FAQs #2 about why a furnace turns on after call for heat has ended:

These questions & answers help diagnose & fix a heating furnace blower fan that cycles on and off after the call for heat has stopped.

For safety this furnace fan cycling problem should be diagnosed and repaired promptly. This article is part of a series of heating furnace blower fan diagnosis & repair procedures given free at InspectApedia.com. Content suggestions are always welcomed.

Questions & Answers: Why Does a Blower Fan Keep Cycling On/Off After Call for Heat Has Stopped

These questions & answers about furnace blower fans that cycle (or should but don't) after the thermostat is satisfied were posted originally at FURNACE FAN CYCLES AFTER HEAT - be sure to see that diagnosis and repair article.

Image: Coleman Intertherm 140 L140-30F Limit Switch Electric Furnace 7945-3281 7945-328 avaliable from hvacpartsstore.com and from many other vendors.

[Click to enlarge any image]

On 2018-12-11 by (mod) - gas furnace purge fan repair

Purge

As we don't know the brand and model of your gas furnace I cannot give a detailed guess at your heating problem and have to offer a general reply.

At the end of a "heat on" cycle a furnace continues to run for a short time to remove the remaining heat in the furnace heat exchanger - otherwise the steel could overheat when the fan stops and the heat exchanger could be cracked or damaged, rendering the heater unsafe.

That process is described at FURNACE FAN CYCLES AFTER HEAT

There are other types of "purge fans" used in heating equipment, in part because the phrase "purge fan" may be used incorrectly or inconsistently.

For example some direct vent furnaces require a fan to force combustion products out of the exhaust flue, and other furnaces or heaters may use a draft inducer fan to assure adequate draft in a tall or cold chimney.

Sorry but I don't know enough about what your system is to say more.

I do not think a novice should try to fix the exhaust venting fans, blowers, or equipment on a furnace as indeed a mistake could risk a fire or even fatal carbon monoxide poisoning.

Attach a photo (one per comment) of your furnace, brand, model, data tag, and of the "purge fan" under discussion and we may be able to comment further.

Let me know what the heating service tech tells you - what we learn may help keep others safer.

On 2018-12-11 by "Purge fan seized up"

My heater all.of a sudden stopped heating. I called the gas company and they sent some one to look into it. He spent some time diagnosing and figured out that the "purge fan" isn't spinning and it should be, and advised that I call an appliance repair person.

I'm waiting for a call back, but, what is involved in identifying cause and repair options. Do you have a sense of how likely is this fix to kill me financially? Is it something I can use YouTube to help me fix myself, or if this something a novice shouldn't dive into?

On 2018-12-11 by (mod) - furnance is doing warm and cold cycles constantly

Summer

What you describe is not normal and not proper operation of a heating furnace.

It is possible that the problem is as simple as a dirty air filter or dirty blower fan in the air handler.

Check to see if an air filter is improperly installed or is of a type that obstructs the air flow more than the furnace can handle.

On 2018-11-28 by Summer

My furnance is doing warm and cold cycles constantly. I told my landlord this and he keeps telling me it's normal. Before he supposedly fixed the problem the furnace ran fine.

Almost going on 2 weeks without proper heat. Can someone please tell me what could be wrong

On 2018-11-01 by (mod) - Fan won't shut off

Long

Please see the diagnostics given at FURNACE FAN WONT STOP

On 2018-11-01 by Long

Fan won't shut off

On 2018-10-21 by (mod) - Heater comes on and blow for about 30 seconds then shut off.

Bernard

With the apology that nobody with any sense will pretend that she can diagnose and advise repair about your heating system from a brief text message, what your case sounds like is an oil or gas fueled furnace whose burner is not operating properly and that is being shut down by a safety control intended to protect you from fire, gas explosion, oil burner puffback, or fatal carbon monoxide poisoning.

Watch out: It would be best to leave the system off and call for repair by a heating service tech who has expertise on your particluar type of equipment and its fuel type and source.

On 2018-10-21 by Bernard

Heater comes on and blow for about 30 seconds then shut off. I switch on the switch on the furnace to off then on. The furnace comes on for a short period then cuts off.

