Bizarre one radiator not working

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Hi, I have a sealed central heating system installed when the house was built in 2007, it's a Baxi MegaFlo system boiler and uses 10mm plastic pipe on to a total of 11 radiators, one of these was added at some point and connects to the 10mm pipes of another radiator. The radiators are spread over three storeys and each one has a drain-off valve.

When I moved in, one of the radiators on the ground floor barely worked. It will eventually warm up a bit but the flow seems to be incredibly slow and it just gets a bit warm. I checked usual things, mostly bleeding it and all the other radiators. The radiator is one of the original radiators and it's in a room that extends out from the rest of the house, so I'm not sure where the pipes come from - there's a concrete floor beneath the radiator.

I asked my plumber to look when he was servicing the boiler to take a look. He tried balancing the system and after eliminating everything else he could think of, suspected that the TRV was jammed.

I decided that if the TRV needed replacement, it would be worth changing the radiator for a larger one. I ended up changing it myself.

When I came to drain the system, I did encounter an odd problem where the radiators on the second floor and ground floor would drain normally but the first floor didn't. The second-floor radiators emptied through the drain-off valves on the ground floor but the first floor did not. I chose to proceed because although the first floor was not empty, it wasn't flowing into the ground floor. The water that came out was almost clear.

I replaced the radiator and the valves with new ones and connected it all up. I refilled the system and flushed out a couple of airlocks. I haven't added any inhibitor back in yet.

Now, the brand new radiator is doing the exact same thing as the old one did. It'll warm up a bit. I've switched off all but one or two of the other radiators and it seems to make no difference. I've tried switch off valves on either side of the radiator and flushing water through from the system. It'll come through quite fast on either side but the radiator still won't warm up much under normal operation.

It seems to me that it's likely a blockage or airlock but the water runs clear and it's a modern system and I've removed the other airlocks I created during my DIY work. I've flushed water through both pipes and it seems to come through fairly well but they disappear behind plasterboard so I don't know where they go.

So, should I get a plumber in? Is there anything I can try? It seems like a hard to solve problem...
 
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Firstly 10 mm off 10 mm is a Nono.
I suspect you have a blockage in the 10mm pipe maybe magnetite build up in stainless steel inserts.
It’s a pig to find and sometimes a pig to replace the blocked bit.
Other causes could be kinked pipe.
Disconnect one valve at a time and compare the flow from each one.
 
I assume the problem rad is on the ground floor,if concrete floor ?
Close off EVERY other radiator ,by its TRV ( don't touch their lockshield valve). Ensure the programmer is not calling for domestic hot water,and is only setting the central heating to come on. Run central heating. If problem rad now heats fully ,you need to balance the system. If it still doesn't heat fully it's a pipework issue.
 
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I assume the problem rad is on the ground floor,if concrete floor ?
Close off EVERY other radiator ,by its TRV ( don't touch their lockshield valve). Ensure the programmer is not calling for domestic hot water,and is only setting the central heating to come on. Run central heating. If problem rad now heats fully ,you need to balance the system. If it still doesn't heat fully it's a pipework issue.

I tried this and it didn't run. I decided to take the radiator off the wall to investigate the pipework. When I disconnected it, a pile of black metallic chunks (not sludge) which I assume are magnetite came out (see photo). I flushed them out until they stopped coming but it didn't resolve the problem. I guess that when the last owners fitted an extra radiator, they neglected to add inhibitor.

So, is there any way I can break up the magnetite without tearing my house apart?
 

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They are the loose bits and there’s a lot of that.
You’ll more than likely have to replace some pipework where the 10 mm meets the main pipework.
Where are you located Benedict?
 

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