Radiator Not Heating Up? Common Causes and How to Fix It

A radiator that won’t heat up is one of the most common heating complaints, and the good news is that the cause is usually easy to identify. Where the radiator is cold tells you a lot about what’s wrong. This guide runs through the most common reasons a radiator isn’t heating up and how to fix each one.

Start by checking where the radiator is cold

Before anything else, feel the radiator while the heating is on. Where it’s cold points straight to the likely cause:

Cold at the top, warm at the bottom: trapped air
Cold at the bottom, warm at the top: sludge build-up
Cold all over (but others are warm): a stuck valve or closed valve
Every radiator cold: a boiler, pump or system-wide issue

1. Trapped air (cold at the top)

If the radiator is warm at the bottom but cold at the top, air is trapped inside and stopping hot water from reaching the top. This is the easiest fix — you simply bleed the radiator to release the air. Follow our step-by-step guide on how to bleed a radiator, then check your boiler pressure afterwards and top it up if needed.

2. A stuck thermostatic radiator valve (TRV)

Each radiator usually has a thermostatic valve (the numbered dial). Over the summer, when the heating is off, the pin inside can stick in the closed position, so the radiator stays cold even when turned up. To check, unscrew the TRV head and look at the small pin underneath — gently moving it up and down with pliers will often free it. If the radiator warms up afterwards, that was the culprit.

3. Sludge build-up (cold at the bottom)

If the radiator is warm at the top but cold at the bottom, the problem is usually a build-up of sludge — rust and debris (magnetite) that settles at the base and blocks the flow of hot water. Bleeding won’t fix this. The system needs a chemical flush or a power flush, which is a job for a professional. Left untreated, sludge reduces the efficiency of your whole system and can damage the boiler and pump.

4. Closed or unbalanced valves

Check that both valves on the radiator are fully open — the TRV at one end and the lockshield valve (usually under a plastic cap) at the other. If a single radiator furthest from the boiler struggles to heat while those nearest run hot, your system may need balancing, which involves adjusting the lockshield valves so hot water is distributed evenly. A heating engineer can balance the system quickly.

5. A boiler or pump problem (all radiators cold)

If none of your radiators are heating up, the issue is more likely with the boiler or circulating pump than the radiators themselves. First, check the boiler is firing and that the pressure gauge reads around 1 to 1.5 bar — if it’s low, top it up, and if it’s high, see our guide on boiler pressure that’s too high. If the boiler is working but the radiators stay cold, the pump may have failed or seized, which needs a Gas Safe registered engineer.

When to call a professional

Bleeding a radiator and freeing a stuck TRV pin are safe DIY jobs. Call a heating engineer if the water from the radiators is dark and dirty (sludge), if radiators need balancing, if the pump has failed, or if no radiators are heating at all. Our heating team can diagnose and fix the problem, and flush the system if needed, to get your heating running efficiently again.

Frequently asked questions

Why is only one radiator not heating up?

A single cold radiator usually points to trapped air, a stuck TRV, or a closed valve on that radiator. Feel where it’s cold to narrow it down — cold at the top means air, cold at the bottom means sludge.

Why are all my radiators cold but the boiler is on?

This usually indicates a circulation problem, such as a failed pump or a system-wide airlock, rather than a fault with the radiators. It’s best checked by an engineer.

Can I fix a radiator that won’t heat up myself?

Often, yes. Bleeding the radiator or freeing a stuck TRV pin can be done safely at home. Sludge removal, balancing and pump faults should be left to a professional.

Need help with your heating in West London?

If your radiator still won’t heat up after these checks, it’s worth having a professional take a look before the problem spreads to the rest of your system. Our Gas Safe registered engineers provide fast plumbing and heating services across West London, including radiator balancing, power flushing and boiler repairs.

Tel: 020 3561 4415