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.

 

Changing the FAN OFF Fixed long blower-on time

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. (Nov 7, 2014) TheFremontMan

Reply:

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.

 

Why does the furnace blower come on after the furnace has shut down?

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 2017-03-04 by chris

by (mod) - dirty air filter, dirty blower fan, loose fan pulley or belt, fan speed set too low

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

- if there is a fan speed dip switch on the blower control board it may need to be set to a higher speed

 

Furnace Blower stays on just sometimes after the burner shuts off - unless I tap on it

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 2017-03-20 by kerrysnow

by (mod) -

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

 

Coleman furnace cycles on and off before room temperature is reached

Our older mobile gas coleman keeps going on and off before reaching the temperature. replace themoset and can't figure it out. do have a damper would that have anything to do with it On 2017-12-11 by Anonymous

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

Intertherm electric furnace blower does not keep running after the heater turns off

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 by Steve -

by (mod) -

Above: a typical strip heater, this one at the hvacpartstore.com

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.

by (mod) - Coleman Intertherm 140 L140-30F Limit Switch Electric Furnace

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

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 above for a Coleman Intertherm 140 L140-30F Limit Switch Electric Furnace 7945-3281 7945-328 (from HVACpartsstore.com )

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

by (mod) - An 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.

 

After call for heat was satisfied, Purge fan seized up - furnace stopped workinmg

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 "Purge fan seized up"

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.

 

Could an induction motor for the furnace blower fan cause the fan to cycle after the thermostat is satisfied

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? On 2016-03-21 by larryinmich.

by (mod) - unlikely

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 FURNACE FAN CYCLES AFTER HEAT. Do keep me posted a what you find will help other readers.

What causes a Long blower on time on a Luxaire Furnace

My Luxaire Furnace works but when it starts cool down the blower stays on for a hour

then turns off and stays off. (May 18, 2014) Gordon said:

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.

...

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

Or see these

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• 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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