35–55 °C a standard unit is built for, so it can replace a boiler without changing radiators. Run permanently at a 65 °C design temperature its seasonal COP would be around 2.5–2.9, but with weather compensation it needs 65–70 °C only on the coldest days and spends most of the season at 40–45 °C — which is why UK field monitoring found real-world efficiency comparable to low temperature units. Most reach those temperatures with R290 propane or an EVI compressor.
What counts as a high temperature heat pump?
A high temperature heat pump is one designed to deliver 65–80 °C flow water continuously, rather than the 35–55 °C a standard air-to-water unit is optimised for. That single number is the entire definition: the same vapour-compression cycle, but with a refrigerant, compressor and pressure envelope chosen so the condensing temperature can go higher without the machine derating or tripping.
Three routes reach it. R290 propane has a critical temperature of 96.7 °C and behaves well at high condensing pressures, which is why it dominates new 70–75 °C domestic units. An EVI compressor injects vapour part-way through compression, lifting both output temperature and cold-weather capacity — the mechanism behind Solimpeks' Varm Up Series EVI models, rated down to −30 °C ambient. Cascade machines stack two refrigeration circuits and reach 80 °C and beyond, at the price of two compressors to maintain.
What does the extra temperature cost in efficiency?
At a fixed design temperature, roughly a third more electricity than a 45 °C system in the same house, because efficiency is governed by the lift between source and output. In practice weather compensation removes most of that gap, as the note below the table explains.
| Design flow temperature | Realistic seasonal COP (air source) | Electricity for 12,000 kWh of heat |
|---|---|---|
| 35 °C | 4.0 – 4.5 | 2,700 – 3,000 kWh |
| 45 °C | 3.5 – 4.0 | 3,000 – 3,400 kWh |
| 55 °C | 2.9 – 3.3 | 3,600 – 4,100 kWh |
| 65 °C | 2.5 – 2.9 | 4,100 – 4,800 kWh |
| 75 °C | 2.1 – 2.5 | 4,800 – 5,700 kWh |
The important qualifier is that the table describes the design flow temperature. A high temperature unit running proper weather compensation only produces 70 °C on the two or three coldest days of the year and sits at 40–45 °C for most of the season. That is why the UK Electrification of Heat monitoring found high temperature air source units returning efficiencies comparable with low temperature ones in real homes — they were rarely asked for their maximum.
When is a high temperature heat pump the right answer?
- Listed and heritage buildings where radiators, pipework and floor finishes genuinely cannot be altered.
- Direct boiler swaps on a tight programme, where the value of not re-plumbing the house exceeds the running-cost difference.
- Restricted or microbore pipework that cannot carry the flow a low temperature design needs; a higher temperature difference moves less water for the same kilowatts.
- Commercial hot water in hotels, care homes and blocks of flats where long dead legs and stored volumes force 60 °C+ without an immersion heater.
- Existing high temperature emitters such as fan convectors or unit heaters that were never sized for 45 °C.
Where enlarging three or four radiators is simple, a standard low temperature design will usually run cheaper still. See do I need bigger radiators for a heat pump? — an emitter upgrade is a one-off cost that lowers the flow temperature every winter afterwards, and a high temperature unit on weather compensation benefits from it too.
Is a bivalent system a better compromise?
Sometimes. A bivalent system keeps the existing boiler for the coldest few per cent of hours and lets a standard heat pump cover everything else at low flow temperature. It sidesteps both the high temperature penalty and the emitter upgrade, at the cost of maintaining two heat generators — and, increasingly, of grant schemes that will not fund a system with a fossil boiler still in it. Where a heat pump must carry the whole load in an unimprovable building, the high temperature machine is the honest choice.
What should I check on a high temperature quote?
Ask for the declared performance at your design point, not at A7/W35. A unit showing COP 4.8 at A7/W35 may show barely half that at A−7/W65, and both figures are true. Then confirm four things: the refrigerant and its GWP, because the EU F-Gas Regulation is progressively closing the door on high-GWP alternatives; the propane charge and the clearance zone it obliges around the unit; whether 70 °C is available at −7 °C or only in mild weather; and whether the cylinder coil is sized for the flow rate the unit actually produces.
Frequently asked questions
What is the maximum temperature a heat pump can produce?
Domestic R290 units reach 70–75 °C flow, cascade and CO2 machines reach 80 °C and above, and industrial high temperature heat pumps exceed 100 °C. Every extra degree costs efficiency, so the maximum is a capability to be used sparingly, not a setting to run all winter.
Do high temperature heat pumps work with old radiators?
Yes — that is their purpose. At 65–70 °C an existing boiler-era radiator delivers close to its original catalogue output, so a like-for-like swap is possible without changing emitters. With weather compensation the unit runs that hot only on the coldest days, so the running-cost difference stays modest.
Are high temperature heat pumps less efficient?
At their maximum output temperature, yes: expect a seasonal COP of 2.5–2.9 at 65 °C against 3.5–4.0 at 45 °C. Run with weather compensation they spend most of the year at moderate temperatures, and UK field monitoring found their real-world efficiency comparable to low temperature units.
Is R290 propane safe in a domestic heat pump?
Yes, when installed to the manufacturer's clearance and ventilation rules. R290 is flammable, so charge sizes are limited and a defined zone around the outdoor unit must stay clear of drains, openings and ignition sources. It is used outdoors in monoblock units precisely to keep the refrigerant outside the building.