Many homeowners assume a heat pump means ripping out every radiator and starting again. In most homes, that simply is not true. In this guide you will learn why heat pumps treat radiators differently, how a heat loss survey decides which ones stay, what happens with narrow microbore pipes and when underfloor heating makes sense. Our MCS certified engineers design and install heat pump systems every week across Surrey, Sussex and Kent, and keeping good radiators is something we do whenever it makes sense.
“We never swap radiators for the sake of it. The survey tells us which ones can stay, and we only change the ones that genuinely cannot keep the room warm at a low flow temperature.”
1. Understand how heat pumps differ — warm water, not hot
A traditional gas or oil boiler often sends very hot water around the system. Radiators get hot to the touch and rooms warm quickly. Your existing radiators were almost certainly sized with that high temperature in mind.
A heat pump is most efficient when it produces water that is warm rather than hot. The lower the flow temperature, the less electricity it uses for each unit of heat. Instead of short bursts of high heat, it runs for longer at a gentler temperature and keeps the house at a steady level of comfort.
That is why the Energy Saving Trust's air source heat pump guide describes heat pumps as more than three times as efficient as a gas or oil boiler. The catch is that cooler water means each radiator gives out less heat, so some may need help to keep up.
2. See what lower flow temperatures mean — size matters more than ever
A radiator's heat output depends on two things: its size and how much warmer it is than the room. Lower the water temperature and you need more radiator surface to deliver the same heat. That is the whole story in one sentence.
In practice, every room falls into one of three groups:
- Already big enough. Generously sized radiators, small rooms or well-insulated rooms often need no change at all.
- Needs upsizing. A larger panel, a double or triple panel in place of a single, or occasionally a second radiator.
- Suits another option. Extensions or ground floors being renovated can work very well with underfloor heating, which covers a large area at low temperature.
The aim is to keep the flow temperature as low as practical across the whole house. One undersized radiator can force the entire system to run hotter and less efficiently, just to warm that single room.
Radiators on a heat pump usually feel warm rather than hot to the touch. That is normal. The room still reaches the same temperature; the heat simply arrives more gently and more evenly.
3. Start with a heat loss survey — how we decide which radiators stay
Guesswork has no place in heat pump design. Before recommending any change, a room-by-room heat loss survey should be carried out. It looks at the size of each room, the walls, floors and ceilings, the windows, the insulation and how draughty the room is.
The process follows a clear order:
- Measure each room. Dimensions, construction and ventilation are recorded.
- Calculate the heat needed. Each room's heat loss is worked out for a cold winter day.
- Check each radiator. Its output is calculated at the planned flow temperature.
- Compare the two. Where a radiator falls short, a replacement is recommended. Where it is fine, it stays.
- Review the pipework. Pipe sizes are checked to make sure every radiator can get enough flow.
This avoids two common mistakes: replacing radiators that did not need it, and leaving undersized ones that make the home feel cold. Our room-by-room heat loss surveys are part of every design, and you can learn more in our post explaining what a heat loss survey involves.
If a heat pump quote does not mention a room-by-room heat loss calculation, ask about it. MCS certified installers must carry one out, and you need an MCS installer to claim the Boiler Upgrade Scheme grant.
4. Check your pipework — what microbore means for a heat pump
Many homes whose heating was installed or updated in the 1970s and 1980s have microbore pipework. These are the narrow pipes, usually around 8 mm or 10 mm, that run to individual radiators.
Heat pumps typically move more water around the system than a boiler does. Because the water is cooler, the system relies on volume to carry enough heat. Narrow pipes can restrict that flow, so some radiators may not get enough warm water to heat the room properly.
Microbore does not automatically rule out a heat pump. Depending on the layout, the length of the pipe runs and the heat each room needs, some sections may be fine. Others, particularly the main runs, may need upgrading to larger pipes. A good engineer checks this during the survey and tells you honestly whether changes are essential or optional.
Water quality matters as well. Older microbore systems can hold sludge that narrows the pipes even further. A clean, treated system with a magnetic filter gives the heat pump the best chance of moving water freely to every room.
