This valve is design to relieve the excess pressure from the boiler
in case of a build up of extra pressure in the boiler. The extra
pressure can come from a number of different things. A few of the causes
may be a bad expansion tank, a bad fill valve or pressure reducing
valve, or an open bypass valve. The bypass is easy, just close it and
relieve the extra pressure. The others are not as easy.
Your expansion tank is simply a tank that allows water from the
system to go in and out according to the pressure in the system. This
tank contains about ? air which acts as a cushion to absorb the extra
volume of water that is created as the water is heated. Hotter water
takes up more space or volume, so it needs a place to go to when it is
heated. If the expansion tank is totally full of water then, it will
push water out of the relief valve. Changing your tank will fix the
problem.

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