Scotland Regional Winner, Heating Installer Awards 2026Highly Commended, Scottish Energy Efficiency Awards 2025
🎉 Scotland Regional Winner at the Heating Installer Awards 2026
Scotland Regional Winner, Heating Installer Awards 2026Highly Commended, Scottish Energy Efficiency Awards 2025
🎉 Scotland Regional Winner at the Heating Installer Awards 2026
Home / Blog / High temperature heat pumps for old Scottish homes
A high temperature heat pump is an air source unit that can send water round your radiators at 70 to 75°C, close to what an old gas or oil boiler delivers. Installers point owners of sandstone tenements, granite terraces and stone villas towards one because it promises to work with the radiators already on the wall. Two of our own old-house jobs point the other way: a 1920s sandstone terrace in Mount Florida runs on a 45°C design, and a 220-year-old stone house with walls 600mm thick needed two radiator changes for its heat loss. A design at a low flow temperature costs far less to run.
Sean Hogan · Heat Geek engineer · Updated October 2026 · 14 min read
The pitch for running hot sounds sensible in a cold stone house. Keep the boiler-era radiators, set the water to boiler temperatures and let the heat pump get on with it. The catch sits in the electricity meter, and you can put a price on it.
Flow temperature is the temperature of the water leaving the heat pump on its way to your radiators or underfloor pipes. A boiler in an older house often sends water out at 70°C or more. A heat pump can heat the same rooms with cooler water when each radiator is big enough to pass the heat into its room, and every degree you take off the flow saves the compressor work.
Vaillant publishes the effect for the 7kW aroTHERM plus, the size we fitted in that Mount Florida terrace. Its installation manual gives the coefficient of performance (COP, the units of heat delivered for each unit of electricity used) at four flow temperatures with the outside air at 7°C.
| Flow temperature | COP at 7°C outside | Electricity used for 10 kWh of heat |
|---|---|---|
| 35°C | 4.8 | 2.1 kWh |
| 45°C | 3.6 | 2.8 kWh |
| 55°C | 2.9 | 3.4 kWh |
| 65°C | 2.3 | 4.3 kWh |
Source: Vaillant UK installation and maintenance instructions for the aroTHERM plus VWL 75/6 A 230V S2 (document 0020330791_03, 06.11.2023), EN 14511 test points at 7°C outdoor air, checked 13 October 2026.
Those are single test points on a mild day. A seasonal figure (SCOP) takes in the frosts as well and comes out lower at every flow temperature, and the certified seasonal listings we checked stop at 55°C. Any air source heat pump follows the same slope: hotter water means more electricity for each unit of heat.
Flow temperature also moves with the weather. Weather compensation reads an outdoor sensor and raises the flow only as far as the day needs, so the design figure on your quote is the temperature for the coldest day the system was sized for, and milder days run cooler. Robert’s house in Cumbernauld sits 400 feet up on an exposed hillside, and we designed his system for a 45°C flow at -4°C outside. In a week that dropped to -5°C, Sean’s readings from the unit showed a 43°C flow holding the house at 20.5°C. Our Cumbernauld heat pump case study covers the old microbore pipework that came out to get there.
“Sean from Aventus was able to face time my dad and talk him through concerns about how his air source heat pump was performing during cold weather . We managed to resolve the issues with Sean’s patient help and expert knowledge.”
Sid Ambrose, ★★★★★ Google Review
No standard fixes the meaning of “high temperature” on a spec sheet. In the UK market the label goes on air source units that reach 70 to 75°C water from the compressor alone, without an electric heater doing the last stretch.
Two routes get a unit there.
One is the refrigerant. R290, which is propane, lets a compressor lift water to higher temperatures. Vaillant lists a heating water range of 20 to 75°C on every size of its aroTHERM plus, and Panasonic quotes up to 75°C for its Aquarea L series with the outdoor air at -10°C.
The other route keeps a conventional refrigerant and engineers around it: Daikin’s Altherma 3 H HT runs on R32 and quotes 70°C water at -15°C outside from the heat pump alone.
A high ceiling and a high running temperature are separate things. An R290 unit such as the aroTHERM plus can run a house designed for 45°C and another designed for 65°C, with the same box on the wall outside. The design flow temperature written into the quote decides which of those two systems you end up with, and the bills follow it. Our Vaillant aroTHERM plus review goes through that unit’s sizes, noise figures and warranty conditions.
A ceiling figure says little about an old house on its own. The useful number is the output a unit holds at the flow temperature your design needs on a cold day, which the manufacturer’s capacity tables show and your installer should quote against.
