Why Is My Radiator Not Getting Hot?

The radiator is cold all over

• Check the thermostatic radiator valve (TRV) isn't turned down or stuck closed
• Check the lockshield valve at the other end hasn't been closed off
• Look for a closed isolation valve if the radiator has recently been removed for decorating
• Confirm your boiler and central heating system are actually calling for heat
• Check that the room thermostat isn't set below the current room temperature, which stops the system calling for heat at all

Cold at the top, hot at the bottom

This is a classic sign of trapped air, which rises to the top of the radiator and stops hot water reaching that section. Bleeding the radiator with a radiator key usually solves this quickly and is a job most people can do safely themselves with the heating switched off and cooled down first.

Hold a cloth ready to catch the small amount of water that escapes, turn the key slowly until you hear a hiss of air, and close it again the moment water starts to dribble out steadily. Check the boiler pressure gauge afterwards, as bleeding a radiator often causes a small drop that needs topping back up via the filling loop.

Cold at the bottom, hot at the top

This pattern usually means sludge or debris has settled at the bottom of the radiator, restricting water flow through that section. Bleeding won't fix this — the system may need a power flush to clear built-up sludge, particularly in older systems that haven't been flushed in years.

If you bleed a radiator with this fault and notice the water that comes out is dark or discoloured, that's a fairly reliable sign of sludge in the system rather than just trapped air.

One radiator cold while others are fine

If only one radiator is affected, the fault is usually local to that radiator: a stuck valve, an isolated pipe, or sludge specific to that circuit. If several radiators around the house are cold, particularly upstairs, it's more likely to be a system-wide issue such as low water flow, a faulty pump, or a boiler problem.

What a heating engineer checks and in what order

• Confirms the boiler is firing and the pump is running before looking at individual radiators
• Checks TRVs and lockshield valves are open and functioning correctly
• Bleeds the radiator to rule out trapped air as the simplest explanation
• Assesses whether sludge is present, often by checking the water drawn off and by feeling the radiator's temperature pattern by hand
• Recommends a power flush, a single radiator flush, or a magnetic filter depending on how widespread the problem is

When to call a heating engineer

If bleeding doesn't resolve a cold top, if the valves seem to be functioning but the radiator still won't heat, or if the problem affects multiple radiators, it's worth having a heating engineer investigate. Persistent sludge problems are best diagnosed properly rather than repeatedly bled, as they point to a system that needs attention.