When a plumber was replacing my CW cistern he noticed that the heating
feed/expansion cistern drained and reported that I have a leak in my
heating system.
He said the leak was so bad that it is definitely downstairs as
otherwise there would be obvious damage to the ceiling below.

I have looked all over for the leak, the only place left to check is
under two expensive floors (the floors are raised on joists about 50cm
above the ground).
Before I start damaging expensive floor boards, is there any other
possibility?

One thing that springs to mind is that the HW cylinder could be
leaking water internally (i.e. leaking hot water from the coil into
the tank). Does this sort of thing happen much or am I just graping at
straws?

Any advice much appreciated!
Chris.

In an earlier contribution to this discussion,
Chris wrote:

When a plumber was replacing my CW cistern he noticed that the heating
feed/expansion cistern drained and reported that I have a leak in my
heating system.
He said the leak was so bad that it is definitely downstairs as
otherwise there would be obvious damage to the ceiling below.

I have looked all over for the leak, the only place left to check is
under two expensive floors (the floors are raised on joists about 50cm
above the ground).
Before I start damaging expensive floor boards, is there any other
possibility?

One thing that springs to mind is that the HW cylinder could be
leaking water internally (i.e. leaking hot water from the coil into
the tank). Does this sort of thing happen much or am I just graping at
straws?

Any advice much appreciated!
Chris.

The first thing to do is to check out the situation for yourself rather than
taking the plumber's word for it.

Presumably he found the F&E tank to be emptyish. This could have happened
over a long period due to a combination of a *small* leak (like a seeping
radiator valve), evaporation, and a sticking ball valve.

So put some water back in the F&E tank - not too much, it only needs 2 or 3
inches over the fill pipe when the system is cold. Make a careful note of
the level, tie off the ball valve so that no fresh water can come in, and
monitor the level every day for a week. If it goes down detectably in that
time, maybe you have got a leak - otherwise, there's not a lot to worry
about - just untie the ball valve and make sure it's working. [I give mine a
tweek every time I go into the attic to make sure it's ok]

A leaking coil in the hot water cylinder certainly *can* cause the primary
and secondary circuits to mix. However, unless your system is unusual, the
water level in the cold header will be higher than that in the F&E tank. In
this case, any leakage would be *into* the primary circuit - causing the
level to *rise* in the F&E tank, rather than the other way round.
--
Cheers,
Set Square
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Thanks for the reply. I took your advice and went up to see the extent
of the problem for myself, it looks bad !

When I went up first thing in the morning I noticed a small flow of
water into the F&E tank. Water was about 3 inches above the out pipe. I
tied off the ballcock and about an hour later the water level had
dropped by nearly 3 inches ! The HW was off at the time, I disabled it
yesterday so I guess this eliminates the HW cylinder leak as there
seems to be motorised valves stopping flow when it is switched off.

The plumber said I should be able to hear the water leak (like a slow
running tap) but I've been unable to so far. Is there anything else I
can do other than start ripping up the floors?

Cheers,
Chris.

In an earlier contribution to this discussion,
Chris wrote:

Thanks for the reply. I took your advice and went up to see the extent
of the problem for myself, it looks bad !

When I went up first thing in the morning I noticed a small flow of
water into the F&E tank. Water was about 3 inches above the out pipe.
I tied off the ballcock and about an hour later the water level had
dropped by nearly 3 inches ! The HW was off at the time, I disabled it
yesterday so I guess this eliminates the HW cylinder leak as there
seems to be motorised valves stopping flow when it is switched off.

The plumber said I should be able to hear the water leak (like a slow
running tap) but I've been unable to so far. Is there anything else I
can do other than start ripping up the floors?

Cheers,
Chris.

Motorised valves won't stop the coil being under pressure - because it's
still connected on the return side. But which tank is higher - the primary
circuit F&E (the one you're having problems with) or the big DHW header
tank? If the water level in the F&E tank is higher than that in the cold
header (unlikely but not impossible if the base of the F&E tank is above
attic floor level) water *could* still be leaking into the DHW system though
a faulty coil - but in that case, I would expect to see some discolouration
of the DHW.

When you do your level check, bear in mind that the level will naturally be
higher due to expansion when the boiler has been on than when the system is
cold. So you need to compare levels under the *same* conditions. Did you do
that?

If you *have* got a leak under the floor boards, there's no easy way to find
it - other than taking boards up - unless you know a tame water diviner! g

Can you see into the under-floor void by looking through an air brick. You
might just get a clue this way, if there's a lot of water visible by so
doing.

--
Cheers,
Set Square
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The F&W tank is on the attic floor and the water level is certainly
much lower than the DHW level. Also the water in the DHW tank is
crystal clear.
The boilder was off before and after my check so the system was cold.
Leaking at 3 inches per hour you'd think it'd be easy to find :-(
I'll start investigating under the floor then.
Thanks for your prompt assistance!

Cheers,
Chris.

One more question, if I were to close off the return valve for all
suspect radiators would this help find/eliminate the leak from these
return pipes? If so this could at least send me to the correct room.

In an earlier contribution to this discussion,
Chris wrote:

One more question, if I were to close off the return valve for all
suspect radiators would this help find/eliminate the leak from these
return pipes? If so this could at least send me to the correct room.

It wouldn't really help - because there's static pressure in the pipes, with
the valves open or closed. In other words there's an alternative route to
each bit of pipe, other than the one which goes through the radiator.
--
Cheers,
Set Square
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Finally did some digging and found the leak under 2" of concrete on the
garage ramp. The cement must have lead to corosion of the copper pipes.

Thanks for the advice !
Chris.