Radiator Not Heating Up? Common Causes and Fixes

Troubleshooting Guide

Radiator Not Heating Up?

A radiator that stays cold, or only warms up in patches, is one of the most common heating complaints we hear. The good news is that most causes are straightforward to diagnose, and some you can even check yourself before booking an engineer. Here's what's usually going on.

Common Causes and What to Do

1

Cold at the Top, Warm at the Bottom

This is almost always trapped air inside the radiator. As water circulates, air can build up at the highest point and stop hot water reaching that section.

FixBleed the radiator using a radiator key: turn the valve at the top slowly until you hear a hiss, then close it as soon as water starts to trickle out. Check your boiler pressure afterwards, bleeding can cause it to drop slightly.

2

Warm at the Top, Cold at the Bottom

This pattern usually points to a buildup of sludge or debris settled at the bottom of the radiator, restricting water flow through the lower section.

FixBleeding won't help here. This usually needs a proper power flush to clear the sludge from the whole system, not just one radiator.

3

One Radiator Completely Cold, Rest of the House Fine

If it's just one radiator not heating at all while others work normally, the most likely cause is a closed or faulty valve, either the manual valve or the thermostatic radiator valve (TRV) may be stuck shut.

FixCheck both valves are fully open. If the TRV pin is stuck, it can sometimes be freed gently with pliers. If it still won't move, the valve likely needs replacing.

4

All Radiators Cold or Lukewarm

When the whole house is affected, the issue is more likely with the boiler or system pressure itself rather than any individual radiator.

FixCheck your boiler pressure gauge first, low pressure is a common culprit. If pressure looks normal and radiators are still cold throughout, it's worth booking an engineer to check the pump and system.

5

Radiator Heats Up, Then Goes Cold Quickly

This can point to a faulty thermostatic valve that's closing too early, or in some cases a wider issue with the boiler's thermostat or programmer.

FixTry setting the TRV to a higher number to see if it stays warm longer. If the pattern continues, it's worth having a full boiler service to rule out a wider system issue.

Worth knowing: if you're regularly bleeding radiators or topping up pressure, that's usually a sign of a slow leak or system sludge build-up rather than something to just keep managing. It's worth getting checked properly rather than repeating the same fix every few weeks.

When to Call an Engineer

Bleeding a radiator or checking a valve is safe to do yourself. But if you're dealing with persistent cold spots across multiple radiators, dropping pressure that keeps returning, or you're not confident diagnosing the cause, it's best to get a Gas Safe registered engineer to take a proper look rather than guessing further.

Our engineers cover Dartford, Kent, Essex, Sussex and London, and can diagnose radiator and heating issues as part of a boiler service or a standalone visit.