One Radiator Not Working in One Room? Causes & How to Fix it

When every radiator in your house is warm except one stubborn unit in one room, it’s frustrating — and confusing. The good news is this: if only one radiator isn’t working, the boiler is almost never the problem. In most UK homes, a single cold radiator points to a local issue that’s usually fixable without major work.

This guide walks through the most common reasons one radiator won’t heat up, what you can safely check yourself, and when it’s time to call a heating engineer.

How Does the Single Radiator Feel?

Before jumping to fixes, notice how the radiator behaves. The pattern tells you a lot.

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

• Hot at the top, cold at the bottom → often sludge or internal blockage

• Stone cold all over → valve, flow or circulation issue

• Warm only when other radiators are off → balancing problem

If all other radiators heat normally, your boiler, pump and main pipework are almost certainly fine.

The most common reasons one radiator isn’t working

1. Trapped air in that radiator

This is the classic and most common cause.

Air rises, so it collects inside individual radiators — especially ones upstairs or furthest from the boiler. When air is trapped, hot water can’t fully circulate.

Typical signs

• Radiator cold at the top

• Gurgling or hissing sounds

• Room slower to warm than others

Fix
Bleeding the radiator usually solves this in minutes. Once the air escapes and water flows evenly again, the radiator should heat normally.

2. Sludge or debris inside the radiator

Over time, rust and debris settle at the bottom of radiators. If only one radiator is affected, it’s often because it’s at a low point in the system or rarely used.

Typical signs

• Radiator hot at the top, cold at the bottom

• Takes ages to warm (if it warms at all)

• Other radiators work fine

Fix

• Remove and flush the radiator (DIY if confident)

• If several radiators show similar symptoms, a system powerflush may be needed

Bleeding alone won’t fix sludge — water can’t flow through solid build-up.

3. Thermostatic radiator valve (TRV) stuck shut

TRVs are meant to open and close automatically, but they can seize — especially after summer when heating hasn’t been used.

Typical signs

• Radiator completely cold

• Other radiators on the same system are hot

• Turning the TRV makes no difference

Quick check

• Turn the TRV fully up

• Remove the plastic head (it usually unscrews)

• Gently press the small metal pin underneath

If the pin doesn’t move freely, it’s likely stuck.

Fix
Often freeing the pin restores flow. If not, the valve may need replacing — a common, low-cost repair.

4. Lockshield valve closed or misadjusted

The lockshield valve (usually hidden under a plastic cap on the opposite side of the TRV) controls how much water flows through the radiator.

If it’s:

• Fully closed

• Almost closed

• Knocked during decorating

…the radiator may not get enough hot water.

Typical signs

• Radiator barely warm or cold

• Problem started after decorating or furniture moving

Fix
Opening the lockshield valve slightly often restores heat. This is also tied closely to radiator balancing.

5. Radiator out of balance

In an unbalanced system, radiators closest to the boiler hog the hot water, leaving others starved.

Typical signs

• One radiator cold while others heat very quickly

• Radiator warms only when others are turned off

• Common in extensions, loft rooms, or end-of-run radiators

Fix
Balancing the system redistributes flow so every radiator heats evenly. This involves adjusting lockshield valves across the house.

6. Radiator pipework issue (less common, but real)

Occasionally, the problem isn’t the radiator itself but the pipes feeding it.

Possible causes

• Airlock in the pipe run

• Crushed or kinked pipe (common in older floors)

• Poor connection after previous work

Typical signs

• Radiator never worked properly

• Issue started after renovations

• No response to bleeding or valve checks

This is when an engineer’s diagnosis is worth it.

What you can safely try yourself (in order)

1. Bleed the radiator

2. Check the TRV pin moves freely

3. Make sure the lockshield valve is open

4. Feel the pipes — are they hot near the valve?

5. See if the radiator warms when others are turned down

If none of these help, stop there — forcing valves or draining systems without experience can create leaks.

When to call a heating engineer

You should get professional help if:

• The radiator stays cold after bleeding and valve checks

• Multiple radiators now show similar symptoms

• You see leaks, corrosion, or constant pressure drops

• The radiator has never worked properly

A qualified heating engineer (often Gas Safe Register registered if gas-related work is involved) can quickly identify whether it’s a valve, sludge, balance or pipework issue.

Can one cold radiator increase heating bills?

Yes — quietly.

A cold room often leads to:

• Thermostat being turned up

• Boiler running longer

• Other rooms overheating to compensate

Fixing one underperforming radiator can improve comfort and reduce wasted energy across the whole system.

The takeaway

If only one radiator in one room isn’t working, it’s almost never a boiler fault. In most UK homes, the cause is air, sludge, valves or flow — all local, all fixable.

Start small. Don’t panic. And if the simple checks don’t work, a targeted repair is far cheaper than living with a cold room all winter.