When our current boiler was installed we were amazed at how quiet it was.
My buddy and I were used to hearing the drone of the old on each time it kicked on and now we hear nothing.
It is a wonderful feature but sometimes I wonder if it is one or not. There is one funny indicator that I appreciate telling people about. My Cat! Animals have a much more sensitive hearing ability and he can still hear the boiler kick on. She immediately runs to the floor vent to wait for the rush of air to come out. She almost seems impatient as he waits. There is consistently a delay between when the boiler turns on and when the air comes out. This is due to the high-limit switch. In simple terms this is part of the boiler and makes sure the air is heated before sending it through the ductwork. If your system did not have this it would blow frosty air until it was able to heat it up each time. This would be like blasting the heat in the motorcar while the engine is still cold. You would be angry during that time and it would make you colder than you were and it would take longer to heat the space or your car. If you ever notice that your boiler is blowing entirely frosty air prior to overheated air you may want to have this part checked to make sure it is still working officially. Because this part gets a lot of use it can wear out over time and should be part of your correct tune up routine too.