2 Feb 2023
Reaction score
United Kingdom
Hi first time post, and appreciate some of the steps below that have already been taken are not standard procedure but I was wondering if anyone here had an interpretation or advice.

Have a 3 storey house with a system boiler, 17 radiators in total fed by microbore pipe, with boiler (Ideal H24) on ground floor and hot water tank, central heating pump and 2x expansion tank setup in the airing cupboard on the top floor. System pressure is 1.5bar, never had any issues with this.

Two of the radiators in the house have never got warm, one on the top floor and one on the ground floor. On each the entire radiator is cold, as is the inlet + outlet pipe. When we moved in a plumber came, turned the bleed key on each radiator, water spurted out and after a few seconds of bleeding they proclaimed that it must be a blockage in the microbore pipes somewhere. They said a powerflush wasn't possible on microbore, the pipe would have to be 'traced back' through the plasterboard - big, expensive and messy job so we just accepted the radiators wouldn't work.

Recently I decided to bleed the radiator with the central heating system operating and keep going until the water stopped coming out of the radiator, or until hot water started flowing (I realise this isn't advised...).

After bleeding about 2L of water from each radiator, the water coming out became hot, the radiator heated up and the microbore inlet pipe became hot to the touch - just like the other radiators in the house. However when the bleed screw was put back in place, the radiator cooled down and the inlet pipe became cold again.

Am I correct in thinking that there is likely no blockage in the pipe and this is instead a radiator balance issue? Does anyone with any experience have any ideas on where to go next? I am unsure about the 'drops' or layout of the central heating pipes behind the plasterboard in my house, but would be happy to provide this info if someone could help me work it out.

Not sure if it helps, but photograph of airing cupboard set up attached.

Many thanks in advance for any help.


  • IMG_3455.jpg
    150.2 KB · Views: 34
Sponsored Links
Water needs to flow in ,and out of the radiator for it to heat up,if there is a blockage it's on the other side ( the return ,the pipe that isn't hot).
Try turning off the TRV on every radiator that heats up ,and see if the cold ones then heat up.
Have c/ heating on only ( no demand for hot water cylinder to heat )
Hi Terry, thanks for the quick reply. Should have included in the original comment, closing the TRV on all the working radiators doesn't cause either of the cold ones to heat up.

I take the point about the blockage being on the return pipe, makes a lot of sense. Is it worth adding a system cleaner if there's a blockage - is this a DIY job?
System cleaning chemical needs to FLOW around the system to work. As it appears that you have no flow to 2 rads ,it would do nowt.
Do you have a magnetic filter on the system ?
Is microbore plastic or copper pipes ?
Are rads fed from manifolds or branched from 22mm flow and return pipes ?
Sponsored Links
Hi Terry, no magnetic filter I'm afraid. Pipes are copper - no idea on how the radiators are fed, is this something that's easy to discern from looking at the pipework/boiler/radiators themselves? Happy to check if it's straightforward.

Appreciate the responses so far.
You would need to follow radiators pipes to their source to establish to what they are connected ,inevitably lifting floorboards etc ,and really not worth the effort.
Establishing if water flows freely from both valves connected to cold rad would at least indicate what's blocked.
Presumably you have lockshield valves on the other end of these rads? Are they actually open? (May need a screwdriver to remove the cover then a spanner to turn the valve. Initially, count and record how many turns clockwise it takes to close the valve, then how many turns to open the valve all the way).
Bleeding as a test. Turn off the control valve, make sure lockshield is open, bleed. If you get water/air out, let it run for a wee while, close bleed valve. Check boiler pressure, top up if needed. Close lockshield valve, open control valve, bleed. Again let it run a wee while. If you get reasonable flow (a jet of water rather than a dribble) in both settings then there's more likely an airlock than a blockage ).

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local