Radiator hot feed, cold return

[Bigphil70]

I live in an "upside-down house", boiler on upper floor, problem with almost the last rad on the downstairs circuit. Combi/condensing boiler.

Last week this rad became 'tepid' but the feed pipe to the rad was very hot (stoplock end), return, stone cold.

First job, remove the head of the TRV and check the pin, all OK, so, next thing was to remove rad as this rad has always been a bit sludgy. Had a Mag filter fitted to the boiler a couple of years ago which removes a lot to the metallic debris, so it shouldn't be sludged up any more. The system was flushed out about 10 years ago, but the pipework is probably 35 years old.

Closed off stoplock, closed TRV, opened bleed valve to remove pressure, undid TRV joint and drained RAD (bleed valve open). Once it stopped emptying, undid stoplock union, and tipped rad to make sure it was empty. Water was fairly black, but everything seemed to have drained out. Whilst disconnected, I opened the TRV and got a very healthy squirt of water out of the pipe (no air lock on return). Unfortunately didn't do the same to the stoplock.

Reconnected everything and with stoplock closed, filled rad from the TRV end. Closed off bleed valve, repressurised the system, opened stoplock and tested. Still the same. Closed off all other rads in the house except this one and ran the system, same fault. Input very hot, rad warming, a little, I assume by convection only, bottom of rad and return pipe cold as a witch's.

I can see almost all the pipes when I look under the floor, and some of the pipes take a devious route. Imagine the feed and return running parallel through bathroom and hall, the feed to the faulty rad comes off next to last, but the return isn't alongside it, then comes the last rad which is a very old miniature rad (feed and return) and it looks as though the return from the faulty rad connects beyond the last rad return. There's no way I can get to see this section of pipework as it's under a built-in wardrobe. It has worked so I don't see how pipe configuration can be a problem.

Sorry to be verbose, but I've done it to answer all your anticipated questions.

I guess my only questions now are... could a blocked stoplock get so hot as to heat a little bit of water via conduction through it's union with the rad.? Do I take the rad off again and make sure that the stoplock is working, do I fit a new rad "in-case" the existing has some sort of blockage?

Thanks in hope of an answer which works.

Read more: //www.diynot.com/diy/conversations/radiator-hot-feed-cold-return.32371/#ixzz4WtwSaS00

 

[jeff the gasman]

It's probabl airlocked. Try turning all the other rads off first and tell us the result.

 

[Bigphil70]

Thanks for replyJeff. See para 4 in original. Everything off but this rad. No change. Also refilled rad after replacing through trv on return side and all filled ok. I would have thought that refilling rad should have cleared airlock.

 

[unclebob1]

Could it be the return from this rad is feeding into another rad? And that Rad is off so no flow?

 

[Bigphil70]

Can't see where the return actually connects, but I refilled the failing rad via the trv (return) and so it looks like the return can't be through the last rad on the circuit.

I'm off to but a drawoff cock to connect in place of the bleed valve to make sure I've got a supply on both flow and return. Should also help drive out an air lock if there is one.

If there's an air lock, it must be on the return, but, as that's how I refilled the rad, I don't think there is one.

Should be doing that tomorrow morning. Will post an update.

 

[Bigphil70]

A frustrating day. First fitted my drawoff cock, (invoiced as a 1/2" Type A draincock, £1.19) There's a very good series of videos on Youtube from Plumberparts shows how to do it under "how to clear airlocks". Both replies mentioned airlocks and I needed to convince myself that I had flow into the rad from both sides, it was the same as clearing airlocks, it seemed a good thing to do. There were no sounds of air being released, and the fault was still the same after doing it, so I decided to remove the rad and give it a good cleanout with the hosepipe as the possibility of sludge was now seeming a possibility. There didn't appear to be much muck coming out, so back it went, top up system, fill rad, bleed, top up system again and test.

It now seems to be WORKING AFTER A FASHION. That's to say, it eventually gets hot enough, but the return isn't as hot as I'd expect.

Again, I had a look at the pipes on the last 2 rads, this one and the mini at the bottom of the stairs.

The flow pipe comes along and there's a 'T' off to the faulty rad 'flow'. Flow pipe seems to continue to the 'flow in' on the mini, and doesn't appear to go anywhere else (end of the line). Return pipe runs parallel but there isn't an equivalent 'T' to the faulty rad return. The first 'T' is up through the floor to the mini rad, and the return from faulty rad seems to join beyond this 'T'. Not quite what UncleBob said but daft enough to be similar.

This is such a cock-eyed way of connecting as it would have been far easier to connect flow and return to the main pipes, next to each other.

Slightest issue now, and I get a chippy in to cut me out a hole in the @£$%%^ stupid chipboard floor so that I can run the return from my faulty rad to where it would be logical, and replace the idiotic mini rad with something a bit more meaningful. Hence I'd have all new pipework in this area, and sufficient traps in the floor to maintain the system.

Fingers crossed that tonight and tomorrow, the bedroom stays warm.

 

[jeff the gasman]

An airlock on boiler return with insufficient flow to clear the flow of water will cause this also. I've spent many a frustrating hour trying to get a system circulating after a drain down.
PITA sometimes.