Air-lock in mains???

[Aidan]

I recently changed a kitchen tap for another, installing a single-hole mixer. I had isolated the water at a stopcock under the kitchen sink unit. After a few days, a loud banging started when the cold water tap was used. It stopped for a couple of weeks but has now returned. I've been asked to get it fixed.

The user had thought that it was caused by an air-lock. I told them that you'd only get an air-lock on a tank-fed system and that it was most probably water hammer. However, they rang the manufacturer's technical help dept & they immediately said it was an air-lock.

I'm a bit baffled by this. I can't imagine an airlock in the 18" of pipe I put in; it rises continuously & there's no high-points. I'm now inclined to think the tap is defective and the air-lock story is an excuse. I have to identify the real cause and get it fixed. Has anyone else ever heard of this air-lock business with mains water?

[Chip]

On 12 Aug 2005 10:14:01 -0700,it is alleged that "Aidan" spake thusly in uk.d-i-y:

Most likely the new tap is much more efficient at turning off quickly than the old one and thus causes perhaps unsecured or poorly secured underfloor pipework to move. Finding the loose section (if this appears to be the case) and securing it with clips etc. would be the permanent fix. This would be identified by the bang sounding somewhat distant, and perhaps louder in another section of the house.

[PC Paul]

Or possibly a dodgy valve anywhere else in the house - tap washer, ball valve, anything.

If you turn off the tap slooowly does it still happen? Or is the noise while the water is running?

I'd suspect a toilet cistern if one is fitted on the mains supply.

Hmm... could the valve on the little-used stopcock have been disturbed when you used it and now be flapping away?

[Aidan]

Don't think so; more the opposite. The old one was a 1/4 turn, the new one is a screw-down type tap.

The makers said it was an air-lock, which seems most odd to me. It might be the 'jumper' in the existing stop-c*ck (or the new tap) that is jumping around & suddenly stopping the flow, but I'm particularly interested if anyone supports the air-lock theory.

[IAN]

try opening the stop tap fully

[David Lang]

"Aidan" wrote;

I had excessive water hammer in a mains pipe feeding a bathroom tap. I fitted one of those flexible braided hose type things with a fair amount of slack and fixed the pipe upstream firmly to the wall.

Don't know which one stopped it, but it did stop.

Dave