On 2018-02-20 by (mod) -

Thank you for the nice comment, Tony. We work hard on this information and are gratified when you find it useful. Questions and content suggestions are welcomed too.

On 2018-02-19 6 by Tony

Thank you for sharing your knowledge. Very helpful article.

On 2018-01-22 by Julie

My heater will work fine sometimes but other times the gas heater will keep turning on and off but no heat will come out. What do you suggest I do.

On 2018-01-21 by Steve

Thank for your help.

On 2018-01-20 by (mod) - And a electric furnace is blowing house air across one more heating elements.

You're right, I should have been more clear. I was speaking to generally. And a electric furnace is blowing house air across one more heating elements.

Another parts of your question were dressed in my previous reply.

It's quite possible that your technician is very smart and Savvy but simply is not a good communicator.

On 2018-01-20 by Steve

Thank you for the comments and advice. I have researched further because I am still confused.

My findings--An electric furnace does not have a heat exchanger (true or false).

The wall thermostat has nothing to do with the fan blowing for a short time after the furnace stops.

Even though that is what I was told by the HVAC tech that replaced the strip heater.(true or false).

Possibly the Limit Switch(which does control the fan, to run a short while after the furnace stops) is not functioning properly. (true or false). If the Limit Switch is possibly the issue, how can it be tested? ------

Additional info--

1.The furnace is functioning totally correctly otherwise.

2.The reason I am so confused is-- Everything that find online contradicts what the HVAC tech told me.

He seemed to be knowledgable at the time. Thanks again, Steve

On 2018-01-19 by (mod) - Coleman Intertherm 140 L140-30F Limit Switch Electric Furnace

Here is an image of the Coleman Intertherm 140 L140-30F Limit Switch Electric Furnace 7945-3281 7945-328

On 2018-01-19 by (mod) -

Steve

I agree that a furnace heat exchanger might be damaged by overheating if a blower fan stops while the heat exchanger is still quite hot.

On an electric furnace we don't have the same safety concern as for a gas or oil fired heater as there's not a risk of blowing combustion gases or carbon monoxide into the home. But still a heat exchanger might be damaged and leak.

The strip heater itself, in an electric furnace, looks often like the example I include below from energyvanguard.com

You will see that the strip heater is an electrical component - a heater - that is the actual heat source. A

The wall thermostat, as far as I know, has absolutely nothing to do with the blower continuing to run past the time that the actual heater stops.

Rather, that run-on of the blower is controlled by a fan limit switch on the heater. That control does the following:

1. It does not turn the blower fan ON until the heat exchanger is warm-enough; otherwise the heater would blow cold air on occupants

2. It turns OFF the whole heating system if temperatures become abnormally high and thus unsafe

3. It keeps the blower fan running for a brief interval (usually less than a minute) AFTER the wall thermostat is satisfied (room temperature has reached the set temperature on the thermostat) in order to purge or clear the remaining heat from the heat exchanger and in order to prevent overheating therein.

You can search InspectApedia.com for FAN LIMIT SWITCH to read more about those devices (using oil and gas furnaces as examples).

The limit switch on an electric furnace will be smaller and simpler - a simple temperature sensor.

My comments about heat exchanger damage also pertain to the strip heater itself and the risk of overheat damage. I *think* that the strip heater is designed with the assumption that air is moving over the heater when it's quite hot. I'm researching that further.

I include an example below for a Coleman Intertherm 140 L140-30F Limit Switch Electric Furnace 7945-3281 7945-328 (from HVACpartsstore.com )

On 2018-01-19 08:03:55.455697 by Steve

Please bear with me...In my manufactured home I have an INTERTHERM electric furnace, which I just had the strip heater replaced.

The furnace tech recommended that I replace the wall thermostat because the blower fan stops at the same time as the furnace. He said" the blower fan should run for a short time after the furnace stops and that may have caused my strip heater to overheat.

"With electric it should run for short while and with gas it will stop immediately".

