Furnace Blower Fan Cycles On/Off During the Heating Cycle
Forced air heating furnace control diagnosis & repair for furnace short cycling
FURNACE FAN CYCLES DURING HEAT - CONTENTS: what to check if the forced warm air furnace blower keeps cycling on and off during the heating cycle - while the thermostat is continuously calling for heat. Forced warm air heat short cycling - furnace short cycling or rocking, or shutting off. Where to buy replacement furnace limit switches, parts or repair parts.
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Furnace diagnostic guide for heater cycling on/off:
This article 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.
Some conditions that cause unexpected furnace fan cycling on and off may be dangerous, risking overheating of the furnace heat exchanger which in turn risks cracks and even carbon monoxide leaks. So it is worth checking out this problem promptly.
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
Why does the blower fan keep turning on and off repeatedly during the heating cycle (the thermostat is calling for heat)?
Our furnace seems to turn off and then back on repeatedly during the heating cycle. I've read that normally on a call for heat the burner should come on, then the blower, and both should keep running until the call for heat is satisfied.
What's going on? - Anonymous
Reply: Abnormally low air temperature at the return plenum can cause blower fan short cycling
Definition of Furnace Short Cycling (also called Shutting Off or Rocking on the High Limit):
Short Cycling is when the burner shuts off before a call for heat is met (thermostat stat setting
is not reached).
It is shut off by the fan limit switch as a safety measure. The air handler
will keep running. The temperature in the plenum will go down and the burner will
come on again, only to shut off again quickly. This rocking back and forth on the hi limit
setting is bad for the furnace. The heat exchanger is seeing temperatures it was not
designed for, and will crack. It is also very inefficient and does not allow proper heat to be distributed to the living space.
Common Caauses of furnace short cycling or rocking
Common causes of heating furnace short cycling or rocking are
Furnace cycling on-off caused by inadequate duct work (ducts are too small, bad heating duct design, layout, damage, constrictions, leaks) and on occasion,
Furnace cycling on-off caused by abnormally cold-air entering the heat exchanger (improperly located return air inlet)
The amount of supply and return ductwork a hot air furnace has, can have a large effect
on the actual efficiency of the unit, as well as the life expectancy of the unit. If there
isn’t enough duct work to deliver the heat the furnace produces, obviously clients are
cold, operating costs go up, efficiency goes down, and eventually a premature crack in
the heat exchanger will result (the furnace can’t get rid of the heat and it stresses out
the metal in the heat exchanger). 
Watch out: while the Vermont government publication that we quote above explains that short cycling can crack the furnace heat exchanger, we need to understand that that means: a cracked heat exchanger is unsafe because it may leak combustion gases or fatal carbon monoxide into a building. Besides leading to an expensive furnace repair or replacement, a cracked-heat exchanger is dangerous.
Abnormally low incoming air temperature entering the heat exchanger after the oil or gas burner has shut off may cool down the supply plenum so rapidly that the fan limit switch, working properly, turns off the blower before enough heat has been extracted from the heat exchanger itself.
If this is happening, rising heat and air (by convection) from the still-hot heat exchanger into the supply plenum may warm the plenum again and turn the blower fan back on.
Blower fan short cycling on and off too frequently during the heating cycle is often traced to this problem.
Why would we have "abnormally low incoming air temperature" at a furnace?
We've seen this problem at buildings with improper HVAC duct design, in particular where some or all of the return air to the furnace is being taken from an opening right at the furnace such as a unit located in a cold basement or crawl space(see our photo, above left).
Sometimes in an effort to improve the total airflow delivered by a furnace a technician will cut an opening in the return ducts in an unheated basement or crawl area.
Watch out: This return air opening boosts the total air output of the system but it is an improper and unsafe design.
Heating Furnace Short Cycling Diagnostic Tests & Inspection Points
In addition to this short list, please read the diagnostic discussion and examples that follow
Dirty air filter: a dirty air filter clogs the flow of air through the system, causing the furnace to overheat and go off on high limit
Dirty air handler blower fan: dirt accumulated on the blades of a typical air handler squirrel cage blower fan assembly can reduce air flow through the system by 30-50% or even more in some cases
Fan limit switch defective: a defective or consumer-damaged fan limit switch may fail to operate normally and in fact may be unsafe - a switch that permits the furnace to overheat can crack the heat exchanger, causing both costly damage and potentially fatal carbon monoxide leaks into building air. Beginning a
t FAN LIMIT SWITCH we describe how this control is installed, tested, diagnosed & adjusted. Warnings include avoiding bending or forcing the control which then pushes it out of calibration.
