Why is my radiator cold at the top or the bottom?

What does the cold patch tell me?

Cold at the top, warm at the bottom

Air rises, so trapped air collects at the top of the radiator and stops hot water filling it. This is the easy one: bleed the radiator. If you have to bleed the same radiators every few weeks, air is getting in somewhere, or gas is forming from corrosion inside the system, and that needs looking at.

Warm at the top, cold at the bottom

Sludge, the black iron oxide that forms inside steel radiators, is heavy and settles at the bottom. It blocks the lower channels so hot water only flows across the top. Bleeding will not help. If one or two radiators are affected, they can sometimes be removed and flushed out. If many are, the whole system may need a chemical clean or power flush, and a magnetic filter. See do I need a system flush or filter?

Cold all over

• The thermostatic radiator valve (TRV) may be turned down or its pin stuck.
• The lockshield valve at the other end may be closed.
• The system may need balancing, so radiators far from the boiler get their share.
• On a system with zones, a zone valve may have failed.
• If every radiator is cold, the problem is likely the boiler, pump or controls.

Cold in the middle

A cold band in the middle can be sludge building up across the radiator, or, on older radiators, internal corrosion. A radiator that is badly corroded may need replacing.

Safe checks you can do

These checks are safe for a homeowner. Bleeding clears air (cold at the top). Warm at the top and cold at the bottom usually means sludge, which bleeding will not fix. Do not open the boiler casing or work on gas parts.

1. Turn the TRV to maximum and check the lockshield is not closed (it usually has a plastic cap). A closed valve leaves the radiator cold all over.
2. With the heating off and the radiators cool, bleed any radiator that is cold at the top. Hold a cloth under the bleed key, open a quarter turn until the hiss stops and water runs, then close. See how to bleed a radiator.
3. Check the boiler pressure after bleeding and top up only as your user guide shows. See boiler losing pressure.
4. If a TRV head comes off, check the pin underneath moves. A gentle tap can free a stuck pin; do not force it or dismantle the valve body.
5. Note which radiators are affected and how (cold at top, cold at bottom, cold all over, upstairs only). That helps the engineer decide between air, sludge and balancing.
6. Stop if bleeding does not fix a cold-at-the-bottom radiator, or if pressure keeps falling. Sludge, leaks and balancing need a Gas Safe engineer or heating engineer — do not open the boiler casing.

When to call a Gas Safe engineer

Call a heating engineer if bleeding does not help, if several radiators are cold at the bottom, if you need to bleed often, or if the pressure keeps dropping. If no radiators heat at all and the boiler shows a fault, look up the code in our fault code search, then book a boiler repair. Anything inside the boiler is work for a Gas Safe registered engineer.

This answer is general information, not a substitute for an on-site assessment by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.