He recommended that I get a Honeywell 5000 programmable thermostat and that it will have a setting to adjust how long the blower fan will continue to run after the furnace stops. I have been searching online and reading thermostat manuals online, so I will know exactly which thermostat to purchase.

I cannot locate any thermostat that has this setting.

I cannot locate any information pertaining to this issue at all. So now I am very confused as to what to do. Thank you very much for you help.

On 2018-01-19 17: by (mod) -

Steve

I agree that a furnace heat exchanger might be damaged by overheating if a blower fan stops while the heat exchanger is still quite hot.

On an electric furnace we don't have the same safety concern as for a gas or oil fired heater as there's not a risk of blowing combustion gases or carbon monoxide into the home. But still a heat exchanger might be damaged and leak.

The strip heater itself, in an electric furnace, looks often like the example I include below from energyvanguard.com .

Some electric furnace strip heaters are solid elements rather than coiled wires.

You will see that the strip heater is an electrical component - a heater - that is the actual heat source.

Building air is blown across the strip heater to heat the air, and as you and your service tech point out, a heater that remains hot without air movement across it might be damaged.

However the wall thermostat or room thermostat, as far as I know, has absolutely nothing to do with the blower continuing to run past the time that the actual heater stops.

Rather, that run-on of the blower is controlled by a fan limit switch on the heater. That control does the following:

1. It does not turn the blower fan ON until the heat exchanger is warm-enough; otherwise the heater would blow cold air on occupants

2. It turns OFF the whole heating system if temperatures become abnormally high and thus unsafe

3. It keeps the blower fan running for a brief interval (usually less than a minute) AFTER the wall thermostat is satisfied (room temperature has reached the set temperature on the thermostat) in order to purge or clear the remaining heat from the heat exchanger and in order to prevent overheating therein.

You can search InspectApedia.com for FAN LIMIT SWITCH to read more about those devices (using oil and gas furnaces as examples).

The limit switch on an electric furnace will be smaller and simpler - a simple temperature sensor.

Above on this page we show an example below for a Coleman Intertherm 140 L140-30F Limit Switch Electric Furnace 7945-3281 7945-328 (from HVACpartsstore.com )

My comments about heat exchanger damage also pertain to the strip heater itself and the risk of overheat damage.

I *think* that the strip heater is designed with the assumption that air is moving over the heater when it's quite hot. I'm researching that further.

Manuals for your heater are at INTERTHERM HVAC manuals.

On 2018-01-19 by Steve - trouble with strip heater in an Intertherm electric furnace in a manufactured home

Please bear with me...In my manufactured home I have an INTERTHERM electric furnace, which I just had the strip heater replaced.

The furnace tech recommended that I replace the wall thermostat because the blower fan stops at the same time as the furnace.

He said" the blower fan should run for a short time after the furnace stops and that may have caused my strip heater to overheat.

"With electric it should run for short while and with gas it will stop immediately". He recommended that I get a Honeywell 5000 programmable thermostat and that it will have a setting to adjust how long the blower fan will continue to run after the furnace stops.

I have been searching online and reading thermostat manuals online, so I will know exactly which thermostat to purchase.

I cannot locate any thermostat that has this setting. I cannot locate any information pertaining to this issue at all. So now I am very confused as to what to do. Thank you very much for you help.

On 2018-01-06 3 by (mod) - when the heat can't reach the thermostat setting

In very cold weather you may be at the capacity limit of your heating system

On 2018-01-04 by Barb

Setting the thermostat at 74 in winter the burner and fan went on and reached the 74 but burners and fan continues to run without increasing house temperature beyond setting even after temp reached. Turning the thermo back down to 69 it then shuts off.

