testing for heating coil leak - will this test work?

Water level in F&E has increased since last inspection some months back. Suspect a very small/slow leak from DHW into Cyl heating coil. Both CW Cistern and F&E tanks are on the same platform in the loft but the CW cistern being bigger has a water level at least a foot higher, so any coil leak means water goes from domestic to CH, not t'other way round - right?

The leak may be very small and the increase in F&E water level very gradual. Changes in the level day to day also misleading due to expansion when CHtg is on?

But if I shut off the valve from the F&E to the DHW Cyl and put a jam jar under the C HTg overflow pipe, I can see if any water collects over say 24 hours. I realise turning off that valve has its risks but if I monitor everything carefully, should be OK?

The jam jar will be postioned over the F&E tank so IF I underestmate and overflow volume, it will not mess up my ceiling.

This way - if there is a small leak, the CHtg overflow will operate and I should see some water in my jar >

mahogany said:

But if I shut off the valve from the F&E to the DHW Cyl

Click to expand...

the f/e don't feed the hw cyl.

and put a jam jar under the C HTg overflow pipe, I can see if any water collects over say 24 hours. I realise turning off that valve has its risks but if I monitor everything carefully, should be OK?

Click to expand...

don't really make any sense

if your talking about the vent pipe a faulty cyl coil won't send water up the vent but up the cold feed in to the bottom of the f/e.

That won't prove anything with the heating and hot water turned on, and the boiler and heating may stop working permanently if you try it.

It sounds like the pipe you are referring to is the open vent, not an overflow.

With the boiler running, water will expand up the vent pipe and into your jam jar instead of taking it's normal route back up the cold feed.
When the boiler shuts down, the water will contract again, and, as there would be no feed from the F&E, it will draw air into the system down the vent. Next time the boiler fires, it will expel even more water, as the air drawn in expands more than the water it replaced.
Etc.
Etc.
Then the pump runs dry, and packs up.
Then the boiler overheat stat fails, as it is no longer surrounded by water.
Then the boiler destroys itself, possibly with a very loud BANG!

OK Guys and thanks for oyur valuable guidance. I will give up on that idea.

So how can I discover whether there is very slow leak from the coil assuming the leakage flow is from the domestic hw to the CHtg because the cold water cistern water level is highest?

You could (but its making a mountain out of a molehill and could cause more problems then its worth) but....

You could drain your CH, disconnect the flow/return pipes from the coil at the cylinder and see if there is a flow or water out of the coil.

IF the coil was leaking (which is a fault that seldom occurs really) then with the CH side drained/disconnected (ie the coil) water would then pass from the cylinder (DHW) through the 'split' into the coil as apposed to from the coil and into the cylinder, and subsequently drip out of the coil joint at the cylinder.

But if I shut off the valve from the F&E to the DHW Cyl

Click to expand...

yeah close the feed to the cylinder

That won't prove anything with the heating and hot water turned on, and the boiler and heating may stop working permanently if you try it.

It sounds like the pipe you are referring to is the open vent, not an overflow.

With the boiler running, water will expand up the vent pipe and into your jam jar instead of taking it's normal route back up the cold feed.
When the boiler shuts down, the water will contract again, and, as there would be no feed from the F&E, it will draw air into the system down the vent. Next time the boiler fires, it will expel even more water, as the air drawn in expands more than the water it replaced.
Etc.
Etc.
Then the pump runs dry, and packs up.
Then the boiler overheat stat fails, as it is no longer surrounded by water.
Then the boiler destroys itself, possibly with a very loud BANG!

Click to expand...

methinks not

worst that will happen is the op will have no hot water at the outlets

not that it will prove anything either

if the cold cistern feeding the cylinder is higher (or the water level is)
just tie the ball valve up in the heating f&e tank (smaller) one

if it still overflows it's the coil