Where a radiator is cold tells you a lot about what’s wrong. Cold at the top and warm at the bottom usually means trapped air, which you can often fix yourself by bleeding it. Warm at the top and cold at the bottom usually means sludge has built up inside, which needs cleaning out. Here’s how to check, and what to do in each case.
How to check your radiators
Put the heating on and give it 15 to 20 minutes to warm up. Then carefully run your hand over each radiator, from top to bottom and side to side. Take care, as radiators can get very hot. Make a note of which radiators have cold patches and where.
Radiator cold at the top: trapped air
Air rises, so when it gets into a heating system it collects at the top of the radiator and stops hot water filling it. You might also hear gurgling or ticking noises. Bleeding the radiator lets the air out.
How to bleed a radiator
- Turn the heating off and let the radiators cool down.
- Find the bleed valve, a small square fitting at one of the top corners.
- Hold a cloth underneath and use a radiator key to turn the valve a quarter turn anticlockwise.
- You’ll hear a hiss as air escapes. As soon as water starts to come out, close the valve again.
- Check your boiler pressure gauge. Bleeding lets some pressure out, so you may need to top it back up using the filling loop, following your boiler manual.
If a radiator needs bleeding every few weeks, air is getting into the system somewhere, which is worth getting checked.
Radiator cold at the bottom: sludge
Over time, the water in your heating system reacts with the metal inside the pipes and radiators. This creates a black, muddy sludge, known as magnetite. Because it’s heavy, it settles at the bottom of radiators and blocks the flow of hot water.
Bleeding won’t help with this. Signs that sludge is the problem include:
- Cold patches along the bottom or in the middle of radiators
- Black or dark brown water when you bleed a radiator
- Radiators that take a long time to warm up
- A boiler that’s noisy or keeps breaking down
The fix is to clean the system. A power flush pumps water and cleaning chemicals through the pipes and radiators at high speed to clear out the sludge. Afterwards, we usually recommend a magnetic filter, which catches new sludge before it can build up again and protects your boiler.
Other cold radiator problems
- Whole radiator cold: check the valves at each end are open. If it has a thermostatic valve, the pin inside can get stuck, especially after the summer.
- Radiators furthest from the boiler are cooler: the system may need balancing, so hot water reaches every radiator evenly.
- All the upstairs radiators are cold: this can point to a problem with the pump or, on older systems, the water tank in the loft.
When to call an engineer
Give us a call if bleeding doesn’t fix a cold top, if you see dark water or cold bottoms, or if several radiators are struggling. Our engineers handle central heating repairs across Birmingham and Solihull, from sticking valves to full system cleans.
MS HEATING AND PLUMBING has been looking after heating systems for over 28 years. Our engineers are Gas Safe Registered, and we’re a Which? Trusted Trader.