On 2017-12-12 by (mod) -

Anon

Given only the information in your note, I can't guess whether the root problem with your gas-fired heater is a burner operating problem or control problem. For safety I have to suggest that you ask for help on site from the heating service tech

On 2017-12-11 by Anonymous

our older mobile gas coleman keeps going on and off before reaching the tempature. replace themoset and can't figure it out. do have a damper would that have anything to do with it

On 2017-09-03 by Javier

Why does the return air on my furnace turns on on a hot day? This is a two story house

On 2017-04-23 by (mod) - furnace fan upper limit OFF setting is typically 200F

THANK you Brian, I've made the correction. Indeed using the L4049 universal control fan limit switch manual as an example, Honeywell points out that the factory-set position of the FAN LIMIT OFF is at 200 degF.

I've made the change to the article above - it may take 24 hours to show up on all servers.

I appreciate that you took the time to let me know of this error.

Daniel

On 2017-04-23 by Brian

Recommend fixing typo regarding the fan limit switch, and in particular the FAN LIMIT OFF temperature (red arrow on the photo of the fan limit switch).

As of today (4/21/17) this article says the FAN LIMIT OFF temperature is typically about 100F. It should say about 200F.

On 2017-03-20 by (mod) -

That makes me think that the control itself is defective OR that it has a loose or corroded electrical connection

On 2017-03-20 by kerrysnow

I have the problem discussed here, where the blower stays on after the burner shuts off. The problem is intermittent, and can last anywhere from a few minutes longer than the normal period to 30 or more minutes (seemingly unrelated to the heat in the supply plenum.

An odd thing about my situation is that if I gently tap the cutoff switch on the outside of the furnace (or on the furnace itself), the blower shuts off (only if the burners are off). Any thoughts on this situation would be appreciated. I've changed the switch (a couple of times) and that doesn't resolve the problem.

On 2017-03-04 by (mod) - the blower will come on one or two times after it first shuts down.

Chris,

The reason is probably one of those explained in the article above.

Check for:
- a dirty air filter
- dirt on the blower fan blades
- a loose blower fan pulley or fan belt if it's a pulley-driven fan

On 2017-03-04 by chris

I have a 30 yr old carrier furnace that runs as normal through the heat cycle. However, the blower will come on one or two times after it first shuts down. Is this a safety concern or is it nothing to worry about. I do not remember it doing this in the past. Any ideas what to check?

On 2017-02-03 by Shannon

I have a Weil McLain hydronic gas furnace installed 1991. It has been working inconsistently. It will work for 12 hours then short cycle and lock out. I cleaned flame sensor but it continued. I replaced flame sensor, still have issues.

The gas burner tubes were removed (one looked crooked in its spot)and cleaned, and again 12 hrs later the fan is running, no heat and temp is 5 degrees below thermostat. I cleaned everything and oiled the vent motor.

I did notice that vent motor was clicking on and spinning quick some cycles and it would barely spin on others for a minute or so. What else can I do to check and maybe fix myself before spending a fortune for a technician? Any advice is greatly appreciated. Thanks

On 2017-01-07 by (mod) - repetitive "pulsing" noise at the end of each cycle.

I don't know, April, from just your description, what's happening.
Is this an oil or gas fired heater?
Is this a high-efficiency heater? or one that uses a draft-inducing fan to blow exhaust to a chimney or directly outdoors?

We need to start by understanding what system we're discussing.

Next we need to see just what part is "pulsing" or short-cycling on and off.

On a conventional gas or oil fired forced-air furnace heating system, if the blower fan (that circulates air through ductwork and into the occupied space) is turning on and off at the end of a heating cycle that's usually an indicator that something's wrong: a dirty air filter or dirty blower fan that restricts the air flow rate through the furnace and its heat exchanger could be at fault.

On 2017-01-07 by april

Hi, Our system is 11 years old and is actually working fine. However, there is a repetitive "pulsing" noise at the end of each cycle. We assume it's an issue with the combustion air fan??
Any ideas? Thank you!

On 2016-11-15 by helen k

my 28 year old oil furnace blower ( new motor 4/16 ) comes back on one time, runs for 10 seconds, then turns off. happens about 30% of the time. this is something new. thanks, helen

On 2016-04-29 by (mod) - what turns on the fan

the limit switch at the air handler senses the temperasture rise and turns on the blower fan

On 2016-04-29 by peter

what causes the fan to turn on, when the heat exchanger gets warm?

