Questions and answers about why a furnace keeps cycling on and off before the thermostat is satisfied.

This article series describes what to check if the warm air heating blower fan cycles on and off after the call for heat has stopped, that is after the room thermostat has been satisfied.

This article series describes how to diagnose & fix just about any problem with forced air heating & cooling systems.

How to Diagnose & Fix a Blower Fan that Cycles On/Off During the Heating Cycle

These questions and answers about how to diagnose and fix a heating furnace blower that keeps cycling on and off during a call for heat, that is, before the thermostat is satisfied, were posted originally

at FURNACE FAN CYCLES DURING HEAT; be sure to review the advice at that article.

On 2019-10-08 by (mod) - Blower fan cycles on and off during heat cycle. Burner stays lit. ?

Anon

Let's start by checking the diagnostic suggestions at FURNACE FAN CYCLES DURING HEAT

On 2019-10-08 by Anonymous

Blower fan cycles on and off during heat cycle. Burner stays lit. ? Nordyne mobile home furnace model # mgha-066aafc-01 furnace is 20 plus years old.

On 2019-04-29 by (mod) -

Tim

Thank you for taking the time to review, comment, fool around with the fan limit, and give feedback - that's very helpful to me and to other readers.

Something to check further: when the blower fan isn't able to cool down the plenum sufficiently to avoid overheating up to the FAN-OFF LIMIT temperature during a normal call for heat,

remember to check with great care the cleanliness of the blower fan itself (as well as checking for inadequate return air or a dirty air filter). Anything that reduces the air flow through the heat exchanger can cause plenum overheating even with all other settings correct.

I have found by direct measurement of air flow CFM and plenum temperatures that even a visually modest amount of dust and dirt on those cupped squirrel cage blower fan blades is sufficient to cut the air flow out of the blower by as much as 40%!

You're right that there's plenty of work to sort through understanding, writing clearly, and giving correct information - on the other hand if we don't keep trying to get it right, what's the point of doing the work at all? Sometimes I've worked so long on the material that my own eyes glaze over and I miss mistakes.

Working together makes us smarter.

I have posted a condensed version of your comment at

CASE 2: the furnace BLOWER FAN cycles ON-OFF while BURNER remains ON - blower short-cycling

Daniel

On 2019-04-29 by Tim - fooling with the tightness of the furnace blower fan belt

Hi Dan. I apologize for the lateness of my reply. I solved my problem that night and, well, basically didn't need you anymore so forgot all about you, lol. sorry buddy, tough gig you got goin' here;) You have corrected a lot, it read much better, and your last comments here, my gosh I feel like I created so much tedious work for you.

And yes I was starting to go cross eyed and made my own confusing observations.The page you were unsure of which I was referring to was this one. Opening 2 paragraphs, first says this page is for 'before call for heat satisfied, and the second paragraph says this page is for 'after'... I thought that was worth an 'lol';)

So, my fix... Actually, it was run of the mill and I had already been addressing the correct component, which was the settings on the fan limit switch. I started pretty much exactly at the values you have listed. 200, 135, 85. So, as you say, logic is it needs more heat before the blower kicks in. So up to 140,nope 150, nope. "oh, maybe turn the 'OFF' tab down.

That's the same logic as the 'on' tab going up right, create distance between the two". So move the low end to 75. As you know that was also a nope. It's going back past the 'ON' switch that is shutting it down. Try top end 160 now, and loosen belt.... nope, lol In fact it got worse, but in direct proportion to my adjustments, so I thought, hmmm could it be?

SO I went completely opposite against what 2 pro contractors and several YouTube videos were telling me, I turned it down, way down to 120-125, and took out the shim so belt was loose again…. Turns out, yes, it could be. haha. And then the true logic became more clear.

If you let it get too hot before blower comes on, then the burner cannot MAINTAIN that temperature when it does engage. At 125, the blower comes on but the oven is still getting hotter, because we know that flame size gets the oven to at least 170- 180.so the blower can't cool it down fast enough, until the flame goes off.

