I have inherited a central heating system with 8mm microbore. All radiators in the system get hot except one problematic radiator. I have taken the old radiotor off, flushed it out with a powerwasher which had no effect. I've replaced the old radiator with a new one so I know the radiator is not full of sludge but it's still not getting very warm. When I had the radiator off, I checked the flow in the supply and return pipe, neither were blocked.
I'm left thinking that there may be a partial block in the supply pipe which is reducing the flow and I have therefore added a cleaner to the system in the hope that this might move any blockage, although after nearly a week of running for four hours a day, the radiator is still cool.
Does anyone have any suggestions as to what to try before I resort to re-piping?
Check whether you have a one or two pipe system for the radiators. If it's a one pipe system the outlet from one radiator will become the feed to the next one. If two pipe there will be two separate pipes at each radiator with the feed to each radiator coming off one and the outlet going into the other. If it is a two pipe system try turning off some or all of the radiators nearer to the boiler than the problem one and see if the dodgy one gets hot. If so leave the system running like this for an hour or two then open up the closed off radiators and see if the fault has cleared. I assume that you've bled the new radiator after installing it. If not it could simply be that there is air in it.
You could try balancing the system. Close all lockshield valves, then open one or two turns only, this will evenly distribute water to the radiators, if they're all fully open the water takes the easiest route. The probelm rad may have more bends in the pipework, longer pipe run both = greater resistance.
One or two not heating as well as the others - open lockshield one more turn on problem radiators until all are the roughly same temp.
To do is properly there should be an 11oc temperture difference between the flow and return of each radiator
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