OSO 20D unvented cylinder, recharge air gap

25 Oct 2010
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United Kingdom

I have an OSO 20D unvented cylinder and it allows cold water to drip through the tundish.

I had a plumber around who recharged the air gap, however I noticed that when he turned off the mains cold and opened a hot tap that only a small amount of water (maybe 3-5 seconds) came out the tap. He then turned both valves on the T&P which initially bypassed a lot of water then settled into a drip for around 40mins before he said the airgap was recharged.

I thought that the hot water tap should pass water until the airgap was replenished and even then the water through the tundish would then drip
to a stop within 15-20 mins.

since then Ive noticed dripping every month or so although doing the above solves the issue for another few weeks.

Ive attached an image of the valve setup on the cylinder, sorry for the quality, if you need more pics then I can do better.

The top turn valve comes off the PRV, the bottom direct from the top part of the cylinder.

The MI says to commission that you should drain from the commissioning cock (which is right at the bottom and not very accessible!). Do you need to commission just to recharge the airgap?

Thanks in advance!

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You may find lowering the temperature of the stored water can help regards frequent replenishing of 'air gap' , the higher the water pressure/temp the quicker the air will be absorbed , this is one of the pitfalls of using this type storage unit over the ones that incorporate seperate expansion vessels.
drain the whole cylinder and refill it. Sometimes the plate/bubble forming thingy can stick and draining it right down allows it to drop down and straighten up.
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Commercialman : how do you set the temperature on an unvented cylinder

mickyg : thanks, Ill have to knock a hole in a plasterboard wall as they have turned the cylinder around when installing so the commissioning cock at the bottom of the cylinder is against the wall.

Would probably have to do this anyway to allow access to the heating elements as iirc they are above the commissioning cock.

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