Sealed System Loosing Pressure with no leaks visible

Boiler make / model = Oil Fired Eurocal Ambassador

Problem= Sealed System still Loosing Pressure with no leaks now apparent.

Initial Symptoms= Heating packed up and pressure in CH system found to have dropped to 0.
Evidence of damp found on ceiling, this involved cutting a hole to find the leak.
Leak found at soldered joint in ceiling – system drained - leak repaired by re-soldering.
System re-filled and bled but it is still loosing pressure, it now drops from 1.0 bar to 0 in about 8 hours and seems to be getting worse.
I have re-pressurized the system many times over the last week or so (takes about 45 seconds to get from 0 to 1.0 bar with the tap on the filling loop fully open).
Radiators bled and system pressure topped up each time(always get loads of air from upstairs radiators)
There are no leaks apparent anywhere. There is absolutely no sign of any water dripping at the overpressure release pipe outside. I have tried re-pressurising the system and leaving it off for a day to see if I could find any leaks that might be evaporating in the boiler or on other hot surfaces, I still could not find any leaks.
House is timber framed 3 bed cottage, 4 X rads downstairs 4 X rads and a towel rail upstairs. Some of the CH pipework is inaccessible in the ceiling and walls but there are no sign of any further staining anywhere.

Could these symptoms be caused by anything other than a water leak?

Is it possible for a hidden leak to be causing this (inside the hot water tank maybe)?

In the pictures, what is the CH device on the right of the motorised valve in the airing cupboard? This has a small red cap with an o ring seal)

What is the similar device just visible at the back of the boiler? (This one has a black cap with no seal and no lid).

What is the purpose of the red stopcock below the motorised valve? (it is normally closed)

Could these symptoms be caused by anything other than a water leak?

Is it possible for a hidden leak to be causing this (inside the hot water tank maybe)?

In the pictures, what is the CH device on the right of the motorised valve in the airing cupboard? This has a small red cap with an o ring seal)

What is the similar device just visible at the back of the boiler? (This one has a black cap with no seal and no lid).

What is the purpose of the red stopcock below the motorised valve? (it is normally closed)

Click to expand...

To be losing that much pressure in that space of time suggests that it is a leak on the system which could be hidden as it is a considerable amount to be losing, it could well be in one of the walls and running down the pipework but is very odd that you don't have any signs whatsoever.
Could require strategic isolation of system to narrow the search down. It is possible that the coil in the cylinder may be faulty.

The device in the airing cupboard is an Automatic Air Vent (AAV). This releases any trapped air in the system and the red cap should be about 1/2 a turn open.

The device on the boiler is also an AAV.

The red handled gate valve below the motorised valve is used as a system bypass that allows a flow of water should all controls shut down. It shouldn't be closed and usually should be around 3 or 4 turns open (roughly!!)

Hi Jonjb,
Thank you for the info.
Red Cap and Valve now positioned as per your suggestions.

As you say my system seem to be loosing a lot of water, I am getting nightmares of walls splitting open and family drowning in the ensuing flood!

Is a fault in the hot water tank coil a common problem and how can I check out if mine has failed? The whole system is only 4 years old so I doubt corrosion problems.

Its not that common a problem on systems of that age but i have to say that the system does look a bit older than that. Are there any drain points under the floors or passing through the walls to outside?

In all honesty its going to more trial and error when tracking down a leak it can be one of those pain in the a** jobs!!

as there are no signs, my first thought would be under the floor downstairs.
check any and all compression joints

Hi,
Thanks for the advice, there system was installed in 2004 when the whole house was rebuilt (long story). Only vent to outside is from the boiler overpressure line. There is nothing at all under the downstairs floor, all pipework for the system is either visible or in the walls or ceiling. The walls are timber framed with celotex insulation, plasterboard and plaster.
How long would you expect this sort of construction to be able to completely hide a leak? (I suppose the insulation in the internal walls could soak up a lot of water)
Is there a way to check out if it is a hot water tank coil problem?

If the coil has gone in the hot water cylinder the level in the storage cistern will slowly rise. If you can crack the valve on the filling loop slightly so that the pressure stabilises you may be able to find the leak. If it's in the cylinder coil the level in the storage cistern will rise slightly and you may be able to feel heat creeping up the cold feed towards the cistern.

If you can't turn the valve on slowly enough it may be worth getting a pressure reducing valve and fitting this inline with the filling loop to enable you to locate the problem. The filling link shouldn't be left on permanently normally but it won't be a problem to leave it connected while you hunt for the leak.

since celotex is impermeable to water, if the leak is behind this then it can be hidden indefinitely as the water could just keep running down onto the foundation blocks/pad and seep out from there, without ever tgetting to a internal wall lining. When you check under the floor, can you see as far as the actual blockwork/base of the timber?

hHi,
I have found the mystery leak at last! After pressurising the system and leaving it go cold a puddle started to form underneath the back of the boiler. The excess water is draining outside through the brickwork at the back so never became visible at the front of the boiler.
There are no apparent leaks in the boiler casing so it seems to be coming out of the bottom of the boiler itself. Could the water part of the heat exchanger have rusted through the bottom in just 4 years?

Check the expansion vessel.

Unscrew the dust cap on top of it, you will find a schroeder valve which is excactly the same as the valve on a bicycle tyre.

Attach a pencil type pressure gauge to it, you should get a reading of 0.75 bar or 10 psi. Any thing less than this it will need recharging with a bicycle foot pump (do this with the system empty).

If there is air or water leaking from the schroeder valve the vessel will need replacing and possibly a larger one fitted incase it is broken because it is undersized for the system.

Hi,
It is not a problem with the expansion vessel, that is quite accesible and has no sign of any leak. There is no sign of any leak anywhere inside the boiler case but there is water seeping out of the bottom of the case somewhere.

I will have to take the complete boiler housing out to find out how. It is a 4 year old Eurocal Ambassador Floor Mounted Simple Boiler (not the condensing Type).
The heat from the oil burner goes into a large box with baffles and this heat transfers to the water chamber through the back plate of the box. I suspect that it is the bottom of this water (chamber/radiator/exchanger ??? correct terminoligy)that is leaking.

Anybody have any experience of this type of leak?