Radiator Not Working? London Guide to Cold Radiators and Quick Fixes

Diagnosing a Cold Radiator: Start Here

The specific pattern of coldness on your radiator points directly to the cause. Before calling a plumber, run through this diagnostic sequence.

Cold at the Top — Needs Bleeding

If the top of the radiator is cold and the bottom is warm or hot, there is air trapped in the radiator. Air rises and collects at the top, displacing water and leaving that section unable to carry heat.

Fix — bleed the radiator:

1. Wait until the heating is off and the system is cold
2. Place a cloth or towel under the bleed valve (usually at the top corner of the radiator)
3. Using a radiator bleed key, turn the bleed valve anticlockwise (only ¼ to ½ turn)
4. Hold the key in place as air escapes (hissing sound)
5. When water starts to drip out (no more air), close the valve clockwise — firmly but not overtightened
6. Check the boiler pressure and repressurise if it has dropped below 1 bar

Cold at the Bottom — Sludge Accumulation

Hot at the top, cold at the bottom is the opposite problem: black magnetite sludge has settled at the bottom of the radiator, blocking circulation of hot water through the lower section.

This is the most common radiator problem in London properties because of hard water — the same mineral content that causes boiler kettling produces oxidation sludge in steel radiators. Properties in London postcodes SW, SE, E, W and N are particularly susceptible.

DIY fix attempt: With the radiator fully heated, use a rubber mallet to gently tap the bottom of the radiator — this can disturb sludge and allow it to circulate out to the magnetic filter (if fitted). This provides temporary improvement only.

Proper fix: The sludge needs to be flushed from the system. For a single affected radiator, a plumber can flush it in isolation. For multiple affected radiators, a system power flush addresses the root cause.

Completely Cold Radiator — Check Valves

A radiator that is completely cold while others in the property are hot usually has a closed valve, not a plumbing problem.

Every radiator has two valves: a thermostatic radiator valve (TRV) — the one with numbers on it — and a lockshield valve at the opposite end (a plain valve with a plastic cap). Check both:

• TRV: Is it set to a temperature level? If set to "0" or turned fully clockwise, the valve is closed. Turn to setting 3 or 4.
• TRV pin stuck: In summer, TRV pins can seize in the closed position. Remove the head of the TRV — the pin should spring up. If stuck, release it with pliers (gently).
• Lockshield valve: Should be open. Remove the cap and use a spanner to open it if it has been fully closed.

All Radiators Cold — Boiler Problem

If none of the radiators are working, the problem is with the boiler, pump or programmer — not individual radiators. Check the boiler for fault codes, check the programmer is calling for heat, and check the pump is running. If the pump sounds like it is running but no heat is circulating, the pump impeller may have seized.