I actually tightened the belt and got it to where I'm comfortable, left the 'off' at 75 and she is running perfect, smoother and quieter than ever! I like the 75 better on the low end because that is the room temp which means the blower is going to empty the vents of all the warm air before it goes off.

Thanks again Dan. The work you do is a godsend to low income ppl like me. I very much appreciate it. I hope I helped to get her running a little smoother anyway. Watch them flock, ;) take care.

Peace

On 2019-04-13 by (mod) -

Tim

I have corrected the links and problem descriptions of the 3 common blower or burner cycling problems described at inspectapedia.com/heat/Furnace_Fan_Cycles_During_Heat.php

regarding your questions

Sooooo, for me, I don't know what to do next, lol..... Assuming it's not the duct/vents map design. Also assume for now it's not a faulty switch....

All I have left is dirty orifaces, or burner not adjusted properly? BUT since the behavior distinctly changed when I started playing with the belt which is spinning the bearings I'm thinking I should be able to solve this with belt adjustments, a belt which is already too loose meaning if I tighten it, the fan will get even faster. bahhh!

Comments: you’re right that one wants to start by noticing what was done right before the problem occurred.

I would turn off power, remove belt and check for

- damaged bearings, wobble or end play at the fan shaft

- same at the motor shaft

then be sure your pulleys are totally secure on each drive shaft - nobody spins


So here is my questions today - Could that belt be poorly made and have stretched significantly? Maybe I'll just go buy a new one. or Can I tighten the belt but counter the effect of the fan getting faster? In general I want to slow down the air. (even if I turn the 'fan on' to 160 it does the same thing. Every furnace would do this... 'Fan On' - right away the temp is going to drop.

And it just got to the temp to turn it on. This must mean the air it's getting is too cold or it's spinning to fast. Both seem impossible. It's the same air intakes for 40yrs. And the belt is way too loose already.. aaarrrgghhh.lol

Comments:

I don’t see where you report an overly-simplistic “too much air” remark.

Take a close look at your new drive belt.

A damaged belt might drive a fan more slowly - glazed surface, slipping on pulleys; I have (rarely) used an automotive belt spray that makes the drive surfaces stickier - that can help

A belt that is the right overall size in “length” (really circumference) might still work improperly if its cross-section is not OEM or doesn’t properly match the pulley profiles. For example a belt with a wider profile will sit higher in a pulley and may make a fan spin a bit fast. So when we buy a drive belt we look not just at length-size but also the cross-section shape and size.

From the revised article for your case:

Causes of Furnace BLOWER FAN Short-Cycling During a Call for Heat

That happens most-often when

• The fan speed is too high, perhaps on a control board the speed is set too high for the system design or the wrong pulley size has been installed on a belt-driven blower fan
  
• The furnace burner is not adjusted properly - weak flame, inadequate combustion air (dangerous), dirty gas orifices, oil burner in need of service, burnt-out electrical heating elements - depending on furnace type
  
• The heat exchanger is dirty or blocked - not enough heat transfer out into building air
  
• Very cold air entering the system - blower on: poor return air design picking up more fresh cold air and not enough return air from the building can cool down the plenum below the FAN ON temperature.
  

This may be due to improper location (even an unsafe one) of a main or additional cold-air return.
I've seen systems that were picking up 100% new cold air in a crawl space - no conditioned building air was returning to the furnace - that 's the most-expensive heating design possible and could be unsafe.

Diagnose Case 1: Blower Short Cycling

Watch the dial on the furnace FAN LIMIT CONTROL switch during a heating cycle. If you see the dial rotate to the FAN ON point you should then hear the blower fan start up.
Keep watching.

If the dial rotates back towards cooler temperatures a bit that's normal, but the blower should keep running until the thermostat says it has been satisfied.
I

F instead the dial continues to rotate towards lower temperatures until it drops below the FAN ON setting,

THEN something is causing the plenum temperature to get so low that the limit switch should TURN THE FAN OFF - that's to avoid blowing cold air on occupants.