The COP figures above come from a unit rated to 75°C, so they already show a high temperature heat pump at each setting. The penalty comes from the flow temperature you run at, and it shows once you put a price on each unit of heat. Ofgem’s price cap for 1 October to 31 December 2026 sets Southern Scotland’s single-rate Direct Debit prices at 26.11p per kWh for electricity and 7.87p per kWh for gas. Electricity carries 0% VAT from 1 October 2026 to 31 March 2027, so the heat pump rows below sit lower than they will once VAT returns.
Divide the electricity rate by the COPs above and set a condensing gas boiler beside them.
| Heat source | Cost per kWh of heat | Cost of 10,000 kWh of heat |
|---|---|---|
| Heat pump at a 35°C flow (COP 4.8) | 5.4p | £544 |
| Heat pump at a 45°C flow (COP 3.6) | 7.3p | £725 |
| Heat pump at a 55°C flow (COP 2.9) | 9.0p | £900 |
| Heat pump at a 65°C flow (COP 2.3) | 11.4p | £1,135 |
| Gas boiler at 90% efficiency | 8.7p | £874 |
Unit rates from Ofgem’s energy price cap for Southern Scotland, 1 October to 31 December 2026, Direct Debit, single rate, standing charges excluded. COPs from Vaillant’s 7°C test points for the aroTHERM plus 7kW. The 90% boiler efficiency and the 10,000 kWh of heat are illustrative assumptions.
Held at 65°C, the heat pump charges more for each unit of heat than the gas boiler it replaced, and about 57% more than the same machine at 45°C. On 10,000 kWh of heat that gap comes to £410 at these rates, and it returns every winter for the life of the system, while radiator work is a one-off cost.
Bringing the flow down from 65°C to 45°C asks each radiator to give out its room’s heat with cooler water, so the undersized ones get swapped for larger or double-panel models. Our guide to sizing radiators for a heat pump sets out that room-by-room check. In an older house the shortfall tends to sit in the rooms with the most glass and outside wall, which keeps the list of changes shorter than owners expect.
Running hot can also happen by accident. We were called out to a heat pump held at a fixed 53°C flow to keep a thermal store hot, with four circulating pumps on the system and two of them running around the clock. The homeowner was paying over £500 a month to run it. We took the thermal store out, fitted a hot water cylinder and a 3-port valve to separate heating from hot water, fitted new filters and reworked the pipework, so the heat pump no longer has to hold a store at 53°C.
“We are also grateful for his knowledge and input into adjusting our heat pump and currently our bills are massively reduced.”
Christine C, ★★★★★ Google Review
Hot water is the one job in the house that needs high temperatures from time to time. The Health and Safety Executive’s legionella guidance, written for those responsible for water systems, says hot water should be stored at 60°C or above. In homes, the normal heat pump practice is to keep the cylinder at a lower everyday temperature and run a periodic sterilisation cycle that lifts it above 60°C.
The ceiling decides how that cycle gets done.
A unit rated to 75°C can complete it on the compressor.
A unit with a lower ceiling hands the last few degrees to the immersion heater in the cylinder, which turns one unit of electricity into one unit of heat.
A cycle that runs on a timetable adds far less to the bill than a heating system held hot all winter.
The radiators never need to share that temperature. Heating and hot water sit on separate circuits, switched by a diverting valve, so the controller heats the cylinder to one setting and the house to another. A 75°C ceiling earns its keep at the cylinder, and the radiators in the same house can still run on a 45°C design.
Sandstone and granite walls have no cavity to fill, tenement flats carry tall ceilings and big sash windows, and Victorian and Edwardian villas have bay-windowed rooms that one radiator under the glass never kept warm. All of that raises heat loss, and higher heat loss calls for bigger radiators or hotter water, which is why these houses hear the high temperature pitch more than any others.
A survey puts a number on that loss for each room instead of assuming the worst for the whole house. People had told Penny and Monique that their 1920s sandstone end-terrace in Mount Florida, Glasgow, couldn’t run on a heat pump. We fitted a 7kW Vaillant aroTHERM plus, the R290 unit that can reach 75°C, and designed it for a 45°C flow. We replaced four radiators with higher-output models, added one upstairs and fitted two in a new rear extension, and the house holds 21°C through the year.
Stone-built properties bring other survey questions too, from wall construction to finding room for the cylinder, and our article on heat pumps in older Scottish homes takes them in turn.
