This is pretty spooky, having posted a very similar query yesterday on behalf of a friend - however, this one relates to my own CH system which I've been working on today.
I flushed it all out and added fresh Sentinel, and set about checking the (separate) expansion vessel before repressurising the system, and I think it's failed. I've checked Ed's FAQ, but am I right?
This is a sealed system with unvented HW tank. Unfortunately having mucked about with the valve on top I'm not entirely sure what the status quo was; the boiler pressure gauge has been fluctuating, though. I'm finding it incredibly hard work to pressurise the expansion vessel by force air in via my car tyre pump, and only a very few strokes of the pump seems to pressurise the 19-litre vessel to about 2.5 bar, implying to me that the available volume within is tiny. Plus when I pump I can hear gurgling noises within the vessel, and some HW is forced out down the tundish. (During this time the water side is de-pressurised, and there's an end-plug removed from the bathroom towel rail). No water comes out of the Schrader valve on the vessel however.
So assuming I need to replace the vessel, it seems to be simply a case of unscrewing it (it's connected via a taper BSP screw fitting) and bunging a new one on? Correct? Are all expansion vessels the same - ie I just need to get one of matching volume?
I'm confused about pressures. Everywhere I read that expansion vessels should be charged to about 0.7 bar, with the wet side depressurised. Indeed, the manual for the boiler (Vaillant EcoMax VU226) says it should be between 0.7 and 0.9 bar; however that refers to an internal vessel - does that make a difference? Mine was fitted with an external vessel. However, there's a label on my HW tank (Vaillant) which states "Expansion vessel charge pressure: 4 bar" and "Expansion valve setting pressure: 6 bar" Furthermore there's a label on the side of the expansion vessel itself Varma) which seems to indicate 3.5 bar is appropriate - see picture here:
Any help welcomed. Thanks David