DHW primary not circulating

[deldol]

Conventional (at least it was 25 years ago) pumped CH and gravity DHW system with F&E tank in loft (two storey house), cylinder on first floor and boiler on ground floor (25 year old Glow-Worm, very reliable), immediately below the cylinder, so very short vertical feed to cylinder. I drained the system last week to fit new radiator valves, filled up again and had some difficulty getting CH to circulate, but eventually managed it. At first the DHW worked fine, but now there is no circulation in the primary circuit from the boiler through the heat exchanger in the cylinder. 28mm pipework to/from boiler, feed from F&E tank tee'd into the return pipe just after cylinder, and vent tee'd into the flow pipe just before cylider. Can't believe the heat exchanger can be blocked as I understand it just a coil of pipe (presumably 28mm like the pipework but maybe its smaller?) Can't think of anything else, though, as the system couldn't be simpler. Any ideas?

 

[seco services]

probably airlocked coil
is the flow pipe hot and return cold ?

 

[deldol]

Thanks for reply. Yes, feed pipe is hot right up to entry to the cylinder. Return pipe is cold. Thought previously about an air lock. There is no bleed valve, so thought I would check by loosening a compression joint in the 15mm air vent pipe, only about 9 ins above the T joint between the air vent pipe and the feed pipe. I did this when the whole system had been turned off for a couple of hours and was cooled down. Only water came out so I assumed there was no air lock. If it is an air lock, how do I get rid of it? I would have thought that it should find its way up through the vent pipe.

 

[seco services]

can get trapped in coil.
if the air vent won't release you can put a mains hose in the cold feed and it should push up the vent pipe. only takes seconds. but be careful if you do it hot as the vent will chuck out hot water.

 

[deldol]

Thanks again. Not entirely sure what you mean by the cold feed. Do you mean the bleed valve I used to drain the CH system (off one of the bottom radiators)? I've also got a bleed valve on the CH return pipe just by the boiler - would it be better to use that? It's a bit awkward to get at as it's between the boiler and the wall so might be difficult to get a jubilee clip on but I can try.

 

[seco services]

cold feed from f/e tank in loft that connects to the cylinder return

 

[deldol]

Sorry if I'm being stupid, but can't see a way of doing that easily as there's no valve to connect the hose to. I suppose I could drain down the system enough to empty the f/e tank, disconnect the feed pipe from the f/e tank and then try to get a hose on that, but may be difficult to get a watertight joint.

 

[seco services]

you don't need it water tight just push it against the hole in the bottom of the tank.
theirs enough pressure in the mains to push out a airlock.

 

[deldol]

Miracles! It suddenly started circulating after 4 days. I had decided to leave it until today to try your hosepipe trick, but it started working last night. Must have been an airlock as you said - had them before when I've drained the system but they usually clear within a few hours. Thanks for your help, it will be useful next time it happens.

 

[ChrisR]

15mm air vent pipe

That should be 22. Very likely to block a 15 with soft goo.
This
//www.diynot.com/forums/viewtopic.php?p=771846#771846
is something you can use to clear it out, and flush your coil at the same time. Only needs a few common parts to put together. If your vent is 15mm you can do it with one long tap connector hose.