The central heating system I installed in my house has worked fine for
5 years or so. Then, last sumer, I installed a larger radiator in one
of the rooms (a double rad, measuring 6' x 18" approx. Since then, the
expansion vessel in the boiler has stopped coping with the expansion
of the heated water. I think the new rad is the problem, beccause if I
isolate that rad, the boiler works ok, and water pressure is
sustained. If I un-isolate that rad, and fill the system to 1.25 bar,
it blows water out of the relief valve as oon as it gets hot, and then
the guage on the boiler soon reads 0 bar, and the boiler starts making
a sort of moaning noise, kinda like an incoming 18" shell from a
battleship, just before it hits you. (At least that's wht my vivid
imagination cooks up). As soon as you increase the pressure ot above 1
bar, the noise stops.

I guess the solution is (a) to switch back to a smaller rad, or (b) to
fit a larger expansion vessel. Do you agree?

Does anyone know if a larger expansion vessel can be fitted to a
Britony 80 combi boiler, and if so, how much one costs, and is it an
easy job to fit it? I'm a bit woried by the manual which says the
expansion vessel is part of the boiler's chassis.

Thank you,
Jake