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Water overflows from Expansion Tank every morning

Postby dajonesny » Wed Mar 09, 2011 2:34 pm


Please help! For some reason, there is water coming out of the cold water tank overflow every morning. I've changed the washer in the ball valve - no leakage there. I think it could be the hot water overflow - possible air lock.

I have a Grant 140 multipass oil boiler. Heating works fine, I have noticed that when heating and hot water switched on together the hot water to tap doesnt get pipping hot. However when I swith the heating off and leave hot water on it's pipping on in 20mins.

I moved in to this old farmhouse 6 years ago, changed the boiler 4 years ago and has been working fine until now. I recently drained the heating system to change a couple of rads. Bled the system and no air locks.


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Postby stoneyboy » Wed Mar 09, 2011 9:42 pm

The symptoms you describe probably mean that the coil in the hot water cylinder has perforated. Check the levels in both the CWT and header tank then turn off the mains overnight. If the levels go up in one tank and down in the other it's the HWC, if the levels remain unchanged you have a link between the mains and the secondary system - look for places where cold mains and hot water supply the same piece of equipment eg kitchen sink tap.
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Postby dajonesny » Wed Mar 09, 2011 10:31 pm


I noticed this evening that when I switched the boiler off, there was a gush of water coming down the pipe towards the boiler - the pipe got really hot.

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Postby htg engineer » Thu Mar 10, 2011 5:55 pm

Do you use a mixer shower or mixer tap on a morning to wash ? it could be the difference in pressure causing the tank to backfill.

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