Expansion Tank Losing Water - Why ?

If it is a small tank - then is it possible that expansion is pushing water out of the overflow - then when the system contracts you are left with an empty tank.

Another thought - are you sure it is only an expansion tank - might someone have plumbed something else into it?

========================================= You might have a leaking (or partly open) drain c*ck somewhere downstairs or under the downstairs floor. Also check all downstairs radiators and their valves especially if there's a possibility that leaking water could drain away without being noticed through a suspended floor.

Cic.

I assume you also have another tank for cold water as well as this smaller tank?

If so, is the wather level in the smaller tank, at a higher level than the cold water tank?

If so, you could have a leak in your hot water cylider, where wather is escaping from the central heating system coild in the cylinder, into the hot water in the cylinder, which is conected to the cold water tank.

Toby...

No - you've got a leak somewhere. And quite a bad one. Might be the boiler itself and it's drying out quickly with the heat?

I think you would notice a whole tank of water in the boiler as it would only evaporate when it was on but I would still think its a leak

In article , snipped-for-privacy@dsl.pipex.com scribeth thus

Odds on that the coil in the hot water tank is leaking into the hot water supply...

But in theory at least if the tank that supplies that is at the same height then it would cause a level change in the water...

Alternatively as he obviously has a leak then it is coming out somewhere he will not notice it. Inside the copper cylinder?

The small tank will traditionally be above the larger DHW tank and any perforation in the coil inside the cylinder will allow the water in the header tank to drain down to the level of water in the DHW tank.

Perhaps he has a primatic cylinder and it is faulty?

Tie a plastic bag over the end of the overflow pipe and see if it collects any water. If not, then you definitely have a leak. :^(

not in my house or my fathers or father-in-laws ,both tanks sit on the same "floor" in the loft, so the expansion tank would fill up and over flow as its lower than the cold water tank, as my father found out when his coil failed the larger DHW tank and any

Took quite a while for me to figure out the cause. New washer - nope. Stopvalve turned off - nope. Disconnect feed - nope. Hmmm... must be...

Andy

but he has a continuous flow of water and it does not overflow "> I cut the supply and

Might be a small leak when cold which gets greater when the boiler heats up. But of course these are all guesses. A header tank may actually hold very little water - depending on how the ball valve is adjusted.

Indeed.

The water is almost certainly going into the cold tap supply tank in the loft via a hole in the heating coil in the immersion cylinder. It's easy to verify. Cut off the supply to the cold tank one evening and mark the water level. Next morning if that level is higher you have found your problem. Obviously don't flush the loo or open any taps in the meantime. Alternatively pour a copious amount of cyanide into the header tank and place a member of the family Carassius Auratus into the cold tap supply tank. Make regular observations of the condition and demeanour of said piscine. Numerous other methods spring to mind.

Is the system pumping over? Does the vent pipe return to the correct header tank?

Have a look at the bottom of this page

Thanks everyone for your responses.

The expansion tank is mounted on a stand which is about a metre higher than the cold water tank which sits on the loft floor. I will do the test with cutting the water supply to see if the water level on the cold tank goes up overnight.

The level of the water never reaches the overflow and water isn't being lost that way.

I've traced the vent pipe and it does go back the expansion tank. Oddly though it turns from 22mm to 15mm as it rises up into the loft. I thought that the vent pipe had to be the same diameter as the inlet ?

There appears to be some history of overpumping when I look back at the service history but it looks like the speed of the pump was reduced which seems to have cured the problem.

If I cut the water the expansion tank takes about a couple of hours to empty itself so wherever the water is going we must be talking about a fair amount.

The hot tank is 10 years old. Is that old enough for such a fault to have developed ?