Over the many years of working as a residential plumbing and heating contractor with a plumbing license, I have worked on many boilers and furnaces, a boiler is a heating system that heats with hot water or steam and a furnace is a heating system that blows warm air, but most home owners call anything that heats a furnace, this is fine for this page.
What I would like to do with this page is give you an overview of how a boiler works and how a furnace works.
First I will start with a gas fired hot water boiler, and give you the basic sequence of events that happen when you turn the thermostat on. I will go from the thermostat to the next control all the way to the baseboard or radiators getting hot. This will also work as a little troubleshooter.
Please note: this is just to help you get an understanding of how a boiler works and not a guide to repair your boiler or furnace, I strongly suggest that you have a licensed professional service your boiler or furnace, as it can be very dangerous doing it yourself, sometimes there are special tools needed, and hidden dangers that only a real professional can find, please see this page
First I will start with the thermostat, this is the first place most home owners go when they don't have heat, surprisingly the thermostat is not often the cause of a no heat problem, of course all things fail sometimes, but more likely than not the thermostat is not the problem. I point this out because so many home owners run out and buy a new thermostat only to find it was not the problem. SEE THERMOSTATS
When a thermostat is turned up or the room cools down to a temperature below the setting of the thermostat a signal is sent by wires
(in most cases) to the first control of the heating system, the first control of the heating system may be a few different things in your boiler room,
depending on how your boiler was set up.
Then I will go to an oil fired hot water boiler and do the same, most of the steps are exactly the same except the controls are different.
Then I will cover gas and oil fired warm air furnaces
And lastly I will cover both gas and oil fired steam boilers.