It can be very frustrating dealing with a boiler that is not heating your space as expected, in most cases, such a boiler has a dire issue that needs to be rectified, however more often than not, it is not something you can fix on your own & may require the services of an Heating & A/C professional or Heating & A/C business. It is even more complicated if the boiler is a gas heater; To fix it, one has to open the registers. This will be the first thing the Heating & A/C serviceman will check, especially if you have kids. They are likely to close most of them as they play, leading to a buildup of heat in the heater. This, in turn, causes the boiler to overheat & trigger a sudden shutoff, sometimes even unexpectedly, sadly, closed vents in registers can cause more problems than just overheating. They are likely to trigger system breakdown & worsen existing leaks since they tend to force the boiler to work harder than it should. The Heating & A/C professional will also check the boiler settings & wash the thermostat, but a slight deviation from the standard setting may direct the boiler to act differently when it shouldn’t, then be careful not to miss it as the thermostat accumulates dust. Always wash the thermostat & ask the Heating & A/C professional in charge of the occasional tune-ups to do the same when they come for the session. It is also essential to check the boiler power; Whenever the circuit breaker is off, the boiler tends to shut down & not heat as expected. If you are standard with such electric works, please leave it to the professional .