Hi, I have a gravity fed central heating system and a radiator in 1 bedroom is cold. I have tried draining and re-filling the system, then I have tried replacing the radiator and still it remains cold.
In the main a cold radiator is due to other radiators not being set correctly rather than anything wrong with the cold one.
Theory is all TRV's wide open and you adjust each radiator starting closest to boiler so fixed temperature drop of around 12 to 16 degrees C.
Practice turn off the lock shield valve on a radiator near to boiler, allow it to cool, turn the valve back on 1/4 turn at a time counting the turns, with at least a minute between each adjustment, until you just feel it getting warm. Normally stopping at that point radiator will heat up enough over time. Although it may need a further 1/4 turn.
Then set all other radiators the same. Turn them off then back on same number of 1/4 turns, often around 2.5 turns. Then you will likely find all radiators now work.
I had one at my sister house would not work, found one pipe blocked, tried speedo cable but micro bore and up stairs they had built a partition wall over the floor boards where the central heating pipes were.
Rather desperate so I got a tyre inflater and tried blowing the muck out of the pipe, not really a safe or good idea, but it worked, and it seems poor quality pipes had scale inside them which had blocked them.
However I think this type of problem is rare, and I would not normally try such methods. Normally flushing out is enough, and of course making sure inhibitor is added when to refill.
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