Let’s cover the components and system operation at the same time. On a call for heat at your thermostat an igniter inside your furnace begins to glow. At the same time a small combustion vent blower begins to pull air through your furnace heat exchanger. After a few seconds gas enters your furnace through the gas valve, mixes with the fresh air from the combustion vent blower, hits the igniter and burns inside the heat exchanger. The blower then starts pushing air past the heat exchanger forcing warm air into your home. The exhaust gases vent up through the flue pipe and into the atmosphere. When the thermostat says the house is warm enough it shuts down the furnace. It’s just that simple. The tricky part? Burning the gas efficiently, to keep your gas bills as low as possible and warming your home comfortably, to avoid drafts and other cycling issues.