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.

Q&A on How to 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.

 

Blower fan cycles on and off during heat cycle. Burner stays lit. ?

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 2019-10-08 by Anonymous

Reply by (mod) - causes of furnace blower cycling on and off during a call for heat

Anon

Typically when the burner keeps going but the furnace blower fan is cycling off, then back on, that tells us that when the blower runs the temperature in the heat exchanger is dropping so much that the fan limit switch is turning the blower fan OFF rather than blow cool or cold air onto building occupants.

In turn that means that the burner (oil or gas?) is not sending enough heat into the heat exchanger to keep it warm enough while the blower is running.

In still another turn, that suggests that either the burner itself isn't operating properly (too cool, flame too low, partly blocked burner) OR the heat exchanger is soot-clogged so is preventing good heat transfer out into building air.

Less likely but possible is that the blower fan speed is set too high.

To fix any of those issues requires a service call from your heating company. Discuss the system's operating problem with the service manager and ask for an experienced heating service tech to insepct and clean or adjust the system as needed.

You could start by checking the diagnostic suggestions at FURNACE FAN CYCLES DURING HEAT

Or go right to

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

It may help you also to review these:

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

This short cycling blower fan problem 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.

Also see

Diagnostic 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.

IF 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

Watch out: 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)

Check blower fan speed settings: 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.

 

Blower Fan stops and restarts before room temperature is satisfied; burner keeps going.

My furnace starts fine in the morning. It starts at 68 deg and should run until it reaches 74 degrees.

After a while, the fan stops but the burners are still lit. It stays that way for 10-15 minutes, then the fan starts again until the set room temperature is reached.

Is the fan limiting switch going bad? On 2020-01-25 by Dan

by (mod) - problem is probably not the fan limit switch: here are some things to check.

Dan

With the apology that I can't know it all, I'm not sure the problem is the fan limit switch; usually such controls will "fail safe" that is by leaving the burner off entirely.

A possibility is that the air flow through the heater's heat exchanger is cooling down the plenum and exchanger enough to drop the temperature below the FAN-ON setting (that's designed to avoid blowing cold air on occupants).

Depending on which fan limit controller you have, yours may have (under the cover it has a metal cover) a round dial with the FAN ON FAN OFF and FAN LIMIT settings

You can remove the cover to look.

Watch out: DO NOT PRESS on or try to turn or adjust the settings as doing so could make your heater fatally dangerous.
And don't touch live electrical wires - you can get shocked or worse.

But you can watch the dial as the system is calling for heat; it will rotate depending on the temperature in the plenum.

That will give us a clue about what's going on.

You might check the FAN-ON temperature setting to see if it is set higher than needed.

Another very important thing to check is a bad burner flame or a dirty heat exchanger that's not letting enough heat get into the heat exchanger and into building air.

And we might also check the fan speed - if it's adjustable. Some furnaces include DIP switches on the control board that can change the fan speed. If the blower speed is too great for the input BTU capacity of the burner it might be that the plenum cools down to below the FAN-ON temperature setting.

Fraser Johnson Furnace blower starts late then cycles at 30 second intervals

Hi I have a slightly different problem. Furnace is about 30 years all. My blower starts late and then cycles on and off at about 30 second intervals.

Burner turns on normally, begins to heat. Fan limit switch moves past " Fan Off", then past "Fan On", Blower doesn't start until about 10 - 20 degrees above "Fan On"

Then blower runs on for 30 seconds and shuts off for a few minutes, turns back on for 30 seconds, and shuts off for a few minutes.

Meanwhile, Fan limit Switch reaches "Fan Limit Off", burner turns off and blower still runs in short intervals until furnace has cooled. All the while no heat makes it into the house. P.S.

Fan Limit Switch was just replaced. Please help! 29 January 2015 Matteo 

Reply: Causes of blower cycling on and off when it shouldn't

Plenum too hot: Dirty air filter or blocked, crimped, leaky return air ducting or dirty blower fan blades on the air handler blower fan all can cause inadequate air passage through the heating plenum - that causes the plenum to get too hot and the fan limit switch dial will rotate to the FAN LIMIT setting and then turn the fan off.

In our photo, a slipping or loose drive belt connecting the blower motor to the blower fan itself can also cause the plenum to get too hot as air movement is reduced.

Plenum too cold: if the heat exchanger is dirty or the duct oversized or unsually cold air is returning to the furnace or the blower fan speed is too high, the plenum temperature may fall below the FAN-ON setting, causing the fan to stop.

