Central Heating expansion tank vent pipe overflowing

Postby rednox5 » Sun Nov 30, 2008 1:02 pm


I have a vented hot water system.
I have just descovered hot water pouring out from my vent pipe into the expansion tank. I dont think I have a blockage and my boiler thermostate is working ok. The water level in the expansion tank remains constant as the water is returning into the system by the feed pipe.
I still get a hot water supply from the storage tank to the taps so the hot water from the boiler seems to have 2 flows. 1 through the storage tank, the other up the vent pipe.
I reduced the pump speed from 3 to 2 with no change.
I reduced the flow from the boiler through a gate valve already fitted. this seems to stop the flow up the vent pipe but now the water in the storage tank doesnt get so hot.
I think my problem may be due to the storage tank returm pipe only being 15mm with 3 right angle bends before converting back to a 28mm pipe, compared to the supply pipe from the boiler converting to 22mm from 28mm just before the storage tank. This may be restricting the flow of water directly back to the boiler forcing the water up the vent pipe instead.
Any comments please
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Joined: Sun Nov 30, 2008 12:42 pm


Simply Build It

Postby plumbbob » Sun Nov 30, 2008 10:12 pm

The fault is caused by the feed and expansion pipes being too far apart on the system. This allows a pressure difference to occur between the two resulting in either water being forced up the expansion and over into the tank, or the reverse of water being drawn down the pipe, sucking air back into the circuit.

See these similar topics.

http://www.diydoctor.org.uk/forums/view ... hp?t=15110

http://www.diydoctor.org.uk/forums/view ... hp?t=15039

Ensure that the header tank is cold. If it heats excessively, it can suddenly rupture and cascade the contents into rooms below.
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Postby Steve the gas » Mon Dec 01, 2008 5:10 am

Also if pump has speed settings - put it on 1
Steve the gas
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Postby rednox5 » Mon Dec 01, 2008 2:07 pm

:) Thanks for your comments. I dont know who done the plumbing around my Storage Tank but if I knew I would ask them to pay back the money to the previous owner. Just one more question.... The Vent and the Service Pipe from the expansion tank are close together. One is placed near the inlet and the other near the outlet of the storage tank, so I guess in reality that means they are not close together. Would you suggest that I re-plumb these on the outlet side of the storage tank which is further away from the pressure generated from the pump and to keep their pressure equal. Its not going to be easy running these pipes back to the return side of the pump which is downstairs.
thanks again
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Joined: Sun Nov 30, 2008 12:42 pm

Postby plumbbob » Mon Dec 01, 2008 9:45 pm

The expansion pipe should lead off the flow pipe usually in the airing cupboard. For best results, it should be the highest point on the system. There must be a unhindered escape for boiling water and steam if the boiler thermostat develops a fault.

The trick is to fit the cold feed near this point where the expansion leads off, but not so as to prevent air escaping when the system is filled.
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