How Does My Gas Furnace Work?

A gas furnace makes heat by burning gas inside sealed metal chambers, then passing your household air over the outside of those chambers and sending it through the ducts. The exhaust from burning goes out through a vent. Your indoor air and the flame never touch.

That happens in a set order, and safety checks run at almost every step. Models differ in the details, but the sequence below covers most gas furnaces found in Tri-Cities homes.

It starts with the thermostat

When the room drops below the temperature you have set, the thermostat sends a low-voltage signal to the furnace's control board. That board acts as the brain of the furnace, running each step in order and shutting things down if a step does not check out.

Draft and ignition

The first thing to move is usually the inducer fan, a small blower that pulls air through the furnace and pushes exhaust toward the vent. A pressure switch confirms the fan is really creating that draft. If it does not, the furnace stops right there, because lighting burners with no way to clear exhaust would be unsafe.

Next comes the ignitor. Most modern furnaces use a hot surface ignitor, a piece of ceramic that glows red-hot, though some use a spark. Once it is hot enough, the control board opens the gas valve.

Burners and the heat exchanger

Gas flows to the burners and lights in the combustion chamber. A flame sensor, a thin metal rod in the flame path, confirms the burners actually lit. If the sensor does not detect a flame, the gas valve closes again within seconds, which is why a dirty flame sensor is a common reason a furnace lights and then shuts off.

The flames heat the heat exchanger, a set of metal passages that carry the hot combustion gases. The metal wall between those gases and your home air is the most important barrier in the furnace.

The blower and the ducts

After a short delay, once the heat exchanger has warmed up, the main blower starts. It pulls house air in through the return ducts and filter, pushes it across the hot metal, and sends warm air out through the supply ducts to your rooms. The delay is deliberate, because it keeps the furnace from blowing cold air at you.

When the house reaches the setpoint, the gas valve closes, the burners go out, and the blower runs a bit longer to pull the last heat out of the exchanger. A high-limit switch also watches the temperature and will shut the burners off if the furnace overheats, often because of a clogged filter or blocked airflow.

Standard and high-efficiency models

Older and standard-efficiency furnaces vent hot exhaust through a metal flue, often into a chimney or a metal vent pipe. High-efficiency condensing furnaces add a second heat exchanger that pulls more heat out of the exhaust, cooling it until water condenses and drains away, and they vent through plastic pipe, usually out a side wall. That is why a condensing furnace has a drain line and a standard one generally does not.

Some furnaces fire at one fixed output, some at two stages, and some modulate. Multi-stage and modulating models run longer at lower output, which tends to even out temperatures and reduce noise, but the sequence is the same. From thermostat call to warm air usually takes only a minute or two.

When the sequence breaks

If any step fails, the control board stops the process and usually flashes an error code. Homeowners can safely check the filter and thermostat. Anything involving gas, wiring or the burners should go to a licensed technician. Air-Tech services all brands, so if your furnace will not complete its cycle, call (509) 392-2021 and we will help you track down the failing step.