Also check for Poor electrical connections: a loose or corroded electrical wire bringing power to the blower fan motor OR at the limit switch or fan control board can cause intermittent fan operation

On 2019-04-13 by (mod) -

Damn

You are right - I was cutting and pasting text and fouled that page up - I've corrected it at

inspectapedia.com/heat/Furnace_Fan_Cycles_During_Heat.php

On 2019-04-13 by (mod) -

PS Tim

You'll see in the page I cite my reply - don't try to adjust fan speed by tightening or loosening the pulley - you risk ruining a bearing (too tight) or melting a belt (too loose)

Some air handlers include a control board with dip switches that set fan speed or speed ranges
On other systems the tech may change the fan speed (blower rotation speed) by changing out the drive pulley. Obviously that's not an option on direct-drive blowers but yours uses a motor and pulley and belt system.

On 2019-04-13 by (mod) -

Thanks for commenting Tim -

Unfortunately many people call "any" kind of heater a "furnace" - even steam boilers or hot water hydronic heating system boilers. In my book a furnace uses warm air to heat a building. The blower assembly includes a fan (usually squirrel cage fan) that actually moves air.

In an UPFLOW furnace, air enters at the furnace bottom and is pushed upwards through the heat exchanger and out through the supply plenum. At the inlet to the blower fan there will be lower pressure or speaking loosely "a vacuum" (though that word is not technically precisely correct).

But keep in mind there are also DOWNFLOW furnaces - air enters at the top and moves down through the system
and there are HORIZONTAL furnaces - air enters at one end and moves horizontally.

It's not bad to call the unit an "air handler" since the same blower assembly and heat exchanger may be combined with an A/C cooling coil.

I agree that a FAN isn't a BURNER and have taken pains to separate those terms - in the article where we discuss that - and where you previously posted your comments

https://inspectapedia.com/heat/Furnace_Fan_Cycles_During_Heat.php

I'll review page titles and descriptions to see if I can find the source of your lol's - in future posts it'd be helpful to either post a comment on the pertinent page or include the page URL in your comment - our website sports thousands of pages.

On 2019-04-13 by Tim

Oh ya! I read somewhere about the bottom being a vacuum? The lower section has always been a vacuum . It sucks that cover plate right in tight, always has.

second paragraph - ** worse than you thought

lol, you're welcome. You give me furnace advice and I will proof read for you:) lol, It's the word 'furnace'. We wouldn't call a 'FAN' a burner, or call the 'BURNER' a fan, but we will call either one the 'furnace'.

Actually it's worth than you thought, buddy. This page is one of two with exact same title, and both have major errors. This is the one I mostly read last night, then linked to the other one where I posted my question and you said you made repairs. I thought they were two different problems that I couldn't distinguish between. Now I can, Whatever you did was great because I felt more on top of things while yesterday I was totally lost. But here are the major things I noticed:

- Other page - you have the cases mislabeled. If I read the short blue link description I am #2, but when I go to #2 it is a different problem. But I noticed case #1 was me. So case 1 and case2 switch the numbers. Also in that case #1 it has a how-to-diagnose at the end. That basically re-states the problem of too much air and says 'something' is causing it, end of article, lol.

It does link you to an example which is non-existent except for a link to guess where? here, lol. And then on This Page the second paragraph says 'after call for heat'. lol so that's where I started grinding my teeth, haha. And actually this page is just a bunch of questions about other issues and not relevant to the issue or it's own title.

Sooooo, for me, I don't know what to do next, lol.....

Assuming it's not the duct/vents map design. Also assume for now it's not a faulty switch.... All I have left is dirty orifaces, or burner not adjusted properly?

BUT since the behavior distinctly changed when I started playing with the belt which is spinning the bearings I'm thinking I should be able to solve this with belt adjustments, a belt which is already too loose meaning if I tighten it, the fan will get even faster. bahhh!!!!!

So here is my questions today - Could that belt be poorly made and have stretched significantly?

Maybe I'll just go buy a new one. or Can I tighten the belt but counter the effect of the fan getting faster? In general I want to slow down the air. (even if I turn the 'fan on' to 160 it does the same thing. Every furnace would do this... 'Fan On' - right away the temp is going to drop. ...

