CH Problem

  morddwyd 16:02 29 Nov 13

Not really Speakers' Corner, but not really digital either.

My Ch system is overflowing from the header tank.

It's the old back boiler system, open to atmosphere, gravity fed from a small header tank, fed through a ball cock.

It;s not the ball cock, which is shutting off well below ythe overflow level, but the expansion tube, which vents into the tank rather than out to the roof, is trickling continuously.

This fills the tank and causes it to overflow, (losing heated water!).

Of course the system continually tops itself up, and the cycle carries on.

I have effected a temporary fix by turning the supply to the header tank off, so that it is just a recirculatory system, but there is obviously a stuck valve somewhere.

Where do I start looking please?

  bumpkin 16:35 29 Nov 13

"Where do I start looking please?"

Sounds like it is overheating try turning the thermostat down a bit.

  bumpkin 16:37 29 Nov 13

You say it is gravity fed but is the heating side pumped?

  the.hick 17:15 29 Nov 13

morddwyd, is this a system with the pump only used for heating, ie, the hot-water tank primary feed is gravity circulation? If so, it may be a blockage in the heating circuit causing the vent pipe to pump-over. Check pump speed is not set too high, usually a 2 or 3 position switch on the pump. Also check boiler thermostat not set too high.

  Forum Editor 17:18 29 Nov 13

"I have effected a temporary fix by turning the supply to the header tank off, so that it is just a recirculatory system"

Under no circumstances should you do this. It means that your header tank could eventually empty. You don't mention what fuel your boiler is burning. If it's a gas-fired model there should be an internal thermostat which reads the return temperature on the primary circuit. That will close a gas valve and prevent boiler damage, but with solid fuel the boiler will just get hotter, and the reduced amount of water in the jacket will turn to steam. Ultimately the boiler jacket will crack open.

The continual venting is possibly caused by scale deposits in the system's pipework. Reduced bores can lead to venting in gravity systems. Alternatively, if you have a pumped heating circuit, the pump could be over-pumping. This can happen when a radiator or radiators get sludged up, and water can no longer circulate efficiently. It's common for people to turn up the pump speed to compensate for the sluggish flow, and the result is over-pumping into the header tank via the primary circuit.

Keep the system supplied with water, no matter how irritating it might be, until you or a heating engineer determine the cause.

  bumpkin 18:17 29 Nov 13

OK, lets start with some basic info.

Is it gas, is the heating side pumped or is everything gravity, when did the problem occur was it suddenly or was there any previous indications, has anything been altered since the time it was working OK.

  bumpkin 18:46 29 Nov 13

Another thought on the matter. Drain off some water from the system and tell us if it is clear, blackish, or at its worst black sludge. Easy to do and a good starting point for analysis of the problem.

  morddwyd 19:50 29 Nov 13

Thanks for all the responses.

I have tried running the pump at all three speeds, with no change. I have also tried changing the thermostat setting from low to mid to high, with no change, in fact the water continues to vent overnight when the CH is switched off.


Thanks for the warning. I was aware of the danger, as coal fired back boilers regularly blew out locally when I were a lad! Mine is gas fired and I know it has automatic shut down. However, I am monitoring the header tank last thing at night and first thing in the morning and every six hours or so in between.

I think the sludge possibility is probably the most accurate, and I shall start a treatment next week. I was just hoping there was a stuck valve I could get to meantime. As the water continues to vent into the tank even when the pump and boiler are off there is something not sealing off the header tank when it should.

  bumpkin 20:53 29 Nov 13

"As the water continues to vent into the tank even when the pump and boiler are off"

Then I would suspect that you have a failed coil in your HW cylinder allowing water from a higher level storage tank to feed back into your system. ie the domestic hot water in your cylinder is leaking into your heating circuit via the heat exchanger (Coil)

  namtas 21:50 29 Nov 13

If you are getting hot water into your header tank then something is very wrong, as well as potentially dangerous it is aerating the system this will lead to excess corrosion In addition to the original problem. I would look at the pipe between the feed and the vent which will most likely be choked up, modern practise is to make this pipe one size up on the other system pipes

  bumpkin 22:22 29 Nov 13

namtas, OP has stated " the water continues to vent into the tank even when the pump and boiler are off"


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