How a Gas Furnace Works
Name the parts of a gas furnace, follow its sequence of operation and explain what each safety device proves.
Table of contents
A gas furnace is a burner and a heat exchanger with a set of safety checks wrapped around them. Before it lights, the control board wants proof that the vent is working. After it lights, the board wants proof that there is a flame. Each step depends on the one before. Once you know the order, a furnace that will not run tells you where to look just by where it stops.
What You Will Be Able to Do
- Name the main parts of a gas furnace and what each one does.
- Recite the normal sequence of operation.
- Explain how a flame sensor proves flame.
- Describe the difference between 80% and condensing furnaces.
- Explain what each safety switch protects against.
The Main Parts
| Part | What it does |
|---|---|
| Control board | Runs the sequence and reads the safety switches |
| Draft inducer | A small fan that pulls combustion gases through the heat exchanger and pushes them out the vent |
| Pressure switch | Proves the inducer has created draft |
| Ignitor | Lights the gas, usually a hot surface ignitor |
| Gas valve | Opens to send gas to the burners |
| Burners | Mix gas and air and hold the flame |
| Flame sensor | Proves flame is present |
| Heat exchanger | Keeps combustion gases sealed away from house air while passing heat through its walls |
| Blower | Pushes house air across the heat exchanger and through the ducts |
| High-limit switch | Shuts off the burners if the furnace overheats |
| Rollout switch | Shuts off the burners if flame escapes the burner area |
The Sequence of Operation
Most modern furnaces follow this order. Timing varies by model.
- The thermostat calls for heat by sending 24 volts to W.
- The control board checks that the safeties are closed.
- The draft inducer starts.
- The pressure switch closes to prove draft.
- The hot surface ignitor heats until it glows, often for 15 to 60 seconds.
- The gas valve opens and the burners light.
- The flame sensor proves flame within a few seconds. If not, the gas valve closes and the board tries again, usually up to three times before it locks out.
- After a short delay, the blower starts, so it does not blow cold air.
- When the thermostat is satisfied, the gas valve closes.
- The inducer runs briefly to clear the heat exchanger, and the blower runs on to pull out leftover heat.
Knowing where in this list the furnace stops tells you what to check next.
How the Ignitor Works
Most furnaces today use hot surface ignition. A silicon carbide or silicon nitride element glows hot enough to light gas. These parts are brittle. Skin oil on a silicon carbide ignitor can create a weak spot that shortens its life, so do not touch the element with bare fingers. Over thousands of heating cycles they wear out and crack.
Older furnaces use a standing pilot, a small flame that burns all the time. A thermocouple proves the pilot is lit by making a tiny voltage that holds the gas valve open. Some furnaces use a spark to light a pilot.
How Flame Sensing Works
Flame conducts a small electric current. The board sends AC voltage to a metal rod in the flame. The flame lets current flow to the burner, which is grounded, and it passes much more easily in one direction. The board reads this as a small DC current, measured in microamps. This is called flame rectification.
A healthy signal is commonly 1 to 6 microamps. Many boards need at least about 1.0 microamp, and 2.0 or more is a comfortable reading. As a rule of thumb, clean a rod that reads below about 1 microamp, and replace one that stays below about 0.7 after cleaning. Check the manufacturer's value.
A thin layer of oxide on the rod weakens that signal. Then the burners light and shut off after a few seconds, because the board cannot see flame. A dirty flame sensor is the most common cause of a no-heat call.
80% and Condensing Furnaces
| Feature | 80% furnace | 90%+ condensing furnace |
|---|---|---|
| Heat exchangers | One | Two, including a secondary that condenses water from the flue gas |
| Flue gas | Hot and dry | Cool and wet |
| Vent | Metal, often Type B vent | PVC, CPVC or polypropylene, as the manual lists |
| Drain | None for flue gas | Condensate drain and trap |
Older atmospheric, or natural draft, furnaces had no inducer and relied on hot gas rising up the flue. They usually ran 60 to 70% AFUE. You will still find them.
A condensing furnace pulls so much heat from the flue gas that water vapor condenses out. That water is mildly acidic. A plugged drain or trap backs up into the inducer area and causes pressure switch faults. The vent also has to slope back toward the furnace so condensate drains out rather than pooling in the pipe.
The Safety Switches
Pressure switch: stays open until the inducer pulls enough draft. It protects against a blocked vent or a failed inducer.
High-limit switch: opens if the air around the heat exchanger gets too hot, usually from low airflow.
Rollout switch: opens if flame comes out of the burner area. A trip usually points to a cracked heat exchanger or a severely blocked flue, and it must not be simply reset.
Warning: Never jumper or bypass a safety switch to keep a furnace running. A safety that opens is reporting a real problem, and bypassing it can put carbon monoxide in the home or start a fire.
The Heat Exchanger and Carbon Monoxide
The heat exchanger separates burned gases from the air people breathe. If it cracks, carbon monoxide and other combustion gases can enter the home's air. Carbon monoxide (CO) is odorless and can kill. Any suspected crack means the furnace must be shut down until a qualified technician checks it.
Fuel and Gas Pressure
Natural gas and propane need different orifices and pressures. A typical natural gas furnace has a manifold pressure of about 3.5 in. w.c. at the gas valve outlet. Propane is usually about 10 to 11 in. w.c. The rating plate and manual list the correct values.
Adjusting gas pressure, repairing gas lines and firing a furnace are work for trained, qualified technicians.
Check Your Understanding
A furnace's inducer runs, the ignitor never glows and the board shows a pressure switch code. Where in the sequence is it stuck? At step 4, proving draft. The board will not go to ignition until the pressure switch closes.
A furnace lights, runs for about four seconds, then goes out. It tries twice more and locks out. What is the most likely cause? A dirty flame sensor, because the board cannot prove flame.
A rollout switch has opened. Is it safe to press reset and leave? No. Find the cause first. It often points to a cracked heat exchanger or a blocked flue.
Why does a 90% furnace have a drain? It condenses water vapor out of the flue gas, and that water has to go somewhere.
Key Takeaways
- The basic order is call, inducer, pressure switch, ignitor, gas valve, flame sensed, blower.
- Each safety proves a condition before the furnace moves on.
- A flame sensor uses a microamp signal, and a dirty one is the top cause of no-heat calls.
- 80% furnaces vent hot dry gas, and condensing furnaces vent cool wet gas with a drain.
- A cracked heat exchanger can put carbon monoxide in the home.
- Never bypass a safety.