How to Figure Out What is Wrong With Your Furnace

How a Furnace Works; How It Turns Itself On and Off

Often the best way to figure out what is wrong with something is to know how it is supposed to work. If you know the sequence of operations, you can pinpoint where the sequence is being disrupted. Your furnace is no exception to the rule.

When your furnace is called into action by the thermostat, there is a rhyme and reason to the procedure it follows in safely turning itself on. Next time your furnace doesn't respond to the call, you will be able to see where the problem lies, and you can either decide that it is within your skills and resources to repair it, or that you need a professional HVAC technician to handle the issue for you.

The following discussion can save you time and money, whether you fix the problem yourself, or end up showing the problem to your service technician.

NOTE: Furnaces burn natural gas, and use electricity, normally 120 volts; both of these present hazards. As always, be sure the power is shut off to your unit before you do any work on it. Also, never assume anything. If what you see in your home differs from what I describe here, do not guess. You could make the problem worse and ultimately more costly to repair.

A fairly standard 80% efficient furnace, easily identified by its metal chimney. Higher efficiency furnaces usually have PVC (plastic) chimneys.
A fairly standard 80% efficient furnace, easily identified by its metal chimney. Higher efficiency furnaces usually have PVC (plastic) chimneys.

Firing Order: Steps in the Operation of a Modern Forced-Air Furnace

When the thermostat asks for heat, the furnace jumps into action, going through the steps and components below. You can see many of these components in the video.

  1. The thermostat tells the furnace to come on.
  2. The inducer motor starts up.
  3. The pressure switch confirms proper venting of the chimney.
  4. The hot-surface ignition module (if you have one, and not a spark ignitor or a pilot) begins to glow.
  5. The gas valve opens and the gas is ignited by the ignition source.
  6. The flame sensor verifies that the gas has been lit.
  7. The high-limit switch reaches its set temperature.
  8. The blower motor comes on.
  9. The furnace runs until the thermostat is satisfied and tells it to shut down.
  10. The gas valve shuts.
  11. The high-limit switch reaches its low temperature setting.
  12. The fan shuts down.

A newer furnace may have even more bells and whistles; an old furnace may have just the minimum, a gas valve and a thermocouple (like an old-fashioned flame sensor). The thermocouple or flame sensor tests whether the pilot is lit, and stops the main gas valve from opening if it is not. Of course a broken thermocouple can also stop the gas valve from opening.

The Thermostat

The thermostat is where it all begins. The thermostat is really just a set of switches that open and close depending on the temperature, allowing power to flow to certain circuits in the heating and cooling system. Think of them like drawbridges that swing up and down. They are normally drawn up, but can be lowered to close the bridge and allow power to pass. When the temperature in your home drops, the thermostat drops its bridge, sending power to the furnace to let it know that heat is needed. As the room temperature rises again, the thermostat raises its bridge and shuts off the power. For air conditioning, the thermostat works the same way but in a mirror image, closing its bridge when the house gets too warm.

If the temperature in the house is lower than the temperature to which you set your thermostat, your thermostat may not be functioning as it should. You can test this by jumping (touching together) the red and white wires to your thermostat. If the furnace comes on, your thermostat is quite likely the problem.

What Do the Wires in the Thermostat Do?

Wire Color
What it Controls
Red is for power. This carries 24-volt power, supplied by the furnace. This power waits at the “bridge” until it is told where to go.
White is for heat. When the bridge (switch) between the red and white wires closes, the thermostat is calling for heat.
Yellow is for cooling. When the bridge (switch) between the red and yellow wires closes, the thermostat is calling for cooling.
Green is for the fan. When the bridge between red and green closes, only the fan runs. No heat or cooling is called for.
Blue is a rogue or wild card. It can be used to power a display or for advanced features. Usually, though, it is wrapped back into the wall and not used at all.
Other colors of wires in a home system may be for advanced functions or future use.

The Inducer Motor and Fan

The inducer motor and fan. The fan draws the "bad" exhaust gases out of the heat exchanger and pushes them up into the chimney.
The inducer motor and fan. The fan draws the "bad" exhaust gases out of the heat exchanger and pushes them up into the chimney.

The inducer motor and fan push exhaust up the chimney, getting rid of carbon monoxide (CO). Older systems relied solely on "natural draft" to vent these gases, but inducers have been a great addition to the furnace system. They extend the life of chimneys, and when used with pressure switches they help prevent CO poisoning.

The Pressure Switches

This unit has two pressure switches because it is a two-stage furnace. Inside the casing is a diaphragm, that when pressurized, completes the connection to the next component.
This unit has two pressure switches because it is a two-stage furnace. Inside the casing is a diaphragm, that when pressurized, completes the connection to the next component.

