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3 Downstairs Radiators not Heating up and Feed Pipe Goes Cold

Postby benfowler15 » Tue Dec 04, 2018 9:27 pm

My 3 downstairs radiators are not heating up, the 22mm pipe feeding them from above is hot at the top but then rapidly goes cold halfway down the pipe. The return pipe from the 3 Rads going back up is cold. My 4 radiators upstairs are heating up fine. Heating pump is working within my Combi boiler and I’ve had all rads off downstairs flushed through via mains. I’m guessing there is an airlock somewhere along the return because of the lack of movement within the flow pipe coming down the wall. Any suggestions would be kindly accepted. It’s been bugging me for weeks now.
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Postby ericmark » Thu Dec 13, 2018 4:23 pm

I would start by adjusting lock shield valves, do remember I am an electrician not a plumber but over years I have been called in error so many times, I have worked some things out.

So start with working radiator nearest to boiler, turn off the lock shield valve and allow at least the pipes to go cold, then turn it back on 1/4 turn at a time with 2 minutes at least between each adjustment until you feel some heat in one of feed pipes, then stop.

Repeat with other working radiators, then see if the non working radiators are now working.

It is common for plumbers not to set up the central heating or for people to fiddle and open up valves. But with thermostatic radiator valves what will happen is when the good rooms get warm the TRV will close and force heat to next easiest radiator, so once hot even with lock shield valves wide open, the system will work OK, however turn off the central heating and it takes ages to start working again.

To make it worse, modern boilers modulate, that is the flame height reduces when the return water is hot, as said once running OK, but when trying to heat a cold house, if the lock shield valve is wide open, then it heats one room at a time, and if one of those rooms has a room thermostat it can turn off heating before whole house warm.
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Postby Browndoff » Sat Jan 12, 2019 9:14 pm

Ericmark may well be correct - but if the system was well set up and the valves were well-tuned - it could STILL happen!
Maybe you have 'hard-water' in your district - i.e. the water comes from a ground-source passing through limestone and picks up calcium carbonate - which comes out of solution and forms a 'crust' inside your pipes. You'd know this if you get 'furring' in your kettle...?
This is true in my house and I recently had a problem - not with any radiator but with my hot-water cylinder's internal heating-coil. I had to dismantle part of the system and forcibly clean out the pipes - a dreadful job!
I'd recommend that you attach a temporary drain-hose to this pipe which 'goes cold' and see if you can get a decent flow of water through it. If it's good, connect your hose further along the circuit and you'll eventually find where it's blocked. Clean or replace the blocked section and the system should be good again - mine was!
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