And it just got to the temp to turn it on. This must mean the air it's getting is too cold or it's spinning to fast. Both seem impossible. It's the same air intakes for 40yrs. And the belt is way too loose already.. aaarrrgghhh.lol

On 2018-11-04 by (mod) - furnace makes a noise when first ignites

As there are many possible noises and causes, some normal, others indicating trouble, Gaetano, I'm sorry but from just "it makes a noise" I can't tell you what the noise is nor what it means.

At HEATING SYSTEM NOISE DIAGNOSIS you will find a series of descriptions of different sorts of noises produced by heating equipment as well as advice on what those mean and what to do. Please take a look there and let me know how that works for you.

On 2018-11-04 by Gaetano Vivenzio

when my burner on my furnace go on make a little noise e go away after 2 minutes

On 2018-11-03 by (mod) - check for a loose wire or bad or corroded connection at the heater control

Richard

Check first for loose or improper wire connections.

If there's no problem there I'd look for a control or thermostat issue

Also don't forget to look for a dirty or blocked air filter. Reduced air-flow through a furnace can result in overheating at the heat exchanger, causing the limit control to turn off the heat.

On 2018-11-03 by Richard - Coleman Evcon Presidential heating element cycles on and off

I have a Coleman Evcon Presidential hvac unit I just changed out one of the two heating elements and not the new element cycles on and off when heater is on the blower stayes on.

Can Anyone tell me why the element is doing this?

On 2018-03-10 by (mod) -

Phat,

Look for a dirty air filter, loose connection, or other causes listed in the

article FURNACE FAN CYCLES DURING HEAT

On 2018-03-10 by PhatJag

Train 3-ton air handler with emergency electric heat strip, short cycles 3-speed fan blower in air handler is turning off and on after the heater coil has been running for 5-10 minutes.

The train blower will bump on and off 15-20 times per minute, in 1/2 to 1.5 second increments but the thermostat is 1-degree short of desired temperature.

On 2018-03-09 by (mod) -

Lucille

Try the diagnosis and repair at FURNACE FAN WONT START

On 2018-03-08 by Lucille

Need a switch that’s on blower furnace not working right will not kick on

On 2018-02-11 by (mod) -

Robert

in this order I would check for a loose wire or electrical connection, it dirty or obstructed temperature sensing probe, a blockage in or reduced air flow which could be cause for example by a dirty filter or a dirty blower fan assembly, or a failing fan limit control relay

On 2018-02-11 by Robert

I have a Modine garage gas hanging heater the turns on and off with within 10to 15 seconds before it reaches the correct temp on the new thermostat any ideas that I can check

On 2018-01-25 by (mod) -

I'd look for an automatic thermostat program that's operating or thermostat mis-wiring.

On 2018-01-23 by TONY WRIGHT

my forced air gas furnace is firing up when the thermostat is not calling for it. I hear it in the middle of the night light, start heating up the chamber and then turning off before the blower comes on.

I am hearing this occurring several times during night. got up last night to verify. According to thermostat temp setting, it should not have been trying to come on. However when the temp falls below thermostat setting, it does come on, starts blowing and shuts down correctly. Any advice would be appreciate. Its a American Standard unit about 5-10 years old

On 2018-01-05 by (mod) -

Sounds like a burner going off on safety reset. If so it needs service, adjustment, repair.

On 2018-01-05 by Anonymous

It's turning on then goes off after a couple minutes ,it's 56* inside, won't blow heat

On 2017-12-30 by (mod) -

Rich

Watch The Fan limit control dial and it will tell you what's actually happening. If the dial rotates until the temperature reaches the high limit then you know the problem is that the blower is not coming on when it should or air flow through the system is restricted

On 2017-12-28 by Rich

it is almost like it is trying to heat up before the blower actually kicks on but then the motor shuts down and does this over and over until it actually needs to blow warm air.

On 2017-12-28 by Anne Goldberg

Hot air furnace sometimes does not stop running when reaches thermostat temp & continues to run with cool air does not stop running with cool air, I have to turn system off to stop it

On 2017-12-27 by (mod) -

I'm unclear about just what is happening during "cycle mode" but if you mean that the heat is cycling on and off repeatedly when the thermostat is not calling for heat, then something's wrong and needs repair: a dirty air filter, an overheating heat exchanger, blocked air flow, or a bad heater control.

