What does the cold patch tell you?
Feel the radiator carefully once the heating has been on for twenty minutes:
- Cold at the top, warm at the bottom: air trapped in the radiator. Bleed it.
- Cold at the bottom or in the middle, warm at the top: sludge, a mix of rust and debris, sitting in the radiator and blocking flow.
- Cold all over, while others are hot: a closed or stuck valve, or a system that needs balancing.
- All radiators cold, boiler running: a pump, valve or controls problem rather than the radiators themselves.
What are the common causes?
- Air in the system. It collects in radiators, especially upstairs. Gurgling pipes are a clue.
- Sludge. Corrosion inside steel radiators produces black sludge. Birmingham’s soft water means limescale is rarely the problem here, but sludge in older systems is common.
- Stuck thermostatic radiator valve. The pin under the TRV head can stick down after a summer of not being used.
- Unbalanced system. Radiators nearest the boiler take most of the flow and the furthest stay lukewarm.
- Pump failure. The pump moves water around the system. If it is weak or seized, radiators stay cold and the boiler may overheat.
- Diverter or motorised valve. On a combi a sticking diverter valve can send heat only to the taps. On system boilers a motorised zone valve can stick shut.
What can you safely do yourself?
- Check the radiator valves are open, both the TRV and the lockshield at the other end.
- Free a stuck TRV pin. Remove the head and gently tap the pin; it should spring up.
- Bleed cold-topped radiators with the heating off, starting downstairs.
- Check and top up the pressure afterwards if your user manual shows how, as bleeding lowers it.
- Turn the heating up at the thermostat to be sure the boiler is being asked for heat.
When do you need an engineer?
If bleeding does not help, if several radiators have cold bottoms, or if all radiators stay cold while the boiler runs, the system needs looking at. Sludge is cleared with a chemical clean or power flush, often followed by fitting a magnetic filter. Pumps and valves are common single-part repairs. If the boiler is also making noise, see boiler making noise.