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.
If the reducing valve or fill valve is leaking by then you can also
get extra pressure in the system. A normal heating system will only run
10-20 psi pressure. Most well systems or city water is way higher than
that. If the pressure in your boiler gets over 30 psi then it will begin
to relieve the extra pressure by dripping water.

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