I am tasked with optimizing the heating system of a quite large building. It currently has two gas boilers working on four separate circuits (all joined together of course when they return to the boilers). I am doing this on a volunteer basis and at the moment I am at the basics of understanding how these systems exactly work.
My current main confusion relates to what exactly makes a boiler to turn on, burn gas and heat water. So far what I've understood is that 1) a thermostat can send a signal for it to turn on, 2) you can just turn it on manually and it will stay on if there's no thermostat control, 3) the temperature of the returning water matters too (I might be wrong on any of these of course). But I'm not sure how these factors are related, how they play together, etc.
E.g. when the thermostat is in a room where the set temp has been reached, can/will the boilers turn on if the returning water is cold from passing through other, less warm rooms? Also, what is the exact role of the temp difference of the outgoing and returning water in general?
I'd much appreciate some basic info on this, even just some pointers/links as to what to read to understand this better.