problems refilling central heating system after repairs


Postby jetway » Sun Sep 21, 2008 4:06 pm

I have a gas central heating system, indirect low pressure. I drained down the system 2 weeks ago replaced 3 radiators upstairs and now i am trying to refill the system but i am having problems. When i release the ball valve which is in a cupboard in a upstairs bedroom the tank fills up but no water will fill the radiators upstairs or downstairs. I have tried refilling twice now with all beed screws open. When the tank has filled there is no noise of water or air escaping anywhere and if you open the drain cock then very little water comes out and it takes ages for the tank to drain, any help would be gratefully received
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Postby htg engineer » Mon Sep 22, 2008 8:23 pm

Could be an airlock or a blockage in the cold feed, With all vents closed and tank full, turn pump for 30 seconds or so - this will hopefully clear any air, and push the water through to the radiators. Don't leave the pump running as all you will do it burn the motor out.


Still no luck ? possible bloskage in cold feed.

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Postby jetway » Sat Sep 27, 2008 12:50 pm

Thanks for your advice you posted I am very gratefull. I tried running pump for a while and still no water coming through so I drained the tank and cut into the cold feed. It was clear back up to the tank but I couldnt blow down towards rad circuit with bleed screw open. I cut in at the bottom of cold feed where it tees off to the hot water cylinder and found a blockage. The system has refilled easily and is running, thanks

Could you advise me on this question also, am I correct in saying that the hot water from the heating also runs through a coil in the hot water cylinder to heat the water? because when the heating is up to temp the top pipe to cylinder was hot and the bottom is cold, could this coil also be blocked? There are no motorised valves or anything of that sort. The system I have is basic in that you cant have the hot water on without the heating. Thanks John
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Postby chris_on_tour2002 » Sat Sep 27, 2008 4:11 pm

"am I correct in saying that the hot water from the heating also runs through a coil in the hot water cylinder to heat the water?"

yes, correct.

"when the heating is up to temp the top pipe to cylinder was hot and the bottom is cold"

firstly are you referring to the pipe at the very top of the cylinder? if so, this is not the heating coil, its your domestic supply. so you'd expect it to be piping hot when the hot water is on.

the pipe at the very bottom is the cold water feed from the tank in the attic. so you'd expect that to be stone cold.

the coil flow and return are usually found on the side of the cylinder, one about half way up and the other directly below it toward the bottom of the cylinder. check that you have identified the correct pipes first and if so:

do you have hot water? if so then it's probably not blocked. you would expect a temp difference between the two parts of the pipe - the coil works on heat exchange, hot water runs through the pipe, gives up it's thermal energy up to the water inside the cylinder thus is cooler on exit, then goes back to the boiler for reheating.

is the return part of the pipe stone cold whilst the flow is piping hot? this may indicate a blockage. but if your hot water is ok and heats up quite quickly then there is probably no blockage.
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Postby Dave From Leeds » Sun Sep 28, 2008 1:23 am

Don't you mean you can't have the heating on without the hot water? For an indirect hot water system you are correct in assuming that there is a heating coil inside the cylinder connected to the heating system. If there is a complete blockage in this coil, you won't get any hot water at all. The same thing would happen with an airlock in it. To check for an airlock, you could try connecting a hosepipe to the vent pipe over the top of the CH top-up (feed & expansion) tank with the other end on a cold tap. Then get someone to slowly turn on the tap whilst you observe for bubbles coming up from the bottom of the tank. If there are some, keep the tap running until the bubbles clear.
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Postby unguided » Sun Oct 05, 2008 12:21 pm

drain as much water out of the f/e tank as you can. cut the cold feed pipe near where it drops into the heating system caution there will still be some water in here, so try and catch it with a bucket. the blockage will be on the tee where it drops into the heating system clear this with a screw driver or scewer then join the pipewith a compression socket and refill the system. once this is done look into having your system flushed and finally put some inhibitor in the system this hould cure your problems.
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