Yes, but not impossible.
I've seen a new boiler that leaked. The makers replaced it when asked to advise, but it seemed there was a known defect that they wouldn't volunteer information about.
Expansion vessel replaced with new boiler?
Expansion vessel big enough?
An undersized or over pressurized one would do as described, but would chuck water out the pressure relief valve when the system is first heated after refilling.
I believe you said at one point that this is a very small system. If I'm correct then I'm beginning to think your problem could be with a wrongly set up pressure vessel. The sealed system installed in my last house and my sons house had a bladder in a cylinder that had to be pressurised to the correct PSI. I'm thinking that a very small system has little volume to pressurise so could quickly lose it with temperature changes such as the boiler shutting down.
Just think of a garden sprayer. If its too full there is too little air to pressurise, and it doesn't spray for long. Half fill it and pump to the same PSI and it sprays for much longer. This is because you can pressurise air to a far greater degree than liquid.
Its the same with pressurised borehole/well pumped water supplies. The pump forces water into a pressure vessel but its not the water in there that is compressed it air inside a bladder within the vessel . That bladder of air then expands forcing the stored water out through the system to the taps etc. That bladder has to be kept at the correct PSI for the system to work.
Perhaps you need a better/more suitable pressure vessel than the one built into the boiler, or it adjusting to a more suitable PSI to allow a larger volume of pressurised air.
Mike
I don't think that it's an expansion vessel issue. The old boiler had it's expansion vessel replaced as a part of the diagnosis, and I personally checked it's bladder pressure as 0.5 bar as specified.
I don't think water is escaping from the over pressure vent, however I picked up (maybe from a respondant to this thread) the idea of putting a balloon on the outlet as a positive test, and I'll certainly do that as in this rainy weather it's hard otherwise to be certain.
Andrew
Where was 0.5 bar specified? It seems low, 1 bar is more usual I think. What water pressure do you fill it to when you refill it?
0.5 bar is spec'd in the Worcester 20-25 installation manual for a system all on one floor with a smallish system volume, with system cold and fully depressurised - I also was surprised. Filling pressure is 1.2 bar. Don't know the spec on my replacement which is a Grant Vortex 21 combi.
Andrew
That may be your problem.
Half (?) the expansion vessel volume is used up when you fill it cold. The remaining volume is then inadequate to accommodate the expansion and the excess water volume goes out the PRV when the water gets hot.
I'd have thought 1 bar air charge (no water pressure) and 1 bar fill pressure.
Before the boiler was changed, and as part of the leak investigation, I charged the (recently replaced as a diagnostic aid) expansion cylinder to 1 bar. It was after that hadn't changed the symptom I googled for Worcesters spec and found it was supposed to be 0.5, as it had been :(
Andrew
I still think that is probably where your problem lies.
The simple solution is to check the expansion vessel pre-charge pressure ( drain boiler first), recharge the heating system and watch the pressure gau ge as it first heats up, with a container/balloon catching water from the P RV discharge.
If it is an expansion volume problem, the PG will rise to the PRV set pre ssure (usually 3 bar, but gauges are often inaccurate) and stay there whils t the boiler is hot. On cooling it will drop to 0. You should get a dischar ge from the PRV; the discharge pipe should have a continuous fall to the ou tlet.
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