Furnace pressure switch tubing leak: a furnace that is cycling on-off can be caused by a leak in pressure switch tubing. Thanks to reader Mark who commented (March 2015 at MOBILE HOME HEATING) that a furnace that cycles on and off may be caused by pressure switch tubing that has a leak.
Improper fan limit switch settings (discussed below)
Inadequate return air ducts: omission of proper return air ducts to deliver building air to the furnace return plenum; some systems take some or all return air from a cold basement or crawl space - expensive and potentially very dangerous if the return air can draw flue gases into building air.
See UNSAFE DUCT OPENINGS)
A seat-of-the-pants test we perform while the air handler is running is to open the air handler door or cover to expose the blower fan and then release it. If the door SLAMS shut the system was running at a vacuum and return air is inadequate; or if opening the door causes the furnace to stop short cycling or shutting off that may be the same diagnosis.
See UNDERSIZED RETURN DUCTS
see RETURN AIR REGISTERS & DUCTS
Watch out: most modern air handlers include a safety interlock on the air handler door - the switch will turn off the furnace if you open the door with equipment running. This is because it's easy to lose a finger or be even more seriously injured by opening the air handler when equipment is running.
Inadequate supply air ducts: warm air supply ducts that are too small, bent, constricted, leaky
(SUPPLY DUCT AIR LEAKS), or
even disconnected. Some rules of thumb on heating air duct requirements are given
at SUPPLY DUCTS & REGISTERS
Furnace defects or air handler defects (wrong blower speed) that prevents the furnace from producing enough heat rise across the heat exchanger. For a furnace with adequate duct
work this heat rise should fall in the range of 45 degrees to 70 degrees. For belt-driven air handler fans, a too-loose or slipping or damaged drive belt can slow the blower and thus reduce air flow through the system.
Oil burner nozzle over-sized: a too-large nozzle (or a change that upped the oil burner fuel unit output pressure) may have increased the oil burner firing rate over that the furnace is designed-for. Check the nozzle size and fuel unit pressure against the furnace manufacturer's recommendations and allowable firing range on the furnace data tag.
See OIL BURNER NOZZLE & ELECTRODES
Oil fired heater BTU output estimating tip: If there is no data tag on the furnace or you can't find the oil burner output BTUH you can guess it as follows: Multiply the oil burner nozzle size (firing rate in gallons per hour) x 138,200 (BTUs per gallon of No. 2 heating oil) x an efficiency estimate (use .75 for older furnaces and .80 for newer furnaces if you don't have a recent efficiency measurement).
Thermostat wiring or device defects: check for loose thermostat wires, debris-clogging of a thermostat's bimetallic spring or thermocouple (and clean them)
Bad thermocouple or dirty thermistor on gas fired equipment: Short cycling or unexplained on-off cycling of heating equipment has also been traced (by one reader) to a dirty or failing thermocouple (possibly the reader meant a dirty thermistor).
Or see THERMOCOUPLES
This condition was reported in the FAQs by a reader commenting at HEAT ANTICIPATOR Adjustment
Abnormally high air temperatures at the supply plenum can also cause blower fan to turn on and off unexpectedly
While this defect is less common than the cold return air problem discussed above, a dirty air filter or any other defect that restricts air flow through the heating system supply or return ductwork can also cause unexpected blower fan on and off cycling.
If the airflow through the heat exchanger and supply plenum is too weak the furnace oil or gas burner may be able to heat the system to the point that the fan limit switch, to protect the unit from overheating damage (warping, cracking) will turn off the burner, even though the thermostat is still calling for heat. In this condition the blower fan may continue to operate (to cool down the "too hot" heat exchanger and supply plenum).
