Does a heat pump work with ordinary radiators? The 50-degree test, how much larger they need to be and which radiators do fit

Short answer: a hybrid heat pump always works with your existing radiators, because the boiler steps in as soon as the radiators are not giving off enough heat. An all-electric heat pump works with ordinary radiators if they are large enough for a flow temperature of 45 to 50 °C; that is the case in roughly half of homes built after 1990. Test it yourself: set the boiler to a 50 °C flow temperature during a cold week. If the house stays warm, it will work. If it does not get warm, replace the radiators in the coldest rooms with a larger type (11 to 22, 22 to 33), a fan-assisted radiator or underfloor heating. Special "heat pump radiators" do not exist; they are ordinary radiators with more surface area or a fan.

Why the flow temperature determines everything

A radiator gives off heat through the difference between its own temperature and the room. The output in the catalogue (for example 1,500 W for a type 22 measuring 60 by 100 cm) is stated at 75 °C flow, 65 °C return and a 20 °C room: a difference of 50 degrees. A heat pump is most efficient at 35 to 45 °C; every 10 degrees higher costs 15 to 25 per cent in efficiency. At a lower flow temperature, the same radiator gives off much less heat.

Flow / return	Difference from room (20 °C)	Output vs catalogue	Example type 22, 1,500 W
75 / 65 °C (classic boiler)	50 K	100%	1.500 W
70 / 50 °C (modern condensing boiler)	40 K	75%	1.125 W
55 / 45 °C (hybride, hoge stand)	30 K	52%	780 W
50 / 40 °C (all-electric with radiators)	25 K	41%	615 W
45 / 35 °C (low temperature)	20 K	31%	465 W
35 / 30 °C (underfloor heating)	12,5 K	17%	255 W

At a 50 °C flow temperature, a radiator therefore gives off roughly 40 per cent of its catalogue output. Whether that is enough depends on how generously the radiators were sized at the time. In many homes built before 1990 they were sized tightly for 90 °C; in homes built after 2000 they tend to be generous, because insulation improved but radiator sizes did not shrink.

The 50-degree test

1. Choose a week with outdoor temperatures around or below freezing.
2. In your boiler's installer menu, set the maximum flow temperature to 50 °C (with weather compensation, lower the heating curve). The manual describes where this setting is; if in doubt, ask your installer or us.
3. Open all radiator valves fully and leave the thermostat at its normal setting.
4. If every room reaches temperature within a few hours and stays there in the evening, your radiators are large enough for all-electric at 50 °C. If it also works at 45 °C, the heat pump will run efficiently.
5. If one or two rooms lag behind, note which ones: only those radiators need to be larger.

The test costs nothing and tells you more than a calculation. It also saves gas straight away: a condensing boiler at 50 °C condenses better and uses 5 to 10 per cent less.

Hybrid: radiators stay as they are

With a hybrid heat pump, the controls set the flow temperature to what the radiators need. As long as 45 to 55 °C is sufficient, the heat pump runs; when it gets colder and more is needed, the boiler takes over. Small radiators only mean that the boiler steps in a little more often and the gas savings are somewhat lower. So you do not need to replace anything. That makes hybrid the logical choice for homes with energy label C to E, see Hybrid or all-electric.

All-electric: four solutions if the test fails

Solution	What it does	When	Indication
Heavier type in the same place	Type 11 to 21 or 22, type 22 to 33: 1.7 to 2.3 times the output at the same height and length	If the brackets and connections can stay; the radiator does become deeper	Type 22, 60 × 100 cm circa € 120 tot € 180; type 33 € 170 tot € 260 in onze ·
Larger radiator	Taller or longer, more surface area	If there is wall space; often under a wide window	Price follows the size; moving connections costs labour
Fan-assisted radiator (dynamic convector)	Small fans blow air across the fins: at 45 °C up to three times the output of a panel radiator of the same size	If space is limited, or for a radiator that has to work at 35 °C	Jaga Strada Hybrid type 16, 80 × 35 cm €775.10; requires 230 V
Underfloor heating	Largest emitting surface, works at 30 to 35 °C	During a renovation or when laying a new floor	Aanleg € 60 tot € 100 per m²; zie Installing underfloor heating

In practice you rarely need to replace all the radiators. The living room and the bathroom are usually the bottlenecks; bedrooms need little output. Replace only the radiators that lagged behind in the test, and use a type 33 or a fan-assisted radiator there.

How much radiator do you need at 45 °C?

First calculate the heat loss of the room: 40 to 130 W per m² depending on insulation, see the table in How many kW heat pump do I need?. Divide that by the factor from the first table. A 30 m² living room at 60 W/m² needs 1,800 W. At a 45 °C flow temperature (factor 0.31) you then need radiators with 5,800 W catalogue output: for example two type 33 radiators of 60 × 140 cm. At 50 °C (factor 0.41), 4,400 W is enough. That is why an all-electric design in existing homes often ends up at 50 °C, and at 35 °C only with underfloor heating. Which type delivers what is listed in Choosing a panel radiator: type 11, 21, 22 or 33.

What else to adjust

• Thermostatic radiator valves open. A heat pump wants a lot of water flowing through a lot of radiators; closed valves raise the return temperature and lower efficiency. Control the temperature with the room thermostat, not per radiator.
• Hydronic balancing. Each radiator must get its share of the water; at low temperatures you notice poor distribution immediately. Have the installer balance the lockshield valves.
• Buffer tank. With a small water volume, the heat pump short-cycles; a 50-litre buffer tank prevents this; discuss the size with the installer when choosing your · .
• Pipework. Thin pipes (10 mm) restrict flow; with a high output per radiator, 15 mm is needed.

Radiators that do not work after the conversion usually have a cause other than the heat pump: air, a pump or an incorrectly set valve. See Radiators stay cold. All types are listed in the · ; the full overview of choices in Choosing radiators or underfloor heating.

If your radiator is too small for a low flow temperature, choose a larger type at buy a panel radiator.

Frequently asked questions

Do special heat pump radiators exist?

Not as a category. What is sold under that name are type 33 panel radiators, radiators with fans (Jaga Strada Hybrid, Radson E.Flow) or low-temperature convectors. All three do the same thing: more output at a small temperature difference.

Can I use a fan-assisted radiator with a boiler?

Yes. It works at any flow temperature and the fans only switch on when the room calls for heat. Useful as an intermediate step: on the boiler now, on the heat pump later.

What if one bedroom stays cold with all-electric?

Replace only the radiator in that room with a type 22 or 33, or choose a small fan-assisted radiator. The heat pump's electric backup heater is not a solution for one room; it heats the whole house at expensive power.

Do I also need to replace the pipes?

Usually not. Changes are only needed with very thin pipes (10 mm) to a radiator that has to deliver a lot of output, or with an old one-pipe system. The installer will see this during the survey.