Re: CH Expansion pipe vents into cold cistern

If the bathroom is downstairs, leaving the hot tap on with the pipes blocked will result in the collapse of the domestic hot water tank - a frequent occurrence in winter where the two pipes are frozen. Depending on the height of the tank you may have a problem with an upstairs bathroom. You may be OK if the tank is very old (they were a lot thicker then). A rather risky technique! If you must use it, as soon as the water slows (ie you notice an effect) shut of the tap and remove the bungs (or one at least).

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Good point! I sort of had that in mind when I said bathroom, and meant an upstairs one. But yes, do be careful as Bob says. It would be a good idea to have someone watch the tank, ready to yell if it shows any signs of inward bucking.

Revising the idea, how about blocking just the header outlet with a bung and then have someone turn on the bathroom tap. Then test which vent is drawing in air. This could be by putting a piece of cloth over it to test if it tends to be sucked in, or maybe by just listening. Don't use the palm of your hand to test for a vacuum as it could be quite a strong one, and the cylinder might buckle with you stuck to the vent. Probably not though.

Or better: Block off the header outlet as above (don't block the vent), then run the hot tap till the flow stops. Any tap will do, there is no risk this way even from a downstairs one. Then turn off the tap. The vent pipe will now be drained down to the level where it joins the top of the hot water cylinder. Now head back to the loft.

With the header bung still in place, put the other bung in one of the vents. Now remove the bung from the header. This will cause the emptied vent pipe to refill with cold water and if you picked the right one it will compress the air in the vent above it. You then pull the bung out and you should hear a decent pop of air rushing out if it is the right one. You could probably detect which one has the air issuing without bothering with the bung.

Yet another scheme: If you could pour some hot water down one of the vents an assistant below should be able to feel which of the two pipes warms up. This would need a bit of hose, a funnel, a kettle and some jury rigging!

-- Phil Addison The uk.d-i-y FAQ is at

what about taping a balloon (or balloon equivalent) onto the end of the vent pipes and see which one reacts?

Probably safer with revised method 2 - you wouldn't really want it being accidentally sucked into the DHW tank!

-- Richard Sampson

email me at richard at olifant d-ot co do-t uk