No pressure - Worcester Bosch combi boiler

15 Mar 2004
Reaction score
United Kingdom
My combi boiler keeps losing pressure. On sunday it dropped to zero, but prior to that it kept dropping despite topping up to 1 bar with the filling loop, and then when heating was on it went right up to 2.5 or even 3 bar. So on sunday engineer came out and said expansion tank had no air in it, and pumped it up so gauge showed 1 bar, then heating went on, pressure rose to 1.5-2 bar ish and it was fine, engineer went on his way.

Then last night the pressure starting dropping again, got up this morning and it was on zero again - argh!!!!!!!!!!!!!!!!!! :evil:

Engineer coming out again tomorrow, but it is showing the same symptom ie top it up and then put heating on = high pressure (he said yesterday that expansuion tank won't work with no air in it hence high pressure on gauge).

So, what causes the expansion tank to lose the air? I'm really worried it is gonna need parts and with Christmas round the corner and a house full of guests I'm getting more than a bit worried.

Also, what would happen if the heating came on when pressure gauge showed zero?
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get the engineer to trap some air in one of the radiators this will act as a
temporary expansion chamber .
expansion vessel membrane may have perished\split.
If so the air will gradually escape once recharged to 1bar.
So it may need a new expansion vessel :D .
install a new one on the system rather than replace the boilers vessel. Always easy to change or re-charge if it fails.

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engineer been - it needs a new expansion vessel, which he is coming to fit tomorrow. In the mean time the boiler is limping along so we have some heat thankfully.

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