Issue with one radiator in open vent heating system no flow

30 Nov 2008
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United Kingdom
Old system with potterton floor stander on GND floor gravity fed cylinder pumped central heating. All radiators get hot apart from one (has previously worked fine but has not worked for about a year unused room for when kids come over)

House is solid floor so downstairs is via drops from 1st floor. Cracked open the flow and drained the radiator and took off the wall and flushed with hosepipe. With the radiator off I got hot water from the flow valve, however on the return side I can remove the valve and get barely a dribble.

Prior to taking off radiator I turned all rads off including lockshields and still no heat. Whilst like this I did get some pumping over in the loft from open vent only a very small dribble.

Emptied the header tank by removing a plastic fitting that had been cut in under the floor 15mm pipe where the header tank cold feed goes under the floor heading towards the boiler which is 7m away. Didn't get a gush of water from that either on the part of the pipe that is heading towards the boiler just a dribble but all radiator valves were off.

Put radiator back on and refilled header tank and bled the rest of the system first before the offending radiator with a small amount of air coming out of those radiators but bled normally. Moved to the offending radiator and that was slow to bled and didn't hear water gushing into it however got there in the end after turning boiler on and getting heat into rest of system. Radiator is now full but still no heat. Seems like the return pipe is blocked? All other radiator get hot.

I have access to compressed air could I give the return pipe a blast with that? Have some x800 but didn't know if any point in introducing that in case cannot flush it out and if no flow through radiator would it clear the blockage. There are two drain cocks on the low points for the drops for the gnd floor.

Thanks for any advice
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Don't use compressed air unless you remove anything in the system with a gland seal such as pumps and valves. You'll blow them out.
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@Madrab the 15mm pipework goes under the floor of the bedroom not sure where it goes from there onwards. There are 3 beds upstairs and one bathroom all with radiators in. The 15mm from the F&E I believe goes to 22mm as there is a box section near the newel post that has 4 22mm pipes in it heading towards the floor mount boiler on the Gnd floor. Two radiators on the ground that are fed from above are on a 15mm drop that feeds 2 rads I know not ideal but previously got everything heating up evenly with careful balancing. Thanks
The reason I suggest it is, if you can then you may be able to use the mains to flush through the length that is be blocked/restricted.

You may have been able to get everything to heat up with balancing because that section was starting to fur up, now it's got to the point whereby it just can't get enough flow, regardless of balancing.

if you have a drain point then a cleaner into the system and a flush may also shift it if there is still some flow through it.
How much of the cold rads return pipework is visible / accessible ? Is it copper pipe ?
OK so have an update. Rigged up a washing machine hose on the return of the offending return valve union this was not flowing other than minor dribble. Gave it a couple of pumps and then got good flow. Put everything back together and still not working. So moved onto the flow pipe which after a few pumps I got a good gush of water but then it stopped to a trickle and no mater what I done I could not get that flowing other than a decent gush after pumping and then a dribble only. I put the bad radiator back on and filled system from header tank without issue. Introduced some x800 and shut all rads off apart from the bad one and got a hot flow pipe but without any meaningful flow. Shut all other radiators off and left running for 10 mins and still no flow. Went back to the return pipe and that flows perfectly with the washing machine hose attached to the valve. Flow pipe no flow still just a dribble. So it is looking like a blocked pipe somewhere for this radiator. Anything else I can do or is it just a case of ripping floors up. Is there a likely place for the blockage? Can I assume that my cold feed is OK as I can fill the system OK? One observation that I have is that despite me seeing 22mm pipes from downstairs to up stairs (remember the gnd floor is all fed by drops from 1st floor) that on the landing it seems like the flow and returns are all in 15mm so it appears that the whole system is 15mm :(
A lot more unusual to have 15mm feeds that block up like that, that's more usually the issue with 8/10mm microbore. My last go to is using a system pressure tester. It allows the pipe run to be put under high pressure but to ensure it gets best effect then it's usually from one end to another.

What are you using to pump it and what are you pumping, air will not be the best and it should really be water which can also help move the blockage.
OK don't have one of those:( I am using a twin barrel footpump. I am guessing I have a blockage that is reducing the diameter of the 15mm pipe but not totally. Then when I pump it up the pressure gets greater beyond the blockage then when I release the pressure the water gushes out due to having greater pressure. Seems like it has been very poorly piped up from the access I have had thus far it seems like the whole system is 15mm apart from the part from the boiler to the upstairs but then the main runs have been reduced to 15mm! What a pain :(
Can get a cheap bucket tester from ebay for 17.99 or do you think be better to start cutting pipes?
If this rads feeds are from upstairs is there no way to lift the floor(ing) upstairs and track the pipework back? Only way you're really going to be able to work you way back until you find the blockage.

yes the air will compress and can force it's way past the blockage easily, with water it's more likely to move the blockage to get the pressure through.
You need to get to both ends to find and flush the blockage if the cleaner didn't shift it, some blockages may go in one direction but not in another, there's no real science to it unfortunately. All you can do is try it from both ends until it shifts. If it doesn't then a re-pipe is the only other fix TBH
OK I even tried to stick a steel drain cleaning tape down the open radiator pipe and only made it 5cm or so that was a fail. Would have expected the tape to have gone around any immediate bends under the floor as it is very flexible and thin. So maybe it is very local to the radiator.

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