Radiators stay cold while the boiler is firing: pump, air or diverter valve

Short answer: if the boiler is firing but the radiators stay cold, no water is circulating. This is caused by a seized or worn circulation pump, air in the system, a diverter valve stuck in the hot water position, or radiator valves that are closed. Start by bleeding and checking the water pressure; if the problem persists, the pump is the most likely cause.

The simple checks first

Check that the thermostat really is calling for heat and that the boiler is actually firing: the display then shows a rising flow temperature. If the boiler itself and the pipe coming out of it get warm, but the radiators do not, the problem is in the flow.

Bleed the radiators, starting on the ground floor and working upwards. Air in the system is by far the most common cause of a single cold radiator. Then check the water pressure and top up to 1.5 to 2 bar if necessary. If the pressure keeps dropping, read our guide on water pressure that keeps dropping.

If all radiators stay cold while the boiler is firing, the problem is not air but circulation.

The circulation pump

The pump pushes the heated water around the system. Two things go wrong: it seizes or it wears out. Seizing mainly happens after the summer, when the pump has stood still for months and deposits have made the shaft stick. You can hear the pump humming, but nothing moves. Wear shows as a droning noise and radiators that only get warm close to the boiler.

Modern boilers often report this themselves. Bosch, for example, shows 281 B for a blocked pump or air in the pump, Vaillant reports F.14 for a circulation problem and Remeha reports E05 when the heat exchanger temperature rises too high because of poor flow. That last one is important: too little circulation makes the exchanger red-hot in places, and in the long run that costs you the most expensive part of the boiler.

What a pump costs

Part	Manufacturer number	Price
Itho Daalderop CH pump including wiring harness, RS15/5-3	07.95.36.013	€ 198.95
Inventum heating pump	S1012788	€ 366.95
ATAG circulation pump	65116765	€ 423.95

All pumps can be found under pumps in the catalogue. Note that pumps differ per boiler series in connection size, pump head and control; a modern modulating pump is not interchangeable with an old three-speed pump.

The diverter valve

In a combi boiler, the diverter valve sends the water to the radiators or to the hot water heat exchanger. If it sticks in the hot water position, the heating gets nothing, while the boiler does fire as soon as you open a tap. You can recognise it because you do have hot water but no heating. Sometimes only the motor is stuck: that costs considerably less than the complete unit, for example the ATAG diverter valve motor 230 V (S4855500, €165.95) compared with a complete ATAG diverter valve (65119728, €303.95). All variants can be found under diverter valves and hydroblocks.

If only the upper floor or one room stays cold

Then it is not the pump but the distribution. Often the system has not been balanced properly: the radiators close to the boiler take all the water and too little is left upstairs. Hydraulic balancing solves this; an installer sets the flow per radiator. The same goes for underfloor heating, but then on the manifold.

A single radiator that stays cold while the rest are warm usually has a stuck thermostatic radiator valve. Unscrew the head and press the pin; if it is stuck, the valve needs replacing.

DIY or call an engineer?

Bleeding, topping up and freeing a thermostatic radiator valve are things you can do yourself. In most boilers the pump sits inside the casing, and that is work for a certified gas engineer. If the pump is separate in the pipework, outside the boiler, replacing it is water-side work that a handy DIYer can manage: drain the system, swap the pump, refill and bleed.

Blijft één radiator koud omdat hij te klein of versleten is, dan vindt u een vervanger bij · .

Frequently asked questions

How do I know whether the pump is running?

Put your hand on the pump housing while the boiler is producing heat. A running pump vibrates slightly and becomes lukewarm. Many pumps also have a light or display. If you only hear a hum without vibration, it is probably stuck.

Can a seized pump be freed?

Sometimes. Many pumps have a bleed screw in the middle with the shaft behind it; an engineer can carefully turn it loose. Once a pump has seized, it usually happens again.

Why does my boiler get hot but the radiators don't?

Because the heat is not being carried away. The boiler produces heat, but without circulation it stays in the heat exchanger and the appliance shuts down on overheating. Do not keep resetting it: you will damage the heat exchanger.

Is a more economical pump worth it?

A modern modulating pump uses considerably less electricity than an old three-speed pump and adjusts its speed to the heat demand. For a separate pump in the pipework that is a sensible replacement; if the pump is inside the boiler, the manufacturer determines which type belongs there.