Radiator Not Heating Up or Getting Hot – Causes & Fixes

Few things are more frustrating than turning the heating on… and finding one radiator stubbornly cold. It’s one of the most common heating complaints in UK homes, and thankfully, it’s usually fixable without replacing half your system.

This guide walks you through why a radiator won’t heat up, what you can safely check yourself, and how to tell when the problem is bigger than a quick DIY fix.

No scare tactics. No jargon dumps. Just practical help.

How is the radiator not getting hot?

Before reaching for tools, take 30 seconds to notice what the radiator is doing. That pattern often tells you the cause straight away.

• Cold at the top, warm at the bottom → air trapped inside

• Hot at the top, cold at the bottom → sludge settled inside

• Cold all over, others fine → valve or flow issue

• Several radiators cold → system-wide problem

• Radiator heats briefly, then goes cold → pressure or circulation issue

That one observation saves a lot of guesswork.

The most common reasons a radiator won’t heat up

Trapped air inside the radiator

Cause: Air enters the system
Effect: Hot water can’t fully circulate
Result: Radiator stays partly or fully cold

This is by far the most common issue in UK homes, especially at the start of winter.

You’ll often hear hissing or gurgling, and the radiator will feel cool at the top even though the pipes are hot.

What usually fixes it:
Bleeding the radiator once the heating is off and cool. After bleeding, the boiler pressure often needs topping up slightly — that’s normal.

Sludge blocking the bottom of the radiator

Cause: Corrosion inside the system
Effect: Heavy debris settles at the lowest point
Result: Bottom stays cold even though the top heats

This shows up a lot in older systems or homes without a magnetic filter.

If the radiator is hot at the top but cold at the bottom, air isn’t the issue — sludge is.

What helps:

• One radiator affected → remove and flush it

• Several affected → system powerflush (engineer job)

Left alone, sludge doesn’t just block radiators — it can damage pumps and boilers too.

Thermostatic radiator valve (TRV) stuck shut

Cause: Valve hasn’t moved for months
Effect: Internal pin sticks closed
Result: Radiator gets no hot water at all

This often happens after summer when the heating’s been off.

The radiator stays stone cold, even though everything else works.

What usually works:
Removing the TRV head and gently freeing the pin underneath. It should spring back when pressed — if it doesn’t, that’s the problem.

Lockshield valve not allowing flow

Cause: Valve partially or fully closed
Effect: Water can’t circulate properly
Result: Radiator stays cold or weak

This often happens after decorating or plumbing work.

Fix:
Opening the lockshield valve slightly and giving the system time to respond.

Heating system needs balancing

Cause: Hot water takes the easiest route
Effect: Nearby radiators steal the heat
Result: Distant rooms stay cold

If downstairs radiators are roasting while upstairs barely warm, balancing is usually the answer.

Balancing doesn’t repair anything — it simply redistributes heat properly.

Low boiler pressure

Cause: Water lost through bleeding or small leaks
Effect: Boiler can’t circulate heat properly
Result: Radiators go lukewarm or cold

Most combi boilers struggle below about 1 bar.

Fix:
Top up pressure using the filling loop, following the boiler’s instructions.

Circulation or pump problems

Cause: Pump struggling or failing
Effect: Heat doesn’t move around the system
Result: Multiple radiators underperform

This is less common, but more serious.

If radiators stay cold even after bleeding and valve checks, this usually needs an engineer.

A simple, safe DIY order to try at home

If you’re comfortable checking things yourself, this order catches most problems:

1. Check boiler pressure

2. Bleed the cold radiator

3. Check the TRV pin moves freely

4. Make sure the lockshield valve isn’t closed

5. Compare with other radiators

If one of these fixes it, you’re done.

When it’s time to call a heating engineer

DIY has limits. Get professional help if:

• Several radiators won’t heat up

• Bleeding makes no difference

• Pressure keeps dropping

• You see leaks or corrosion

• The boiler makes banging or kettling noises

At that point, the issue is usually sludge, pump performance, or internal boiler components — not something you should guess at.

How to stop the problem coming back

Most radiator problems return for predictable reasons. A few habits make a big difference:

• Bleed radiators once a year before winter

• Keep boiler pressure in the recommended range

• Add inhibitor after any drain-down

• Fit a magnetic filter if you don’t have one

• Don’t ignore small cold spots — they’re early warnings

The takeaway

A radiator not heating up is annoying — but rarely mysterious.

In most UK homes, the problem comes down to air, sludge, or a stuck valve, not a failed boiler. Start with simple checks, fix what you safely can, and bring in an engineer if the problem spreads or keeps returning.

Getting it sorted doesn’t just make the room warmer — it protects your system, saves energy, and makes winter far more comfortable.