The function of the pressure switch is to verify that the inducer motor is actually pushing air up the chimney. If the chimney is blocked, for example by a bird's nest, the pressure switch shuts down the system in order to eliminate the risk of CO entering the home. Of course the pressure switch may also shut down if the motor is old and running too slow to satisfy the switch, or the hose from the fan to the switch is pinched or broken, or the switch itself is bad.

Again, a rather simple device in a complex system. The hot surface ignitor uses electrical power to heat a very fragile ceramic element, charcoal-like in appearance, to such a high temperature that natural gas or propane bursts into flame as it flows by. The hot surface ignitor is located at the orifice at the first burner port, in order to light the gas immediately as it's introduced to the port. This prevents a buildup of unburned gas in the unit.

Some furnaces employ what is called spark ignition. Instead of a red-hot surface, a spark ignitor creates a series of sparks to ignite the gas.

It's not important whether you have hot-surface or electronic ignition in your furnace; the important thing is that you are avoiding using a pilot light. Pilot lights waste gas; they burn gas constantly during the heating season, and some people let them burn year round. Want to save a few dollars? Make sure you shut off the pilot on your furnace when it's not being used for the season. Just be sure you know how to relight it next year before you do.

The hot surface ignitor is very fragile, and when working, very hot.
The hot surface ignitor is very fragile, and when working, very hot.

What type of ignition source do you have?

  • Hot Surface
  • Electronic
  • Standing Pilot
See results without voting

The Gas Valve

The gas valve is basically an electronically controlled gateway. When the unit calls for heat, and the circuit board confirms that the right conditions have been met, the unit passes energy to the valve, causing it to open and release gas to be ignited by the ignitor or pilot. When the heat cycle has run its course, the energy to the valve is cut, and the valve once again shuts, cutting off the gas supply to the burners.

The Flame Sensor

The flame sensor is very simple, yet causes a lot of problems for homeowners. Its only job is to verify that the gas has been lit, by sensing the heat. If there's no heat, it shuts off the gas, to avoid any dangerous buildup of gas in the unit. The flame sensor is simple to fix when it goes bad or just needs some TLC.

Video: Cleaning a Flame Sensor

The High-Limit Switch

This high-limit switch is not adjustable. Older switches had a dial that could be adjusted, which in reality was not a good idea.
This high-limit switch is not adjustable. Older switches had a dial that could be adjusted, which in reality was not a good idea.

The high-limit switch serves several purposes.

First, it keeps the furnace fan from turning on until it detects that a set temperature has been reached. Otherwise, the fan would come on prematurely, and blow cold air into the house each time the thermostat asks for heat.

Second, it also tells the blower how long it should continue to run before shutting off, once the thermostat has been satisfied. Otherwise, hot air remaining in the duct work and heat exchanger, after the furnace has cut off, would just go to waste.

Lastly, the limit switch will turn the unit off if the heat exchanger reaches a certain high temperature, to prevent dangerous overheating.

The Furnace Fan and Blower Motor

Here's a good exposed shot of what your blower assembly looks like outside of the unit.
Here's a good exposed shot of what your blower assembly looks like outside of the unit.

Ah, finally. Heat for us all. Now that all the sensors have been satisfied, and the limit switch tells the blower that the air is warm enough to send into the living space, it will start the air flowing.

Our furnace's blower will typically use 120 volts and push anywhere from 800 to 2000 cubic feet per minute (cfm). Most blowers require the use of a capacitor, and have a half-inch drive that hooks directly to the fan, unless you have a belt-driven motor.

Tid-Bit: In the case of a malfunction, or a cut in power to the unit, it is common for the blower to turn on and run for a period of time. This may be because there is too much heat, and a cool down is called for, or there may be a build up of "bad gases" which need to be spead out or dispersed by the fan.

The Roll-Out Switch

One other little safety gadget is the roll-out switch. This little guy can sense if flames "roll out" of their proper domain, and shut down the system as well.
One other little safety gadget is the roll-out switch. This little guy can sense if flames "roll out" of their proper domain, and shut down the system as well.

Save Time and Money By Knowing Your Furnace

I hope this helped you to understand your heating system a bit better. Here are some troubleshooting ideas involving the parts you just read about, and here are a few more parts tips.