The oldest house on our list is a 220-year-old stone property in rural Scotland with walls 600mm thick, half of it uninsulated, and upstairs radiators fed by 8mm microbore under new oak floors. The owner wanted to keep his oil boiler as a backup. The heat loss calculations called for two radiator changes in the living and dining rooms, the microbore stayed where it was, and in a week of -4 and -5°C Sean’s readings showed the system running above 400% efficiency. Sean’s promise to the owner before the work started was blunt: “If a heat pump doesn’t work, it’s not the heat pump’s fault, it’s my fault.”
Glasgow’s tenements add their own puzzles, such as a shared back court for the outdoor unit and neighbours above and below. Our heat pump installation in Glasgow starts every tenement, terrace and villa with the same room-by-room survey, and the design flow temperature comes out of it.
“They arrived on time, respected our property, and completed the job to a high standard. The heat pump is performing wonderfully, and we’re enjoying a more comfortable home environment.”
Robyn McDougall, ★★★★★ Google Review
Home Energy Scotland’s heat pump grant of up to £7,500 (£9,000 with the rural and island uplift), and the optional £7,500 interest-free loan beside it, both require an MCS-certified installer and an MCS-certified product. The heat pump must also provide 100% of the home’s heating and hot water. A high temperature unit qualifies on the same terms as any other heat pump, so the ceiling on the spec sheet has no bearing on the funding.
MCS’s design standard, MIS 3005-D, asks the installer to choose a heat pump that covers at least 100% of the calculated heat loss at the flow temperature the system will run, with no help from an electric heater, and to back that choice with the manufacturer’s performance data. That ties the design flow temperature to the size of the unit, because output at 65°C has to be read from the 65°C column. MCS changed the standard in December 2024 so hybrids can be sized to 55% of the heat load at a 55°C flow, and Home Energy Scotland still funds no hybrid, so in Scotland that route gives up the grant.
The MCS Heat Emitter Guide scores each design with a temperature star rating of up to six stars, awarding more stars for lower emitter temperatures. Its guidance table runs all the way to a 61 to 65°C band, so MCS allows a hot design and marks it down. The installer works through every heated room, and the room needing the hottest water sets the flow temperature for the house.
Ask for the design flow temperature and the star rating in writing with each quote, and if two quotes for the same house sit far apart, ask each installer to show you the room that set their figure.
A high temperature design earns its place in a short list of houses.
Even in those houses the choice is seldom all or nothing. The room that needs the hottest water sets the flow for every other room, so changing the two or three radiators holding the design back can lower the design flow temperature for the whole house. A 75°C ceiling then works as headroom for the hot water cycle and a hard frost, and on a quote it sits alongside a design figure well below it.
A £0 survey measures your home’s heat loss room by room, names the radiators that set the flow temperature and shows the running cost at the design figure. It also confirms whether the system meets Home Energy Scotland’s 100% heating and hot water condition.
Not in the old houses we have named here. Our 1920s sandstone end-terrace in Mount Florida runs on a 45°C design after we replaced the radiators that fell short, and our 220-year-old stone house job needed two radiator changes for its heat loss. A high temperature design fits houses where radiators or pipework cannot change.
Running any heat pump hot costs efficiency, and the high temperature models follow the same rule. Vaillant’s figures for its 7kW aroTHERM plus, a unit rated to 75°C, show a COP of 3.6 at a 45°C flow and 2.3 at 65°C, with 7°C outdoor air.
A heat pump should run at the lowest flow temperature the radiators allow on the coldest design day. Our Mount Florida terrace and Cumbernauld radiator system are designed at 45°C, our underfloor heating new build in Bathgate at 35°C, and weather compensation lowers the flow further on milder days.
Often it can, at a running cost. Keeping every radiator and running at 65°C means paying about 57% more for each unit of heat than at 45°C, so compare that yearly cost with the one-off price of changing the radiators that limit the design.
Yes, on the same terms as other heat pumps. The installer and product must be MCS certified and the system must provide 100% of heating and hot water, which opens a grant of up to £7,500 (£9,000 for eligible rural and island homes) plus an optional £7,500 interest-free loan.
HSE’s legionella guidance for those responsible for water systems says hot water should be stored at 60°C or above. Domestic heat pumps keep the cylinder cooler day to day and run a periodic cycle above 60°C, using the compressor on units rated to 75°C or the immersion heater for the last few degrees on others.
Inside Scotland's Most Resilient Village Hall
What Solar Salesmen Don’t Tell You
Was My Own Heat Pump Install a DISASTER? The Results Are In...
Panasonic L Series Heat Pump Masterclass
Heat Pump vs My House: The Truth About New Build Installs | Part 1
This Is What Kills Heat Pumps Early (And How to Stop It)
From Council houses to £2,000,000 Mansions we do it all with Heat Pumps…
How to DESTROY a heat pump in 3 years....ECO4 install
