Im back with another question. lol. So i successfully replaced my circulator. I drained about a bucket full of water from the boiler in order to install but Im sure there was more to drain. After installation i opened up all the shut off valves, turned on the boiler, and it started to repressurize from 0 to 25 psi until it reached the set temperature of the thermostat and slowly dropped back down to 12 psi (normal pressure).
My question is that i didnt add water back to the boiler manually (just opened up the valves that where closed), how does the system get back to 12 psi (its normal value)?