masterplumber.netthe logo

How does my furnace work?

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.
For example if you have more than one thermostat in your home you may have a separate electronic zone valve for each thermostat, in this case the thermostat wires will go to the electronic zone valve that controls the heat for that particular section of your home, I call that a zone. Other thermostats will be wired to other electronic zone valves that control other particular sections of your home or as I call them zones.

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.