All radiators cold except one or two
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Most likely cause & what to check
When most radiators are cold but one or two are scorching hot, the system is not circulating correctly. The boiler is likely firing and the pump is running, but water is taking the path of least resistance through only a couple of radiators.
Check all radiator valves are open — both the TRV (set to 3 or 4) and the lockshield valve. A common cause is several lockshield valves having been fully closed at some point.
Bleed all radiators, starting with those furthest from the boiler — trapped air can cause severe flow restriction.
Check the pump speed setting at the boiler — on some systems the pump speed can be adjusted. A higher speed setting will push water through a sludged system more effectively as a temporary measure.
If all valves are open and the radiators are bled, the system likely has significant sludge blockage. A heating engineer can carry out a power flush (£300–£600) to restore circulation.
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