  • In case of a malfunction, your furnace will usually try to restart three times. If on the third try it cannot reach successful ignition, it will go into a lockout mode for a specified amount of time, usually an hour. Resetting the power (turning it off and on) will usually reset the board and allow the unit to show you what was happening in case you missed it.
  • The little blinking light in the unit isn't a Christmas decoration or little disco in your furnace. This light blinks a sort of Morse Code that can lead you to the problem, or say "Hey, all is well here." Look inside and outside your unit. There is likely a code chart to tell you what the dot, dot, dash, dash sequence you're seeing means.
  • Some annoying noises your home furnace makes may be simple and cheap to quiet down.
  • Take care of your furnace and it will likely take care of you. But if you don't clean the unit and change the filter like you're supposed to, your furnace may break down, or break your bank.

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Comments 18 comments

Dee 3 weeks ago

I am completely ignorant with this, please be patient. When I do not have my thermoatat set the furnace sounda like it wants to turn on then shuts off. This is continuous. During this past summer I just turned off the furnace. Help.

sicxto cruz 4 months ago


will there be anything wrong with the furnace if while everything supposed to be off that part of the house seems too hot ? or would have been a plumbing issue?

Franc75 10 months ago

Hello, My Hot surface ignitor works intermittently while my thermostat is on the cold setting and the room temperature is higher than the set temperature. The Induction fan kicks on and then turns off, it does this a few times in a row and then stops for a while then it does it again. 2 AC guys have come check it and they replace the main control panel, and install a new thermostat and still doing it. I will appreciate your input on this since I'm at lost with this unit.

Cre8tor profile image

Cre8tor 20 months ago from Ohio Author

You need a technician. No offense but booms on gas ignition with smoke alarms going need someone to look at this ASAP.

raymond 20 months ago

My rheem furnace goes boom at ignition and the only stays lit for 45 seconds....then cuts off the fire alarm comes on also

Cre8tor profile image

Cre8tor 21 months ago from Ohio Author

Scott - I'm sorry for such a delay but it sounds like there is an issue at your control board. I've never seen an issue like yours but it seems the board is allowing voltage to go direct to the inducer without command so that's why my initial reaction is this. Thank you for reading.

Scott 21 months ago

The reason I noticed anything was wrong with my furnace was when I turned the system thermostat off, the inducer motor stayed on. My furnace light was flashing that the roll out switch was open. I replaced both roll out switches with new ones, but it is still flashing this message and having the problem that the inducer motor won't turn off even when the system is off. I really don't know what to try next. Any ideas?

Cre8tor profile image

Cre8tor 23 months ago from Ohio Author

ADam Geary - Electrical testing would have to be run to see exactly what is causing the issue. I see no reason it should be if the furnace isn't asking it to.

ADam Geary 24 months ago

why does my electric furnace heating element cycle on and off after furnace heats and shuts off

TeddydaBear 2 years ago

Thanks anyway for responding.

Cre8tor profile image

Cre8tor 2 years ago from Ohio Author

@TeddydaBear - I read it...just wanted to make sure. At any rate, I'm not real comfortable suggesting you add anything to the tank or alter the regulator. What you have isn't a real cut and dry situation and as much as I'd like to provide you an answer, I'm inclined to say this may be something you want to at least call someone out and let them offer advice after checking some things first hand. It may cost you a trip charge but then you sound like you can learn from them and take it from there. Might cost you a few extra bucks but you'd still save and be confident nothing you do will cause further problems. Sorry I haven't been of more help on this one. Thanks for reading and giving it a shot. Let me know what you find if possible.

TeddydaBear 2 years ago

While I appreciate your comments and help.

I don't feel as though you read my text thoroughly. I mentioned that the unit has been in place and running fine for about 13 years.

It is only after replacing the 500 gallon tank

with a 1000 gallon tank and new regulators

that this problem came up.

I visually can tell by opening the valve and trap

(Pipe plug) at the end of the gas line (just

before the line enters the furnace) that the

flow of gas is 1/2 of what it was when the

system was new. This has nothing to do with

the furnace. It has to do with either water in

the LP itself....or a malfunctioning regulator.

The furnace is starved for fuel.

It cannot achieve operating temperature if

it is only getting 1/2 the propane necessary

to operate.

I just wanted to know if there is a way to add

more ethanol to the tank as an extra drying

agent, or which regulator is causing the

restricted air flow (e. g. malfunctioning or

freezing up).

Of course it was properly set up for LP and

adjusted properly. I had an HVAC guy come

in and check my work as well as it needed to

pass all state and local codes.

Cre8tor profile image

Cre8tor 2 years ago from Ohio Author

@TeddydaBear - Being online I can only do/say so much. They stopped making these the same year you installed yours and first I'll ask, when near the unit, do you smell say a rotten egg like smell? Perhaps the exchanger is bad and it needs replaced. If clogged, particularly the secondary, the unit will act like this. I just replaced one doing very much the same thing and there's no quick fix.

