Ignite Plumbing & Heating
Radiator Cold at Bottom, Top, or Not Heating
A radiator that's cold at the bottom usually means sludge buildup. Cold at the top points to trapped air. Cold all over often means a system circulation problem.
Sludge and magnetite buildup
Black iron oxide (magnetite) settles at the base of radiators over time, blocking water circulation. The radiator heats at the top but stays cold at the bottom. Very common in systems over 10 years old without a magnetic filter.
Trapped air
Air pockets rise to the top of the radiator and stop hot water filling that section. The radiator is cold at the top but warm at the bottom. Fixes with a bleed key in under 2 minutes.
Faulty thermostatic radiator valve (TRV)
The pin inside the TRV head can seize, preventing the valve from opening. The radiator stays cold even when others are hot. You may be able to free the pin manually.
Balancing issue
If one radiator gets much more heat than others, the system may need balancing — adjusting the lockshield valves to distribute flow evenly.
Failed pump or zone valve
If multiple radiators are cold or the whole system is underperforming, the circulating pump or a zone valve may have failed.
Practical checks and next steps
If the radiator is cold at the top, bleeding it yourself with a radiator key is safe and free. If it's cold at the bottom or only partially warm, that's sludge — it needs a power flush, which requires professional equipment.
- Check if other radiators are working — is this the only cold one?
- Feel the top and bottom of the radiator. Cold at top = trapped air. Cold at bottom = sludge.
- If cold at top: turn off the heating, let it cool, then bleed the radiator with a bleed key.
- Check the TRV is not turned to the 'frost' setting (usually a snowflake symbol).
- Press the pin inside the TRV valve head — if stuck, this may free it.
- If still cold after bleeding, or cold at the bottom, call a plumber.
Why is my radiator cold at the bottom but hot at the top?
Cold at the bottom almost always means sludge (magnetite) has settled at the base, blocking water flow. This needs a power flush to clear properly.
Can I bleed a radiator myself?
Yes — if the radiator is cold at the top, bleeding it yourself is safe and straightforward. You need a radiator bleed key (available at any hardware shop for under £2) and a cloth. Turn the heating off first.
How much does a power flush cost in Norwich?
A power flush for a typical Norwich home (8–12 radiators) costs £320–£550 including a magnetic filter fitting and inhibitor dose.