Do I need new radiators for a heat pump?

Some of them, usually. Rarely all of them. A heat pump runs your radiators cooler than a gas boiler does, so each one gives out less heat. Where a radiator was only just big enough with the boiler, that room will often need a bigger one. Rooms that already have plenty of radiator can normally keep what’s there.

The only way to know for your house is a room by room heat loss calculation. It works out how much heat each room needs on a cold day. Then every radiator gets checked against that number.

Why do heat pumps need bigger radiators?

A gas boiler typically sends very hot water round your radiators. A heat pump is happiest sending cooler water. Cooler water, less electricity for the same heat.

The catch is simple. Fill a radiator with cooler water and it puts less heat into the room. Same radiator. Lower temperature. To get the same heat out you need more radiator surface touching the air, so either a bigger radiator or one with more panels and fins packed into the same size.

Could you just run the heat pump hotter and keep all the old radiators? You could. Some installers do. But then the heat pump works harder and costs more to run, which rather defeats the point of having one. On an air source heat pump installation we’d sooner size the radiators properly and let it run cool.

Something people don’t expect. A radiator that looks a bit small on paper isn’t always a problem. The heat loss calculation is done for a very cold day and most of the winter isn’t that cold. Heat pumps are normally set up with weather compensation, which nudges the water temperature up as it gets colder outside. So a borderline radiator copes for most of the year. Is that OK for a particular room? That’s a judgement call and we’d talk it through with you. A bedroom you like cool isn’t the same as the living room you sit in every evening.

How many radiators usually need changing?

No fixed number. It comes down to how much heat each room loses and how big the radiators already are.

Houses that have been insulated since the radiators went in often do well here. The rooms need less heat now than when the radiators were sized, so the old ones have spare capacity. Small radiators put in by a previous owner are a different story. So are rooms that were always a bit chilly. Those usually need changes.

Look at the rooms with lots of outside wall and glass. A lounge with a bay window. A back extension with bifold doors. They’re the ones that tend to need attention, while hallways and small bedrooms are often fine as they are.

You should see all this worked out before you agree to anything. A good quote lists every room, the heat it needs and whether its radiator stays or changes. “Replace all radiators” with no workings? Ask why. The room figures come from the same heat loss survey that’s used for heat pump sizing.

Do double or triple panel radiators help?

Yes. Often this is how you get more heat without giving up more wall.

A single panel radiator is one flat sheet of metal. A double has two, usually with fins in between. A triple has three. More panels and fins give the heat more surface to leave from, so a double the same width and height as your old single can put out a lot more heat. It just sits further off the wall.

We’d usually look at that first. Swap a single for a double or triple in the same spot and quite often the pipes don’t move and there’s no fresh patch of wall to redecorate. Where it works it’s the tidiest fix.

It doesn’t work when the radiator’s already a double or triple and still falls short. Then it’s a wider or taller radiator, or a second one in the room. Under a window you can’t go much higher, so the extra normally comes from width.

When you’re choosing, think about:

• Depth. A triple panel can stick out a fair way. Check it won’t clash with curtains, furniture or a door.
• Look. Column radiators and designer ones work with heat pumps too. They just need sizing for the lower temperature like any other.
• Valves. New radiators usually get new valves. Good moment to fit decent thermostatic ones.

Do I need underfloor heating?

No. Heat pumps work with radiators, and plenty of homes run one on radiators alone.

Underfloor heating does suit a heat pump well, mind. It covers the whole floor so there’s a huge surface area, and it can run on very cool water. Already lifting floors for an extension or a kitchen refit? Then it’s well worth considering for that space.

Ripping up floors in a finished house just to fit it is another matter. Lots of mess, lots of cost. Bigger radiators will usually do the job for far less. We’d only suggest underfloor heating where there’s a clear reason in that particular room.

Mixing the two is fine. Some homes have underfloor downstairs and radiators upstairs, with the system set up to handle both.

Do microbore pipes rule out a heat pump?

Not automatically. Microbore is the very thin pipework running to the radiators in a lot of older homes, much narrower than standard heating pipe. People often get told it rules a heat pump out.

The real issue is flow. A heat pump pushes more water round the system than a boiler does, at a lower temperature, and thin pipes limit how much reaches each radiator. Does that matter in your house? Depends how much heat that radiator has to deliver and how long the pipe run is.

Sometimes the microbore’s fine for a little bedroom radiator but too tight for the big one in the lounge. Then only the main runs or the bigger rooms need new pipe. Sometimes it’s more. Either way it’s checked as part of the design. Not guessed.

Got microbore? Mention it when you book the assessment. It helps us plan the visit.

Radiators and pipes are only part of whether your home suits a heat pump. Insulation matters too, and so does space for the outdoor unit and somewhere for a hot water cylinder.

Frequently asked questions

Do heat pumps work with normal radiators?

Yes. A heat pump heats ordinary radiators. You don’t have to buy some special “heat pump radiator”. They just need to be big enough for the room at the lower water temperature a heat pump runs at, and lots of existing ones already are.

Will I need new radiators in every room?

That would be unusual. Most homes end up with a mix, some radiators staying and some getting swapped, and the heat loss calculation shows which is which. Be wary of a quote that changes every radiator without showing why.

Does changing radiators add much to the cost?

It can add a fair bit. How much depends on how many change and whether pipes need moving. It’s one of the reasons heat pump quotes vary so much. Our guide to air source heat pump cost goes through what’s in a quote line by line.

Want to know which of your radiators would need to change? A home energy assessment looks at your home room by room. Book a free home assessment

Sources

• No external figures, prices or statistics are used in this guide. It explains how radiators and heat pumps work in general terms. Any job details will come from Greenbox’s own installs once James supplies them.