What could be wrong with our boiler?

shamac

It's a vailant, 5 years old. Whenever there is a sudden change in temperature, such as a frost, the pressure drops and the boiler cuts out. We have a refill valve on it, you top the water up and it relights fine. We thought we must have a leak, but this only happens intermittently. Any ideas?

Mr_Ted

Expansion vessel air cushion top up may be required?

southcoastrgi

if you can't find a leak on the rads or pipework it's prob the pressure relief valve, locate the 15mm copper pipe outside (prob comes out the wall then bends back on it's self), get a clear freezer bag & tape it around the pipe (so rain won't get in) & see if you get water in it, if you do it's either a faulty valve or some muck on the valve seating.

gas4you

I would go with both of the above.

Expansion vessel flat, very common problem on Vaillant, especially if it is an Ecotec Plus, then change out the PRV, as this will probably have some crud on the valve seat after operating so many times.

I find all the Ecotec Plus boilers I go to need the exp vessel re-charged at least every 2 years.