Why a radiator stays cold, and what to do about it

First, look at where it’s cold

The pattern tells you a lot. Put the heating on and feel the radiator from top to bottom.

• Cold at the top, warm at the bottom: air has collected at the top. Bleed it.
• Warm at the top, cold at the bottom: often sludge (slib) settled in the bottom. Bleeding won’t help.
• Cold all over, while the others are warm: a valve is closed or stuck, or the water isn’t reaching it.
• Lukewarm in the rooms furthest from the boiler: the system probably needs balancing.
• Every radiator cold: start at the boiler instead. My guide to boiler pressure and fault codes covers that.

How to bleed a radiator

Bleeding (ontluchten) lets trapped air out through a small valve at the top corner of the radiator. You need a radiator key (ontluchtingssleutel) from any DIY shop, a cloth and a cup.

1. Switch the heating off and let the radiators cool a little, so the water isn’t scalding.
2. Hold the cloth under the bleed valve and turn the key a quarter turn anticlockwise.
3. You’ll hear a hiss as the air comes out. Wait.
4. Close the valve as soon as water comes out in a steady stream.
5. Do the same on every radiator that was cold at the top.

Milieu Centraal advises bleeding an older system a few times a year so it keeps working well and quietly.

Check the pressure afterwards

Letting air out lowers the pressure in the heating system. Intergas says to check that the pressure is still between 1 and 2 bar after bleeding, and to top up and bleed again if it isn’t.

My pressure guide explains how to top up with the vulkraan (filling valve). If you find yourself bleeding and topping up again and again, something is letting air in or water out, and that needs finding.

A thermostatic valve that won’t open

A thermostatic radiator valve (thermostaatkraan) closes the radiator once the room reaches the set temperature. Milieu Centraal puts setting 3 at about 20 degrees.

After a summer switched off, the small pin under the head can stick in the closed position. To check it:

1. Turn the head fully open, then unscrew or unclip it. The valve body stays on the pipe, so no water comes out.
2. Press the pin gently with your thumb. It should spring back up.
3. If it stays down, move it in and out gently a few times. If it won’t free, the valve needs replacing.

Then look at the valve at the other end, often under a plastic cap. That’s the inregelventiel (balancing valve). If someone closed it, for example to decorate, the radiator stays cold. Count the turns before you change it.

One more thing Milieu Centraal points out: a thermostatic valve can’t ask the boiler for heat. If the living room, where the room thermostat hangs, is warm, the boiler stops, and a cold bedroom gets nothing. That’s also why the radiators in the thermostat’s room should have no thermostatic valves and be fully open.

Some rooms warm, others lukewarm

Water takes the easiest route. Radiators close to the boiler get plenty, and the ones at the end of the run get what’s left. Turning the thermostat up only makes the warm rooms warmer, and Milieu Centraal reckons every extra degree costs about 10 percent more energy over a year.

The fix is balancing, called waterzijdig inregelen in Dutch. Each radiator’s balancing valve is set so the hot water is shared out evenly. Milieu Centraal notes that every radiator needs such a valve for this to work. It takes measuring and patience, and it’s part of my radiator work.

When the radiator is simply too small

Has the room never really got warm, even with the radiator hot all over? Then the radiator may not have enough output for the room. Milieu Centraal explains that an old column radiator gives less heat than a panel radiator with two or three panels and fins.

Water temperature matters too. If the boiler’s flow temperature has been turned down, a small radiator is the first to struggle. I can check the room against the radiator’s output and suggest a larger or different model.

Get it looked at

Tried bleeding and the valves, and it’s still cold? Send me photos on WhatsApp: the whole radiator, both valves close up, and the boiler’s pressure reading, plus your postcode. My rate is €114.95 incl. BTW per hour, with no call-out charge. Materials are extra.

If every radiator is cold and the boiler shows a fault, see boiler repair. Before the heating season, the winter boiler check and a boiler service are worth a look.

If you smell gas, leave the property and call 0800 9009 from outside. If a CO alarm goes off or you suspect carbon monoxide, get everyone outside and call 112.