How A Gas Furnace Works (A Quick Tour)

Glowing Hot Surface Igniter

It’s the old gig, we’ve all heard it……..the hip bones connected to the back bone and so on. Gas furnaces are no different. The gas lines connected to the gas valve, the gas valves connected to the burner and so on.

All parts to gas furnace are essential to a safe operating appliance. Each manufacturer packages pretty much the same parts together and likes to think their warehouse full of furnaces are the best for your home.

The gas furnace parts in this list are connected either physically or electrically in one way or another. Each has a specific purpose in protecting the life and property of the occupants of the home or business. Here they are in sort of a sequence of operation.

  1. Inducer motor
  2. Pressure switch
  3. Igniter or pilot light
  4. Gas valve
  5. Burners
  6. Flame sensor
  7. Limits
  8. Heat exchanger
  9. Recoup coil (90%+ furnaces)
  10. Control board
  11. Blower or fan

Each manufacturer packages pretty much the same parts together and likes to think they have manufactured the best furnace on the market.

Many of the same parts exist in either an 80% or 90%+ furnaces. The essential difference between these two is the 90%+ furnaces have a recuperative coil that fits into the exhaust system. The recuperative coil recovers more of the heat from the exhaust or flue into the air stream that heats the house.

What Is The Inducer Motor Assembly?

The first thing that starts in the sequence of operation and usually the first thing you hear when the furnace starts, is the inducer motor.

Depending on your application, meaning upflow, downflow or horizontal, this inducer assembly can be located in a different place in the furnace cabinet. You’ll notice it right next to the flue pipe with a small fan that cools the inducer motor just like the radiator fan in your car cools the engine.

Inducer motor assembly and flue pipe

Some models of inducer motors have no fan to cool the motor or it could be internal of the motor housing just to keep it confusing.

The purpose of the inducer is to assist the flue gases though the heat exchanger. When the flue gases reach the flue pipe, natural draft exists to take these gases into the atmosphere.

You may have a gas furnace with no inducer which means it’s a natural draft furnace and less efficient.

Just a side note: these flue gases are mostly carbon dioxide which is in the air we breath every day. By the process of photosynthesis, plants absorb carbon dioxide.

Not to be confused with carbon monoxide which is called the silent killer. You can learn about carbon monoxide here.

What Is The Purpose Of A Pressure Switch On A Gas Furnace?

Suppose the inducer motor fails to start or a bird goes down the flue and gets stuck in the inducers blower wheel (I’ve removed several). Although this happens mostly in the newer high efficiency gas furnaces that vent in PVC pipe.

The pressure switch senses that their is a loss of pressure in the exhaust system and in turn will not allow the gas valve to open. You may have more than one pressure switch sensing different ports or locations within the furnace. Manufacturers are looking for safety and dependability to build a better product.

Understanding The Types Of Gas Furnace Igniters

The earliest and most commonly referred to type of ignition is a pilot light. A constant flame that burned onto a thermocouple or pilot generator that produced a very small voltage.

Notice the pilot light to the left of this radiant heater

This pilot assembly was the main safety for vintage gas appliances and furnaces.

Radiant heater in my shop

If the pilot went out or the thermocouple fails your gas valve will not open. It has been a very dependable safety for many years.

Just a side note. These Enerco Radiant heaters are an easy install with no venting or power wiring required and their are some very useful portable units available at Just mount it on the wall and plumb in the gas and you got heat.

The next style of igniter to dominate the market was the spark ignition. Just as the name connotes a spark was generated upon a call for heat and a port would open to deliver gas to a pilot orifice.

The pilot would light and burn on a thermocouple or flame sensor which would obviously sense the flame and allow the gas valve to open.

Next entered the hot surface igniter or HSI. For the first few years these igniters were introduced it seemed they were doomed. Failure was there middle name.

They have, however become quite dependable. One of the weaknesses is they are very brittle and break very easily, mostly upon installation if you get just a little clumsy while trying to get it into place.

Hot surface igniters are also sensitive to the touch. Instructions explain to keep fingers off the tip of the igniter as the oil on our fingers can cause early failure.

My favorite, because there are so many types of HSI’s, is a universal replacement made by Honeywell called Glowfly. It’s a no brainer for service techs to carry on board the truck to keep from having all those various other types of igniters.

Replaces over 110 igniter models

Glowfly comes with a cross reference guide so you can compare your existing igniter and find the correct bracket to match.

You can read more about Honeywell’s Glowfly here and discover the availability at Keeping one of these around for a spare is smart, especially if your tech comes to replace it and he just used his last HSI on his last call.

What Is A Gas Valve On A Furnace?

If a circuit board is the brain then the gas valve would be the heart of a gas furnace.

