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I've just bought a house in a hard water area and found that one of the radiators doesnt heat up (it's in the spare room so not that much of an issue, but still frustrating.)

The system is a microbore central heating system with 7 radiators, and the spare room radiator is the only one that doesn't work. The radiator has been bled, all the other radiators were switched off with this one one to try and clear the blockage. the radiator itself is not blocked as he took it off the wall and cleaned it and it was fine.
While the radiator was off the wall he checked to see if there was water coming up through the pipes and it ran from one side of the radiator but not the other. He said that this suggests that there is a blockage in the microbore (8 mm) pipe. He also didnt recommend a power flush on the system as it could burst the seals. What he did say was to get some cleaner and put it in the header tank and run it for 24hrs to try and clean the system, which is alot cheaper. Now my question is how do I do this? I think this (below) is how you do it but want to check before I do.

  1. tie up float valve in the header tank.
  2. locate the drain valve (by the front door) and drain the header tank, clean the tank out while i'm at it.
  3. add some cleaner to the header tank (he suggested sentinal x800), and drain the system until the cleaner gets into the central heating system (making sure no air gets in).
  4. fill up the header tank and run the central heating system on high for a few hours, how many hours? or is it until the radiator starts working again?
  5. drain and refill the system at the same time, until the water runs clear?
  6. repeat no5 a few times, turning different radiators on and off so the water goes round the system.
  7. add x100 inhibitor to the header tank and drain into the system. Do I actually need to do this?

Are these the correct steps?

Many thanks in advance.


Does the radiator get warm at all?

Could be a piping issue or a valve issue.

Get someone to check it out...too many variables to give correct advice on this problem.
You could go through the process you have detailed and find the problem still exists

chris watkins

Hi mjw,
The steps are correct but it will not work !!! for the cleaner to work there needs to be some circulation otherwise how is it going to get to the restriction ??? How do you know the pipe is not damaged (squashed) rather than an internal blockage.
X800 is strong stuff, if they are worried about power flushing causing damage I would think twice about using it !!

P.S. Yes you need the inhibitor, if you get it clear & don't want it to happen again.
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Oz plumber. The radiator does not get warm at all. When the plumber took the radiator off the wall the valve was moving fine, which suggests to me its a pipe issue or something further up stream.

Chris. You make a good point about the circulation, how would the cleaner would get to the blockage? The only hope is that the blockage is near the manifold, so that it sees some flow when i'm running it. I don't know if the pipe is damaged/crushed without taking carpet, floorboards up, it's pretty well hidden, the manifold is underneath the hot water tank in the airing cupboard which is on the top of plywood with pipes going through screwed down and under skirting, so getting the plywood up (and back down again in one piece) would be a mission. When you say X800 is strong stuff, what sort of damage could it cause?

What would the options be if i didn't do this?


Gas Engineer
putting some pressure on the blocked pipe may shift the blockage but it could also cause more issues to where it shifts too. best to trace along the pipe, look for any restrictions/pinch points and repipe that bit or repipe the complete run.

flushing with x800 is a good idea, do this after youve sorted the blockage out.
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