If this has always been intermittently a did convert the unit to LP when installed right? A gas burning furnace is set up for NG usually unless specified out of the box and would need an LP kit and adjustments to the gas valve to use LP.

You should have a code light on the circuit board that can narrow this down. Read the code like morse code and there is a chart on the unit to help with potential problems. We would have more to go on then. I'll be watching here today so if I can assist with the code interpretation I will do my best.

TeddydaBear 2 years ago

I need help accurately diagnosing what is

wrong with my furnace!

I have a carrier 9200 weather maker on LPG.

During mild periods of weather (around 30 or

above) the furnace cycles normally.

Usually, when shuts off and goes in

lockdown/out mode.

I went from a 500 gallon tank to a 1000 gallon

tank and new regulators.

I installed the furnace, ductwork, tank, lines and

programmable thermostat myself, 13 years ago.

So I don't think it is workmanship.

I suspect two things. A malfunctioning or

freezing regulator, or water in the propane.

One thing I checked is visually inspected the

force the propane has while exiting the trap

below the shutoff valve to the furnace.

It is approximately 1/2 the force I remember

when the system was new. This leads me to

think the regulator either on the tank or second

one on the house is restricting the flow to the


If left to run the normal program from the will come on and run for a short

amount of time, but flame out.....and lock out

after a few minutes.

But if I override the program and put temperature

on "hold" and set it very high.....say 85 degrees?

The furnace will run. But will never achieve the

high temperature set. It will run constantly,

all day long and only achieve maybe 72 degrees.

This could be due to inefficient burning of LPG,

starving for fuel and unable to achieve the temp

called for by the thermostat.

Question is........because I now have a 1000

gallon tank, I do not use all the propane every

season........Does the drying agent (ethanol)

become ineffective over time? Thus allowing

moisture to accumulate in the tank? Or is it

simply a malfunctioning regulator? And which

one is malfunctioning? The one on the tank?

Or the second regulator on the house?

Can any "extra" preventative be added to the

tank to keep it dryer?

Help! It is really too cold to experiment now.

But I am burning off my propane too fast as the

furnace is running all the time now to keep the

house warm.

I am also big expenses are

too be avoided.


pattyejones 3 years ago

Hey Cre8tor, thanks for the reply.

After reading your post I went to the basement and checked to see if the fan was getting any juice by disconnecting the plug and using my tester - nada. When I reconnected the connector to the inducer motor it fired right up. I watched it go through all the steps you outlined.

So I turned the power off and traced the wires from the inducer motor back to each connector and cleaned them with a little sandpaper and it's working now. This weekend I plan on going back down there and cleaning up the rest of them. The basement I have is more like a pit in the ground and pretty damp and dusty.

This unit was here when I moved in 16 years ago, I'm hoping to keep it going a little while longer. Now, armed with a little more insight, thanks to your post, that might just happen.


Cre8tor profile image

Cre8tor 3 years ago from Ohio Author

Well, I like that your thinking in order of process but if the inducer isn't working, the furnace wouldn't/shouldn't fire at all. Without knowing what it's doing it's hard for me to guide you beyond that however I feel quite confident in saying the furnace won't run if the inducer isn't getting power and will shut down if there is a problem there during a cycle.

Thanks so much for reading and commenting.

pattyejones 3 years ago

Thanks for posting this. It's a tremendous resource!

I have been having intermittent issues with my furnace for a few years now. I poke around inside and it starts working for a while, then stops. I have cleaned the flame sensor, checked the ignitor, they seem good. But it seems, after reading your post, that my inducer motor and fan may be the real problem, or maybe it's not getting power all the time.

Since that is the first thing that's supposed to happen and it doesn't, that gives me a real place to start. I bypassed the thermostat but it didn't help so tells me the thermostat isn't the problem.

The fan seems free, not frozen, so maybe it's not getting power? We'll see.

Thanks again for taking the time to put this together!

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Marcy Goodfleisch 4 years ago from Planet Earth

Can I just order an entire book of your hubs? You are like an encyclopedia of terrific knowledge for maintaining our homes! Thank you for yet another helpful article - I know my furnaces are getting old, and I honestly don't know what to do about them! Or I didn't before reading this. Voted up and up.

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    Daniel Robbins (Cre8tor)318 Followers
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    Dan has been in the HVAC industry for 22 years with experience in aspects ranging from installation and service to sales and distribution.

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