confounding central heating pressure drop problem

[chippersproblem]

Here we go. I don't often get stumped by something but this is doing my head in. Her we go:
System:
Sealed non condensing combi system with seven radiators all on the same level in a bungalow. All pipe work runs through the loft area and drops to the radiators by way of spurs.
The boiler is a Grant oil fired external boiler that sits at ground level.

The problem:
There is an inconsistent pressure problem in the system which manifests itself by a complete pressure loss only when the system is off. I can charge the system and watch the pressure fall before my eyes until I turn on the circulation pump. The pressure drop then stops instantly. This happens regardless of whether the burner is on or not. The problem started after the pressure vessel had lost all pressure and so had probably blown out through the PRV .

Whats been done so far:
The pressure vessel has been checked and pressurised to 1 bar. The automatic air bleed valve has been checked and is working and not leaking. The PRV has had a plastic bottle under it and has not leaked any water at all.

The only thing I can think of is that there is a leak somewhere in the system but I can find no sign of water loss anywhere and the fact that there is no loss as soon as the pump is turned on leads me to think that it is not a leak.

I am hoping that someone has come across this before and found a solution before I start cutting holes in the plaster board to see if there are any leaks in the radiator spurs.

 

[Bunnyman]

If the pressures goin down there is a leak end of. Does this model have isolation valve on flow an return? If so pressure to 1.5 bar. Turn valves off and leave for a day. Check pressure if it's drooped it's the boiler . If not then open the valves if pressure drops its on system

 

[chippersproblem]

Thank you for the quick response Bunnyman. Unfortunately there are no isolation valves on the boiler or system.

I would also think automatically that it is a leak. If it is I am fairly sure it isn't the boiler as the heat exchanger is off the bottom of the boiler body so you can see all around under it and there is no water. There is also no damp in the combustion chamber (even when the boiler is stone cold). The thing that doesn't add up is that it doesn't do it all the time and I cannot see how turning on the pump would instantly stop a leak.

 

[andytw]

Are the expansion vessel and pump installed as per the Grant manual and within the boiler casing or are they on the system somewhere external to the boiler ?

andytw

 

[chippersproblem]

Hi andytw,
They are as per manual and sit within the boiler casing.

 

[The831Bunny]

You need to confirm the expansion vessel hasn't lost its charge again (they can take a few days to a month to lose air pressure if diaphragm faulty), or indeed check it hasn't been re pressurised to over 1 bar or you'll get very similar symptoms. Check it with the water pressure at 0. If it's a small 8L vessel 0.8 Bar air pressure may be enough. I'd check Grant's MI if I were you.

 

[chippersproblem]

Yep, I've checked the expansion tank pressure again and all is good. I have had the system sit at a constant 20psi for the last 6 hours with no drop. I'm beginning to wonder if it is the fact that a lot of the pipework sits well above any air bleed point and it is taking ages to purge itself out.

 

[oilhead]

check around any auto air vents you may have. These can be problematic. try closing them all to see what effect it has.

 

[chippersproblem]

Yep it was a leak. The original installer had placed a speedfit joint within the wall just 30cm from the boiler outlet and so over time the high temperature had caused the seal to break down. I'm amazed it even lasted the eight years the system has been in. The leak stopped when the pump was running due to the venturi effect of the water rushing through the joint.
First two metres from boiler now replaced with copper and all running fine. Thank you for all your help folks.