hi there I have a pressure prob with a sealed gfch system.
The pressure shoots up and down - according to the FAQ, this prob means that the expansion chamber has failed, but when I check the chamber, there is no water coming out of the valve - only air. Air pressure is approx 10/11 PSI. I keep topping up water, but there is no apparent leak...
Any ideas?
It will vary - describe the range of "shooting up and down"
I fill the boiler to approx 1.25 bar. Once the heat comes on, the pressure shoots up to 3 bar. Once the heat comes off it settles back. While the boiler continues to operate, the pressure will drop slowly to 0 and then I have to top-up again.
All very painful when i have to get up at 5am to top up...
thanks for your quick response
Is is a combi boiler or do you have a separate DHW tank?
If the latter there could be a leak of primary water from the tank coil into your hot water system.
Its a combi, Potterton Puma 80 boiler.
It could be that there's not *enough* air in your expansion vessel. If it has a diaphragm you won't see water coming out the air top up valve unless the diaphragm has failed. If there is too little air then the expansion vessel might not have enough capacity to absorb the water expansion in the system.
Tim
Good point.
Release the system pressure and check the "precharge" at the valve on the pressure vessel.
IIRC this should measure at about 1-1.5 bar when there is no system pressure. If it's too low pump it up with a foot pump.
That's a bit high unless you have a 4 storey or more house. 0.7-1 bar is more normal, but /definitely/ with the system pressure at zero, otherwise you are just measuring the system pressure.
Blocked connection pipe between the expansion vessel and the system. Ideal Classic combis were a bugger for blocking where the connection flex tees off the boiler internal pipes. Many others with small bore connections blocked within the small bore section. HTH
John
Under what conditions did you measure the air pressure? It needs to be done when the water system is *not* pressurised.
The water is almost certainly being lost through the pressure relief valve, which opens at 3bar. You need to find where this discharges to the outside world, and hang a container under the pipe to see how much water you collect.
As others have said, the problem is probably just that there's insufficient charge pressure (air) in the expansion vessel. This needs to be pumped up to about 10 PSI (0.7 bar) when there's no pressure in the water system, and then you need to use the filling loop to pressurise the water system to about 1 bar (cold).
Ok thanks for all that.
From what I can tell having a closer look, the auto air vent is unwell. The little red knob was tight, when I loosen it, it started leakin.... in the absence of anything obvious wrong with the expansion vessel, I will take a gander at that first. Even when sealed tight, could this be the cause of the problm?
No If it has a red cap on the auto air vent its probable that it also has a fibre washer. the idea with those was to let the air out until the float inside shut the outlet then close the cap. Subsequent accumulation of air would allow the fibre to dry out and air to escape then if water followed the fibre would expand and shut the escape to liquid. The fact that water was behind the cap does not suggest a system problem. There were even some auto air outlets that only used fibre washers and no float valve. Your problem is either as others have said with your expansion vessel precharge or as I said the connection to the expansion vessel is blocked.
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