On 2017-12-27 by Rich

I have a gas furnace that runs, heats up the house efficiently but as soon as it shuts off it goes into a cycle mode until thermostat falls below set temp and then turns back on runs fine without issue. Not sure if this is the furnace staying at temperature or the inducer?

On 2017-11-29 by (mod) - The fan keeps running after heat is satosfied

NIck,

at FURNACE FAN CYCLES DURING HEAT in the Continue Reading suggested links I've posted two diagnostic articles for you

Or see

• FAN WONT STOP - LIMIT SWITCH
• FAN WONT STOP - THERMOSTAT SWITCH
• UNDERSIZED RETURN DUCTS - not enough return air

On 2017-11-29 by Nick

Gas furnace. The fan keeps running after heat is satosfied..fan should cycle off about 3 to 4 min after heat exchanger cools but continues to run

On 2017-11-09 by Anonymous

Oil burn heater fan asiegler

On 2017-11-09 by Anonymous

How do you cut the fan on

On 2017-11-08 by Kay

I have a new Trane HVAC system, installed during the summer. I leave the fan running continuously 24/7, (set to "on" on the thermostat). Now the weather is cold and the furnace is running. When the furnace comes on, it turns off the fan for a second or two then starts the fan again.

Shouldn't the fan just be continuously running without shutting down. Why is does the fan turn off, then turn back on again before blowing warm air?

On 2017-01-30 by Patallen

I have a Coleman Presidential 7660 gas furnace in an older mobile home. It was not used for years.

After cleaning it, I started it and it seems fine but after the main burner comes on, the fan goes on for 30 seconds, then off 30 seconds. This happens non stop. Is it the old valve assembly?
Thanks for your help.

On 2017-01-26 by Deborah

My ex has the heater set to 70 all day 62 at night a 2 story house the ThermaStat is up stairs he does not have the fan on anytime why ? I have a cough and dry skin now always is it from the heater?

On 2016-12-22 by (mod) -

Lee:

The blower fan will normally turn OFF at the end of a call for heat, after the thermostat is satisfied.

On 2016-12-22 by Lee

Does the fan kick off when the house is warm

On 2016-12-16 by Maria K

I have a new forced air furnace and honeywell pro 4000 programmable thermostat. Now that it is below 30 degrees outside, I notice my heat goes on for 15 min, off for 10 which seems to be a lot but not sure. This annoys me at night when the heat is set low. I guess that is about 4 cycles an hour and my heat is pretty good but house drafty.

On 2016-12-16 by John

Hi,

I have a problem that I dont think is mentioned here. I have a very old furnace (about 30 years old probably) that will sometimes cycle on and off. Sometimes it will heat the house up to whatever it's set on, other times it will short cycle once, twice, thrice, whatever, then run correctly. It's completely random. So it short cycles, but not consistently. Thanks for any information, your website is great!

On 2016-11-09 by (mod) -

Wendy:

Often there are un-used thermostat wires that were included in the original wire bundle. You need the wiring chart for your specific model heater.

When a fan runs faster than before I have trouble evaluating that by e-text. For example, cleaning dirt off of the louvers of a squirrel cage fan can substantially increase the amount of air the fan moves even at the original speed of rotation.

Lubricating a fan might slightly improve its speed. Some fan motors are capable of running at two or more speeds (as we want different speeds for heating than for cooling) so simply changing a wire connection at or to the fan might change its run speed.

We discuss fan speeds at BLOWER FAN OPERATION & TESTING https://inspectapedia.com/aircond/HVAC_Blower_Fan.php

and at FAN LIMIT SWITCH https://inspectapedia.com/heat/Fan_Limit_Switch_Guide.php we discuss fan switch wiring

Look at your fan motor data tag for a marking similar to RPM 840/2SPD

Also a change in voltage will change motor speed for motors rated for multiple input voltage levels.

