Cold feed dribbling.

[omits]

If I close off the cold feed in the loft (feeds hot tank floor below and bathroom) it dribbles and eventually lukewarm water comes out. If I let that run long enough there is an air lock in the hot feed when I restore the cold. Am I right in thinking that the dribble is water coming up from the bottom of the hot tank being vacuum sucked out up its cold feed pipe (the cold feed divides in the loft just below the base of the cold tank there)? There will be no cold head as the cold tank is shut off (gate valve on output)? Thanks.

PS the airlock wiill clear on its own if I leave the hot tap on for a while.

 

[terryplumb]

Are you referring to the cold feed to a hot water cylinder ,or a Seperate cold feed to taps ?
Either way ,water will not be sucked up from the bottom of a hot water cylinder by closing a gate valve ( which very often don't close fully by the way)

 

[blup]

Seems about right, classic siphoning.

Blup

 

[omits]

Are you referring to the cold feed to a hot water cylinder ,or a Seperate cold feed to taps ?
Either way ,water will not be sucked up from the bottom of a hot water cylinder by closing a gate valve ( which very often don't close fully by the way)

AHHHH, I see! The gate valves I have on the output of my cold tanks do close but they get clogged with scale. One of them did close off fully again when I operated it a number of times. The other persists in leaking. Yes, siphoning then If I close off the cold feed valve nearer the hot tank that should stop the siphon (if the blighter shuts off properly). Thanks.