The above image shows a new replacement gas valve. The old one refused to open when 24 volt power was applied. Gas valves just stick sometimes. Gas valves are normally a very dependable part of a gas furnace.

If you can see the black iron pipe leaving the right side of the valve it loops down, around and becomes the manifold that feeds the burners with natural gas or propane fuel.

Some Types Of Burners For A Gas Furnace

Directly on the manifold mentioned above you may notice a small brass fitting. This fitting has a small hole which is the orifice that feeds the proper amount of fuel into each burner.

The size of the orifice is different for propane than it is for natural gas. That’s the reason for conversion kits. Some of these kits also include parts for the gas valve.

Burner types vary from heavy cast iron with holes or slots. This type of burner was prone to corrosion and needs constant maintenance to burn clean.

The ribbon type burner is no exception to corrosion. They need to be removed occasionally and cleaned. Ribbon burners are long and are inserted into the heat exchanger.

Most furnaces today are using the inshot burner. These burners are short stubby things that are mounted just outside the heat exchanger. They are less likely to corrode but have a definite noise problem. Inshot burners sound like a jet engine when burning.

The high efficiency 90%+ furnaces are much less noisy. Manufacturers have done some things to reduce the noise on the 80% furnaces.

Where Is My Gas Furnace Flame Sensor, What Does It Look Like?

Mounted near the burners and the igniter, the flame sensor will have a single wire leading connected coming from the control board. Yours will be very similar to the one on the left. Sometimes the tip will be bent a an angle. Look for the single wire leading to the burner area and it will usually be mounted by one quarter inch screw.

Flame Sensor

A wise tip would be to remove it and use a bit of emery cloth and clean the metal tip. Because of where flame sensors are located, in the flame, they do become corroded and cause false starts of the furnace. So if your burners light and shut off repeatedly, it would be a good idea to clean that flame sensor.

Where Is The Limit Switch On A Gas Furnace?

Just like a lot of other parts in products one type does not fit all. In gas furnaces limit switches have different purposes and a furnace could have more than just a couple types of limit switches.

Three Types Of Gas Furnace Limits Serve Three Different Purposes

Limits typically have two wires connected. The large black limit is a high limit switch. It’s purpose and specific location allow it to shut down the furnace if something blocks the airflow like a dirty filter or failed fan motor.

The other two limits are flame roll out limiting devices. Say you had a cracked heat exchanger, the force of air from the blower fan could blow the flame back out of the heat exchanger and the limits would trip and shut off the gas flow to the burner.

How Does A Heat Exchanger In A Gas Furnace Work?

Picture in your mind a wood stove with the door shut. That’s a heat exchanger. Some type of fire on the inside and a barrier to contain the flame.

The heat exchanger in your gas furnace is a chamber that the gas flame burns in and the air from your blower moves the air around the outside of the chamber.

If your heat exchanger is cracked then flue gases can enter the house or the force of the fan can blow the flame out of the heat exchanger and cause the flame roll-out safety switch to open and shut off the flow of gas to the burner.

Cracked Heat Exchanger

Be sure your home is equipped with a current carbon monoxide detector. The heat exchanger in your furnace is a major player in keeping dangerous gases like CO out of your home.

What Is An Integrated Furnace Control Or IFC?

This is the brain. Again several manufacturers provide a multitude of control boards. You can find yours closer to the blower fan and away from the heat of the burner.

IFC’s of today have diagnostics built right into them. Look for a light on the board and then locate the legend somewhere inside the furnace.

Diagnostic Light

The legend will give faults for different blinking sequences of the light. Turning off the power will likely reset the board and then you have to go through the cycle again for a light to blink out the code.

So look for a code first if you think you have a problem.

These controls also control other features of the furnace such as the inducer motor and the blower fan.

Where Is The Gas Furnace Blower Or Fan Located?

Looking at your furnace with the doors off makes it much easier to locate the fan compartment. The opposite end from the burners and gas valve will have an interior panel that will need to be removed to access the blower.

Downflow furnaces are slightly more difficult to get at the fan. If yours is a downflow you will notice the flue goes straight up through the exterior of the fan compartment and the panels are right behind the flue pipe.

Because of the harder access these blowers in downflow furnaces are more neglected. The fan motor capacitors are usually failing due to time. I remember when capacitors were of a lot higher integrity and seldom failed as compared to the caps on the market and in furnaces today.

Some Final Thoughts About Your Gas Furnace

I tried to write about gas furnaces in a generic way and because of that you may have noticed some things unmentioned, like thermostats, transformers, and filters.

You can learn quite a bit more about gas furnaces by going to my home page and entering something like filters or gas pipe installation or any other questions in the search bar.

Chad Peterson

Chad Peterson is a veteran of the HVAC industry since 1977. "I like to explain heating and air conditioning problems in a way the average home owner can understand. "

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