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.

 

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.

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

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

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

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

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

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-

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

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.

 

Thank you to our readers for their generous comments

On 2018-11-15 by DGaines

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 (mod) -

DG

Thank you so much for the nice comments. We work hard to provide useful unbiased information so I'm particularly grateful when a reader finds that it works for them. Your questions, critique, or content suggestions are also always helpful and always welcome. Working together benefits us both.

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

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INDEX to RELATED ARTICLES: ARTICLE INDEX to HEATING FURNACES

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