Electrical problems: Loose wire connection at the limit switch or at the blower assembly

Improperly-installed fan limit switch, helix in contact with something inside the plenum or wiring error or bent control components while someone messed with the switch adjustments

Defective fan limit switch

A blower that cycles back on after turning off can also mean that air flow is the opposite (not enough air) of what I posed above. In this alternative we are seeing a too-fast, cooling down of the plenum temperature sensor (the fan limit swith helix spring) before the plenum steel itself has cooled.

A result is that the heat exchanger or plenum re-heats the air in the supply plenum thus turning the fan back on.

Use our CONTACT US link to find our email to send me photos of your system and of the control.

Reader follow-up: Matteo wrote:

I’ve attached a couple photos of the furnace for you.

I've taken off the cover panels on the furnace and removed the air filter for the photos. Our conversation that I started on the site is pasted below.

The air filter is new, just changed it. The fan and probably the motor is very dirty.

I plan on cleaning it this weekend. Up till about 2 weeks ago the furnace was working fine. Of course, hadn’t been using it very much and then all of a sudden we did have it on everyday.

The wiring at the Limit switch seems fine. Followed the connections and seem alright. The switch was wired like this and worked fine until recently. But I will check to make sure the helix is not in contact with anything.

The Fan limit switch, does seem to be functioning fine. The temperature does not seem to be dropping too fast. Also, when I pull the switch to turn on the fan manually, it turns right on and runs with the same problem, 30 secs or so and off for a few minutes before cycling back on.

One thing I forgot to mention, there was a slight electrical almost burning smell right when this problem first started.

Could the problem be with the fan motor? Or maybe the fan motor heats up and loses it connection to the power until it cools back down? Thanks again! Matteo

Reply: look for a burned start/run capacitor at the blower fan assembly

Take a look at the blower itself when the unit should be starting - I see in your photos signs that there could be a pulley or belt problem: black debris is being deposited where it would spin off of the blower motor pulley.

Watch out: if your system does not have a blower door interlock switch or if you wire the switch off to watch the blower run, you could easily get fingers or a hand cut-off - stay away from an open blower compartment.

Next:

Also, when I pull the switch to turn on the fan manually, it turns right on and runs with the same problem, 30 secs or so and off for a few minutes before cycling back on.

in your note is suggestive of a fan circuit or motor problem since pulling the FAN ON switch, IF all other wiring is OK, should run the fan constantly - the limit switch won't consider temperature at all in making the fan run:

I suspect there may be a bad fan motor start/run capacitor. See if there is one you can find in the fan compartment (with power off) and if so just replace it.

 

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 2018-11-03 by Richard -

Reply 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.

Air Handler Blower Short Cycles in Backup Heat Mode

Trane 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 2018-03-10 by PhatJag

Reply by (mod) -

Phat,

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

article FURNACE FAN CYCLES DURING HEAT

 

Gas Furnace heater turns off shortly before reaching the set temperature on the thermostat

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 2018-02-11 by Robert

Reply 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

Furnace turns on when our thermostat is not calling for heat

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 2018-01-23 by TONY WRIGHT

Reply by (mod) -

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

 

Furnace Blower Fan keeps running after heat is satisfied

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 2017-11-29 by Nick -

Reply by (mod) -

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

Blower slowed down and burner cycles on and off after I cleaned the blower fan

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-stat 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 2016-11-08 by Wendy

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

Reply 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 OR you changed a dip switch or other fan speed control setting.

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

Reply 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.

by Wendy

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

Reply 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.

Indoor Air Handler Blower cycles every 30 seconds

What will cause a blower motor to cycle every 30 seconds yet run smooth when the aux heat is used.

When the thermostat calls for a 1 or 2 degree rise in temperature the blower motor short cycles but when the thermostat call for 3 degrees or more it doesn't short cycle until the thermostat setting is within 2 degrees.

Than the short cycling begins On 2018-12-07 by Gary-

Reply by (mod) -

The cause is probably one of the reasons given at

FURNACE FAN CYCLES DURING HEAT 

Watch the temperature dial on the fan limit control switch as the furnace goes through a cycle and let me know at what temperature the dial is pointing and which direction does it moves when you see the short cycling.

