One Radiator Cold, The Rest Hot

What's happening?

When one radiator stays cold and every other one heats, the boiler and pump are fine, so the problem is at that radiator. Where the water stops is the key. If the pipe to the valve is cold, nothing is reaching it: the valve is shut, or the pin inside the thermostatic head has stuck down. If the pipes are hot and the radiator is cold, water is arriving but cannot get through, so the cause is either a lump of sludge or scale blocking the radiator, air locked at the top, or a lockshield valve set so tight that hardly any flows. This is different from a radiator that is cold only at the top or only at the bottom, which are air and sludge patterns on their own pages. Here the whole radiator is cold.

What usually causes it?

• A thermostatic valve pin that has stuck down and holds the valve shut
• The lockshield valve closed or nearly closed after balancing or decorating
• Air locked in the radiator that a quick bleed has not cleared
• Sludge or scale blocking the radiator, which gets worse over years
• A new radiator or valve fitted with the flow and return pipes crossed
• A valve that has been fitted the wrong way round, or a valve insert that has failed
• A blocked pipe or a stray lump of solder or debris in the pipework to that radiator

Is it dangerous?

No. The risk is minor: bleeding releases hot water and sometimes dirty black water, so use a cloth and a container and keep the boiler off while you do it.

Can I fix it myself?

Feel the two pipes near the valves. Take the head off the thermostatic valve by unscrewing it and press the small pin that sits in the body of the valve with a fingertip: it should spring back up. If it is stuck down, work it gently until it moves, or replace the head, which does not need the system drained. Check the lockshield valve on the other end is open, by taking off its cap and turning it anticlockwise a little. Then bleed the radiator with the heating off.

When should you call a heating engineer?

Call a heating engineer if the radiator is still cold after the pin, lockshield and bleed, if black water comes out, or if this is a newly fitted radiator. They can remove and flush it, or find a blockage in the pipework.

What will the heating engineer actually do?

• Check the thermostatic and lockshield valves for flow and free the pin or replace the head or body
• Remove and flush the radiator to clear sludge and scale
• Test the flow and return pipes to the radiator and correct crossed or blocked connections
• Balance the system so the radiator gets its share
• Fit a magnetic filter and inhibitor where sludge is the cause

Who fixes this

• Heating not working
• Power flush

Questions people ask

Sources & References

1. Vaillant: heating controls
2. Vaillant: what is a magnetic boiler filter?
3. Vaillant: most common boiler problems