On 2016-03-21 by (mod) -

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies,

On 2016-03-21 by (mod) - induction blower on Intertherm mobile home furnace

Larry I'm not sure what you're describing. If you mean that your furnace uses a draft inducer fan to vent the furnace oil or gas burner, that might indeed be drawing room air through the heat exchanger after the burner cut off.

But that would cool DOWN the heat exchanger and plenum faster rather than more slowly, so it should reduce rather than increase the extra air-circulating blower-fan cycling on and off after the burner has turned off.

I'd start by checking the suggestions in the article above. Do keep me posted a what you find will help other readers.

On 2016-03-21 by larryinmich.

my intertherm mobile home furnace has an induction blower. could this have an effect on fan blower cycling after the stat is satisfied on the heat cycle?

Question: Long blower on time on Luxaire Furnace

(May 18, 2014) Gordon said:
My Luxaire Furnace works but when it starts cool down the blower stays on for a hour
then turns off and stays off.

Reply:

If the blower stays on for a long time but then turns off on its own AFTER the burner has shut down I suspect that the air filter is dirty, the blower is dirty, or ductwork is crimped or disconnected - something is restricting air flow so that not only does it take a long time for enough air to pass through the heating plenum to cool it down, you are also unnecessarily increasing heating costs.

Question: success at getting long-on blower time to work properly by adjusting the FAN OFF temperature on the limit switch

(Nov 7, 2014) TheFremontMan said:
My blower used to stay on for more than an hour after the burner turned off. It turned out that the lowest setting (fan-off temperature) was set around 60 F.

I adjusted it to 90 F as suggested in this article, and the fan turned off two minutes after the burner turned off.

Reply:

Wow that sounds right.

That after-blow cycle is designed to cool down the heat exchanger to reduce the risk of cracks that might occur if the blower stopped while the furnace heat exchanger were still quite hot.

Surely an hour is more than needed to cool down the equipment. If you can find a shut-down temperature recommendation from your furnace manufacturer or from the limit swith manufacturer we'd end up spot-on.

...

Continue reading at FAN LIMIT SWITCH or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see this

HVAC Blower Fan Articles

• BLOWER FAN OPERATION & TESTING - check for a dirty blower fan
  • BLOWER FAN CONTINUOUS OPERATION
  • BLOWER DOOR SAFETY SWITCH
  • FURNACE BLOWS COLD AIR
  • FURNACE FAN CYCLES AFTER HEAT
  • FURNACE FAN CYCLES DURING HEAT
  • FURNACE FAN STOPS EARLY
  • FURNACE FAN WONT START
  • FURNACE FAN WONT STOP
  • WOOD FURNACE FAN LIMIT CONTROL
  • CAD CELL RELAY SWITCH
• FAN, AIR HANDLER BLOWER UNIT
• FAN LIMIT SWITCH TROUBLESHOOTING
• FAN MOTOR START CAPACITORS
• FAN LIMIT SWITCH
  • FAN LIMIT CONTROL SETTINGS
  • FAN LIMIT SWITCH INSTALLATION & WIRING
  • FAN LIMIT SWITCH TROUBLESHOOTING
  • FAN RUNS ONLY ON FAN-ON / MAN
  • FAN WONT STOP - LIMIT SWITCH
  • FAN WONT STOP - THERMOSTAT SWITCH
  • FURNACE FAN WONT START
  • FURNACE BLOWS COLD AIR
  • FURNACE FAN CYCLES AFTER HEAT
  • FURNACE FAN CYCLES DURING HEAT
  • FURNACE FAN STOPS EARLY
• FAN MOTOR START CAPACITORS
• FAN NOISES, HVAC
• FAN ON AUTO MAN THERMOSTAT SWITCH
• FAN ON SWITCH ADDITION
• FAN WONT STOP - THERMOSTAT SWITCH
• UNDERSIZED RETURN DUCTS - not enough return air

Suggested citation for this web page

FURNACE FAN CYCLES AFTER HEAT FAQs-2 at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to HEATING FURNACES

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