Can water travel back from hot water tank into cold tank?

[runningboygreen]

Hi
I woke up this morning to find my overflow leaking from my cold water tank (after the hot waater had come for the morning). I've adjusted the ball valve & it's stopped but I've got a lingering doubt.

At the bottom of the cold water tank there's an outlet down into the hot tank in the airing cupboard. The outlet to this from the cold tank is warm & the water in the cold tank immediately by the outlet is also warm.

Is that normal or is it somehow possible that the warm water is coming back into the cold water tank causing it to fill up from the bottom?

Any help would be much appreciated. Thanks

 

[JohnD]

firstly verify that the ball-valve is not sticking, dribbling or wrongly set. that is the easiest and cheapest fix.

however yes it is possible

a few possibilities:

(1) the hot cylinder is getting exceptionally hot and boiling or expanding up the pipe

(2) you have a mixer tap, probably a Ceramic one in the kitchen but maybe a shower, which has mains-pressure cold and tank-pressure hot, where an internal leak is allowing water to pass up the hot pipe

(3) you have a cylinder where the pressure on the boiler side (inside the coil) is different to the pressure in the cold tank, and a leak in the coil allows water to move from the higher to the lower pressure (however usually the F&E is lower than the cold tank, in which case it is the F&E that overflows)

Diagnosis:
You should be able to spot (1) easily enough

for (3) turn of the mains stop-cock and see if the overflow stops immediately or continues until the two tanks equalise.

I bet it's (2). If you have service valves on your mixer tap, close them.

 

[runningboygreen]

Thanks for that.

I don't think it can be 1) as nothing's been changed to make the water heat any more than it was (unless I suppose the thermostat's gone wrong).

I don't think it's 3) either as the water level in the header tank seems fine & nowhere near the level of the F&E tank, and the F&E tank's on the same level as the expansion tank.

It could be 2) so I'll take a look & see if I can find a leak.

Thanks again.