Furnace blower fan startup trouble - intermittent slows or stops

The furnace supply fan comes on after a call to heat. Then it slows (or turns off for a second or two) then the burner fires. while the supply fan runs. I can feel the air go from stone cold to warm. Then the motor stops. Complete cycle is about 3 to 4 minutes. On 2019-11-16 by Mathew Sgattoni

Reply by (mod) -

If this is a direct drive blower fan motor this sounds like

- loose fan wiring connector

- bad fan relay

- bad fan motor or bad motor start/run capacitor


IF your fan is driven by a pulley and belt I'd look for a loose pulley or bel

Coleman Evcon Presidential HVAC unit heater not working: blower stays on, heater element cycles on and off

I have a Coleman Evcon presidential hvac unit.

I just changed out one of the two heating elements and now the new element cycles on and off when heater is on the blower stays on. Can Anyone tell me why the element is doing this? On 2018-11-03 by Richard

Reply by (mod) -

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 a call for heat, Furnace fan cycles on and off several times before staying on

When I turn my furnace on it heats up and then the blower fan kicks on it runs for about a minute the fan kicks off it does this several times before eventually the fan kicks on and stays on. On 2021-01-02 by Scott

by danjoefriedman (mod)

Scott

If your system has always had this problem then it's possible that the fan speed is set too high or that the return air and Supply Air Duct systems are not properly balanced.

I'm speculating on that second point that the return air is cooling off the heat exchanger and shutting off the blower

 

During a Heat-On Cycle, Furnace blower turns on too soon and blows cold air

I have an electric furnace that is bugging me. I’m sure that the problem is probably the fact that whoever installed it run the duct work in the attic and without adding a bunch more insulation around the duct there’s probably nothing else to do.

Anyway, what does seem off though is that it appears that as soon as the digital thermostat goes to turn on the heat coils that the blower comes on instantly, so it starts off blowing cold air for around 30-45 seconds before it gets warm.

Once it gets warm it’s working great, so the coils don’t stay on very long at all causing it to start blowing cooler air after about 1 minute tops of running.

Then it blows cold air again for about 1 minute or more before the blower stops. With the old school thermostats, at least on gas furnaces, you could adjust the cycle of the blower to work with the installation, if the blower started too soon you could change when it started and if it ran too long you could shorten it, but these digital thermostats don’t have that, at least the one I have on this electric furnace doesn’t.

So, without having to add a pile of insulation around all these ducts in the attic, is there a way with my setup to make the blower motor kick on like 30 seconds or close to that time later than it does and to kick off around 30 seconds sooner, because the way it’s set up now I get around twice as much cold air out of the unit as I do warm air which bugs me since when the heat coils catch up to the blower it’s blasting some really nice heat out, but I’m freezing for a minute before and after that nice heat.

Maybe I should add too that it also seems that the blower runs on what appears to be high speed only

Maybe I’m thinking wrong, but it seems to me that it should start on low then switch to high with the heat cycle, but it just comes on at the same time the thermostat clicks signaling the heat cycle in what seems to be high speed and runs that same speed until it shuts off. Could it simply be a case of it should start on low then cycle to high when the heat coils catch up?

I’m a certified residential electrician (was going to college for electrical engineering, passed the residential electric part, started electric motors and 3-phase stuff and the college was shut down, so I never finished my degree) and have done several furnace installs, mostly gas installs though, and have repaired a few electric furnaces (usually obvious stuff, like wires melted and limit switches with holes burnt in them, not much testing needed there),

but I have the basic knowledge of being able to read the electric diagram and I own a very expensive HVAC/industrial electric tester with an amp clamp and leads, so I can probably handle the technical explanation you’d have, plus I live in an apartment where I have to tell the maintenance guy how to fix stuff (I’ve done residential construction from the footing up for 25 years)

and I’m sure they’re not interested in hiring a furnace guy to work on it, so if there is a way to cycle the kick on time, kick on speed, and cycle run time on this newer electric furnace with the digital thermostat I can probably understand your explanation of how to do it. Thanks. On 2019-01-16 by Dan W. Johnston

by (mod) -

Dan

We need first to check the dip switch or pin settings on the blower control board.

Can you ID the brand and model and thus find the manual?

 

Thank you to our readers for their generous comments

Much appreciated you taking the time and effort to help diya! I own an auto repair shop and know your shooting yourself in the foot giving out free info.

Just obtaining a flow chart for my furnace is impossible, so specific test results not even an option. Gave me the knowledge to repair my furnace with some parts I retained, but did so in confidence (1 part and fixed 1st try) thanks to you vast knowledge and willingness to help. Much appreciated! - On 2018-11-15 by DGaines

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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 MANUALS
  • 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

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