Gas-fired hot water heat is an interesting overlapping set of semi-independent systems that work together to keep your house warm.
First is the burner. There is a thermostat (called the aquastat to distinguish it from the other kind of thermostat) inside the boiler and when the pump is running, if the water in there gets too cold to do any good, the burner starts.
Next is the pressure/level system. There’s a pressure valve on the intake that keeps the water level in the system high enough. There’s a Safety Relief Valve that releases water (onto the floor of the basement) if the system is malfunctioning somehow and the pressure gets too high. Overpressure valves fail over time by getting weaker until they open at lower and lower pressures. There is an expansion pressure tank that is above the boiler. It is there to moderate the pressure and collect the trapped air.
Then there is your actual thermostat. When a demand for heat comes, it starts the pump. Once the pump is on, the burner/aquastat runs, regulating the temperature of the water and thus of your radiators.
On my system, there are actually two thermostats and a pair of clever motorized valves. If thermostat A demands heat, valve A opens and passes the heat demand to the boiler. If thermostat B demands heat, valve B opens and sends on the demand signal. The valves are all wired together in a chain and there’s a clever electromechanical switch on each valve that passes the demand down the line. So if anyone wants heat the pump runs, and the same motor that opens the valve works the switchgear. When all the valves are closed, the demand for heat is satisfied and the pump shuts off.
And sitting in my office in a quiet house, I can hear each part running. If a thermostat demands heat I can hear the valve motor start cranking the valve over, and when it’s open I hear the pump start up (if I listen very carefully. Normally the pump is basically silent). And after a moment I can usually hear the little “thunk” as expansion makes the improperly-braced pipes in the newer section shift as they expand. And then in reverse when the room is warm enough: the valve cranks over, the pump stops, the pipes shrink and shift with another thunk. All the parts are of course attached to the pipes, which run through the whole house and carry the vibrations everywhere.
So for instance when the impeller started failing, I could hear that the pump was dying. And when one of the valves’ motors got weak, I knew because I could hear it cranking for way too long. And when the overpressure valve got old and weak, the BANG of it opening up and spewing water everywhere got me down into the basement in a hurry.
The fact that there are many parts makes it seem daunting. But the fact that there are many parts means that (usually) each individual part is easy to understand. When the pump’s bearings failed (Because I’d over-oiled it and softened the rubber parts) I replaced the pump. When the pump’s impeller failed I replaced the impeller. When the valve’s motor failed I detached it from the physical valve and replaced it at leisure.
That being said, there are plenty of things about this system that will kill you dead if you get them wrong. Also maybe burn down your house. Especially gas pipes, fittings, etc. Do not mess with these if you don’t know exactly what you are doing. I personally have only modified gas piping once, it was a minor change, and I was biting my nails the whole time. But I digress.
It’s not just the heating. Listen to your house and it will tell you what’s going on. I can hear when the washer is running. I can hear when the dryer stops. I can hear when the dishwasher changes stages in its cycle (except annoyingly the last stage, which is heated drying and makes no sound. So I can’t actually hear when it’s done.) And I can hear when something is dripping that shouldn’t be.
Listen to your house and it will tell you what’s wrong.