Odd boiler behaviour help please

[TMC]

I have an old Vaillant combi boiler

the model is a vcw-sine 18 t3 w

Apart from replacing a leaking automatic air vent 6 years ago the boiler has given trouble free service for many years

It is on the jobs list to be replaced but has not yet got near to the top

For the last 2 winters the following has occurred

When the boiler runs on central heating the water pressure rises from 0.8 bar at 20C to almost 3 bar at 60C and spits some drips out of the relief valve

as soon as the flame cuts off the pressure drops over maybe 30 seconds to the normal working pressure of about 1.2 bar

When the flames came up again the pressure started to rise again. I switched the boiler off at this point but accept that I may need to do a longer test to see if the rise and fall continues

The system does not need regular topping up and I can find no leaks and all af the radiators are air free

My initial thought was that the expansion vessel had packed up but had that been the case I would have thought that the water expansion in the whole system would have caused a greater water loss and little or no drop in pressure when the burner was idle because all of the water in the system was hot

My next thought was that the air pressure in the dry side of the expansion tank had dropped such that when the system was full there was water in the wet side at resting pressure and some expansion but less that before the problem

The air valve on the expansion tank is a b****r to get at due partly to the location of the boiler but should show 0.7 to 0.9 bar when there is zero pressure in the heating circuit.

If the pressure is too low would this give the symptoms described above?

If so I will strip out what I need to in order to check it

any help appreciated

Regards

[Roger Mills]

It sounds to me as if there's too little charge pressure in the expansion vessel. If that is the case, when you pressurise the wet system it uses up too much of the expansion volume achieving the static pressure. When the system subsequently gets hot, there is insufficient expansion volume left to prevent the pressure rising more than it should. It is seemingly not rising that much that it operates the pressure relief valve and spills water - but is still not good. My guess is that there's *some* charge pressure, but not enough - maybe 0.3 or

0.4 bar rather than 0.7 It sounds as if the Schrader valve has been leaking a bit, rather than the vessel being totally shot and full of water on the air side.

[TMC]

Thanks for this that confirms my thinking

thats Saturday taken care of then

regards

[TMC]

A result there was almost no presure in the dry side but no water either all pumped up now and looking good