On 2016-11-08 by Wendy

So I guess ,My ? is why is my fan running so fast is it because it was that dirty which it was but I judt don't understand why the fan is running that fast we used to have air flow coming out the vents but now it's alot of air flow but cooler so do I need to get a new blower motor or can I changed the settings

On 2016-11-08 by Anonymous

this is my primary heating source Electric and no heat pump just a split Nordyne system with central air That's the issue the wires come in through the wall of the furnace and those are the ones that i'm having the problems with mine is a 5 wire control I have Blue Rc, Red RH, W white , C white , G green this is how it was before my problems is the wires coming in none are labeled

i also found more wires so now these are the colors Red, Orange, Yellow, Green, White, Black ,Brown, we currently have it spliced to the white because At first we had no heat at all the only other wires with nothing connected are orange ,black, brown, Now my heat works its just that ever since it was cleaned it faster and loader then when t-state clicks heat off it continues to run a a slower better speed as b4 I hope this helps

On 2016-11-08 by (mod) -

I see. There should be terminal markings in your thermostat that help sort out wiring connections. Or armed with the thermostat brand and model we ought to be able to find a wiring guide for it. That info combined with an identification of the wires in your system ought to be what's needed.

You don't give your location, nor do I quite know if the electric heat is a backup for a heat pump, backup for some other heating system, or primary heat. The controls may vary depending on the heater's role.

Watch out: for electrical shock hazards.

On 2016-11-08 by Wendy

Yes that what I'm think its a wire issue when we was hooking all the wires back up I have what to me looks like a jumper white wire ( thick white wire connected to a thin white wire running to the white ON THE CONTROL BOARD for the t-stat but when I was hooking them up the white wire pulled off of one and I didn't know which one it is

I have a orange, yellow,green, brown,Red hope this helps I did not mess with the motor or anything I just vacuumed and used a tooth brush on the cage it was dirty

On 2016-11-08 by (mod) -

Yes the faster blower speed is cooling down the heat exchanger. I'd look first for a wiring error and second for a damaged control.

On 2016-11-08 by Wendy

Hello I have a Nordyne Electric heat/ac unit now this system is 23 years old with no problems at all is is model 901282 Mobile home unit now I needed to clean the blower housing and squirrel cage

now I just unhooked the wires took photo and all hooked back up But now it seems the the blowers runs on high and it seems to make the heat cooler

then the t-state clicks to off and the blower still runs at a slower regular heat flow and it runs like that for about 40 seconds I have noticed that my heats keeps kicking on alot please help thanks Wendy

On 2016-10-27 by Anonymous

I have a heater model # 7655 856 my blower motor goes on and off while flame stays on any help

On 2016-02-15 by (mod) -

Anon

Take the old switch along with a photo of the data tag information on your Bryant heater unit to your local HVAC supplier to ask if they can offer a generic replacement.

On 2016-02-15 by Anonymous

Where can I find a substitute for a Bryant heater 821 fan control relay switch circa 1974.

On 2015-11-23 by (mod) -

Thanks for the tip, Blair.

On 2015-11-23 by Blair

I had this problem, It was just dust in the sensor switch, But My furnace is a older one back when they still made a real furnace, So It had no cracks from the constant kicking on and off.

...

Continue reading at FURNACE FAN CYCLES DURING HEAT - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• AIR HANDLER / BLOWER UNITS - home
• FAN LIMIT SWITCH - home
  • FAN LIMIT CONTROL SETTINGS
  • FAN LIMIT SWITCH INSTALLATION & WIRING
  • FAN LIMIT SWITCH TROUBLESHOOTING
  • FAN RUNS ONLY ON FAN-ON / MAN
  • FURNACE BLOWS COLD AIR
  • FURNACE FAN CYCLES AFTER HEAT
  • FURNACE FAN CYCLES DURING HEAT
  • FURNACE FAN STOPS EARLY
  • ISOLATION SWITCH WIRING
• UNDERSIZED RETURN DUCTS - not enough return air

Suggested citation for this web page

FURNACE FAN CYCLES DURING HEAT FAQs 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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