Today I want to give you a basic Sequence Of Operation for a standard gas furnace aka 80% Efficiency furnaces. This will not apply to every single one out there but typically will cover about 90% of what you will find.
The Furnace is in standby mode waiting for a call for heat, the Thermostat has a call for heat, which closes a switch inside the thermostat and sends 24 Volts to the W (Heat) Terminal on the control board.
The Inducer draft/combustion blower motor then starts.
The Air flow switch will then close and complete a circuit through the safeties as long as everything is okay.
Depending on the furnace either the Hot Surface Igniter (HSI) or Spark Igniter will start. If you have a standing pilot then skip this part.
The Gas Valve will open and the burner will ignite, once the flame sensor senses flame it shuts off the ignition source, if it doesn’t sense a flame it shuts the gas valve off and then tries again usually 3 times before it goes into a lock out.
Next the indoor fan/blower motor will start, some are immediately and others will be delayed by a timer or thermostat.
Once the thermostat is satisfied it will open the switch cutting off the 24 Volt signal to the control board.
The Gas valve will shut off and then either at the same time or a with a delay will shut off the Inducer draft/combustion blower motor
At this point the indoor fan/blower motor may continue to run to get all the heat it can or it might shut off after a short delay.
The furnace is now in standby mode waiting for another call for heat.
That’s basically it, of course there are variations on these, but it should help anyone with understanding what’s going on inside that furnace of yours. I will of course do a series on high efficiency furnaces, but until then if you have any questions let me know.
Posts related to Gas Furnace Sequence Of Operation
On many occasions, my furnace will come on……inducer fan…and will fire, but when the main blower fan comes on, the flame goes out and it reignites seconds later while the main fan is on. It also short cycles once in awhile and it seems to be on a regular 5 minuet cycle, almost timed ! What can be causing this ? Once in full operation the furnace works fine.
Could be the temperature setting/limit for your fan coming on, it’s getting too hot and tripping on over temp. Once the fan is running then it will maintain the proper temperature in the furnace..
I would look into the fan cycle control…most of the new ones are a non adjustable bi-metal switch…some older furnaces have adjustable versions…and both frequently fail…hope that helps..sorry for the delay in responding.
Hi Rick, our gas furnace quit running the other morning. It was bitter cold. I didn’t have a clue on what I was looking for. Anyway, I called a tech and when I come home 3 hours later and turned it on it worked. I cancelled the tech. Someone at work said I could have flooded it. Any thoughts? Thx and enjoy your vids!
I don’t think you can flood a Natural Gas or Propane Furnace…, even with Propane being heavier than air I’ve never heard nor seen it, the most common problem is the air flow switch. If the line or switch plug up a little bit it could cause the furnace to not come on, sometimes just the vibration is enough to open it up for a little while, causing an intermittent tripping. Another issue could be as simple as a loose connection or a problem with the ignition sequence, if it’s a spark igniter they have intermittent startup issues, HSI – Hot Surface Igniter can have hairline cracks and look good and work off/on…I would check on the air switch. I’ll be posting a video soon on more troubleshooting. Hope that helps you out.
I've been an HVAC/R Mechanic working in the Seattle area for over 15 years, specializing in the commercial service industry.. I’m also a Licensed Electrician & Gas Piping Mechanic and have numerous other trade related certifications. I’ve instructed at local trade schools and now continue teaching through this site.
Today I want to give you a basic Sequence Of Operation for a standard gas furnace aka 80% Efficiency furnaces. This will not apply to every single one out there but typically will cover about 90% of what you will find.
4 comments
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Gerry Eisert
November 20, 2012 at 5:36 pm (UTC 0) Link to this comment
On many occasions, my furnace will come on……inducer fan…and will fire, but when the main blower fan comes on, the flame goes out and it reignites seconds later while the main fan is on. It also short cycles once in awhile and it seems to be on a regular 5 minuet cycle, almost timed ! What can be causing this ? Once in full operation the furnace works fine.
Rick
January 12, 2013 at 5:18 am (UTC 0) Link to this comment
Could be the temperature setting/limit for your fan coming on, it’s getting too hot and tripping on over temp. Once the fan is running then it will maintain the proper temperature in the furnace..
I would look into the fan cycle control…most of the new ones are a non adjustable bi-metal switch…some older furnaces have adjustable versions…and both frequently fail…hope that helps..sorry for the delay in responding.
Rick
January 26, 2013 at 1:34 am (UTC 0) Link to this comment
Hi Rick, our gas furnace quit running the other morning. It was bitter cold. I didn’t have a clue on what I was looking for. Anyway, I called a tech and when I come home 3 hours later and turned it on it worked. I cancelled the tech. Someone at work said I could have flooded it. Any thoughts? Thx and enjoy your vids!
Rick
January 26, 2013 at 4:51 pm (UTC 0) Link to this comment
I don’t think you can flood a Natural Gas or Propane Furnace…, even with Propane being heavier than air I’ve never heard nor seen it, the most common problem is the air flow switch. If the line or switch plug up a little bit it could cause the furnace to not come on, sometimes just the vibration is enough to open it up for a little while, causing an intermittent tripping. Another issue could be as simple as a loose connection or a problem with the ignition sequence, if it’s a spark igniter they have intermittent startup issues, HSI – Hot Surface Igniter can have hairline cracks and look good and work off/on…I would check on the air switch. I’ll be posting a video soon on more troubleshooting. Hope that helps you out.