5. Consider underfloor heating — useful, but not essential
Underfloor heating works superbly with heat pumps because it spreads gentle heat across the whole floor. It is a natural fit for extensions, new builds and rooms where the floor is coming up anyway.
It is not a requirement, though. Correctly sized radiators work perfectly well with a heat pump, and many homes run happily on radiators alone. If you are already planning new flooring or a kitchen extension, it is a good moment to consider our underfloor heating installation service. Otherwise, sticking with well-chosen radiators is usually simpler and less disruptive.
Floor coverings matter if you do go down this route. Thick carpet and underlay act like a blanket over the pipes, so the Energy Saving Trust recommends a combined tog of 2.5 or lower. Tiles, stone and engineered wood all pass heat through well. Many homes end up with a mix: underfloor heating in a new open-plan kitchen and upsized radiators upstairs, all running from the same heat pump at the same low flow temperature.
6. Weigh up cost and efficiency — getting the balance right
There is always a trade-off. A lower design flow temperature gives better running costs but may mean more radiator upgrades. A slightly higher one keeps more of your existing radiators but costs a little more to run each year.
A good designer talks you through that balance rather than picking one extreme. The same principles apply to our low-temperature heating system upgrades, which can also make a modern boiler run more efficiently today.
Funding helps too. Under the Boiler Upgrade Scheme in England and Wales, you can get £7,500 towards an air or ground source heat pump, taken off the quote by your MCS installer. Eligible homes currently heated by oil or LPG can get £9,000 until 31 March 2027. See our page on heating grants you may qualify for, or check an installer through the MCS find an installer directory.
7. Prepare before your survey — a practical checklist
A little homework makes the survey quicker and the design more accurate. It also helps you ask the right questions.
- Note any rooms that already feel cold or take a long time to warm up.
- Find out what insulation you have, such as loft depth, cavity wall insulation and double glazing.
- Look at where your pipework runs if it is visible, and whether the pipes look narrow.
- Think about plans for extensions, new floors or redecorating, which are good moments to change radiators.
- Check your radiators work properly now, using Baxi's guide to bleeding a radiator if any are cold at the top.
“Feel each radiator on a cold morning before the survey and jot down any that are slow, patchy or cold at the bottom. It gives us a head start on spotting sludge or flow problems that need fixing first.”
8. Get an honest design — keeping as much as sensibly possible
When we design an air source heat pump installation, we start with the survey and a close look at your radiators and pipework. Then we recommend a design that keeps as much of your current system as sensibly possible, while making sure every room reaches a comfortable temperature at a low flow temperature.
Where radiators need upsizing, we explain which ones and why. If you would like an independent view first, Citizens Advice on choosing a heat pump is a helpful starting point. We work across the region, including homes in East Grinstead and nearby.
Frequently asked questions
Will I have to replace all my radiators?
Usually not. Many homes keep a good number of their existing radiators when a heat pump is fitted. The room-by-room heat loss survey shows exactly which rooms need larger radiators at the planned flow temperature and which do not. Rooms with generous radiators, good insulation or a small floor area often need no change at all.
Will my radiators feel as hot as they did with a boiler?
They will usually feel warm rather than hot to the touch, and that is completely normal. The heat pump runs for longer at a lower temperature, keeping rooms comfortable without the peaks and dips you get with a boiler. The room itself still reaches the temperature you set on the thermostat.
Can I keep my microbore pipes?
Sometimes. It depends on the layout of your system, the length of the pipe runs and how much heat each room needs. Your engineer will check whether the flow through the pipes is adequate during the survey, and tell you whether any sections, often the main runs, should be upgraded to larger pipes.
Do I need underfloor heating for a heat pump?
No. Underfloor heating works very well with heat pumps because it delivers gentle heat over a large area, but correctly sized radiators work too. It is often worth considering when you are already replacing floors or building an extension. Otherwise, a well-designed radiator system is usually simpler and less disruptive to install.
Written by the NW Heating Services team
NW Heating Services installs, services and repairs boilers, heat pumps and underfloor heating for homes and landlords across Surrey, Sussex and Kent, with more than ten years of local experience. Our guides are written from the jobs our engineers carry out every week. More about our team.