The blower fan will continue to run until the supply plenum temperature drops to the low limit, then the fan limit switch will turn off the blower. As the thermostat is continuing to call for heat, when the furnace temperature drops, the fan limit switch will permit the burner to turn back on.
Question: the furnace comes on, heats, but the fan then cycles off and back on - what's wrong?
My Grandmother old furnace comes on and heats but the fan cycles off and on. If I put the unit to run it works great heats the house. Is something I can trouble shoot with and electrical back ground or should I call a pro? - Harry Doucet 10/8/2012
I haven't seen my exact problem. My furnace will come on, run it's whole cycle but where You think it's going to shut completely off, the fan or motor or whatever keeps running & within about 30 seconds the furnace goes thru another cycle where I hear the click to tell me the flames have come on then I hear the blower click on then the furnace runs it's usual 12 min cycle before I hear the click of the flames go out leaving the blower on & then when the blower is supposed to shut completely off it doesn't & so again in 30 seconds I hear everything click & it starts up another cycle.
And sometimes I can Stop it by turning the thermostat down slightly & if that doesn't stop it I have to shut off the Power Source by the Wall Main Furnace Switch, Not the Breaker, but the Furnace Wall Switch.
Would this still be the Limit Switch if it Doesn't completely Stop? I was hospitalized for 1 month & shut my thermostat off & when I came home & put the thermostat up to 63 where I usually keep it about, this is when all the trouble started with the furnace Not shutting down completely but it was Fine prior to that. And every summer I shut the thermostat off so I'm not using gas & this has never happened before.
My furnace is made by Intertherm for a Mobile Home & it's a Natural Gas Furnace. Thank You, Kimberlee :) PS... I'm asking You first because as a single Mom I always seem to get ripped off big time by repair men! - Kimberlee 11/6/2012
-- and --
I have an old house, circa 1963, original gas furnace, but blower was
replaced about 20? years ago and ignition converted (by professional)
to electronic, vs. pilot a few years ago. No A/C. Has worked fine since all that. This year (furnace not used often
here in San Diego), when heat turned on, all works as normal, burner
comes on, heats up, then fan comes on. However, the fan keeps cycling
off, then on. Sometimes as often as < 10 seconds on, and usually very
short ( < than 5 seconds) off.
I've replaced the filter just before this season, and even tried
removing it to ensure no restriction there, same problem.
There's a fan ON switch, and when turned on, the fan runs continuously
so not likely it's the fan itself.
My limit switch (a Therm-O-Disc AF 301) only shows a temp for turning the blower on, see
the Therm-O-Disc™ limit switch photo below, and it's set to 130.
I'm guessing the switch is malfunctioning and turning blower off too
soon since the burner stays on while the fan cycles on and off way too
often. - R.E. 12/6/2013
Reply: what to check if the furnace fan seems to cycle back on shortly after it shuts off
Kimberlee and R.E.,
Furnace short cycling with burner ON:
This article, beginning above at FURNACE FAN CYCLES DURING HEAT describes the causes of furnace short cycling while the burner is on. A good starting point for diagnosing this trouble is to watch the fan limit switch while the burner is operating.
[Click any image for an enlarged, detailed version]
Furnace cycles off at HI LIMIT: If you see that the limit switch is rotating so that its indicator or pointer is at the HI LIMIT (red arrow in our photo) then the furnace is overheating, usually for one of the reasons we list in the article above.
Furnace cycles off at FAN OFF: If you see that the limit switch is rotating so that its indicator is at the FAN OFF (blue arrow in our photo) temperature then there is so much cold return air arriving at the furnace that it is shutting down to avoid blowing cold air on room occupants. If the burner is really "ON" then I expect that there is a problem with the burner (flame too low, flame incomplete on a gas burner), or some fool has routed freezing cold outside air into the furnace instead of routing building air to its return plenum.
Therm-O-Disc Fan Limit Switch: this switch has no visible fan limit HI and OFF settings: In R.E.'s example, because the furnace used to work normally, we would not think that there is an underlying inadequate duct work design. And R.E. made sure that a dirty filter is not blocking air flow through the system.
But the other defects in the list above, such as a dirty blower fan assembly (that can drastically reduce air flow) or damaged or crimped return ducting, or crimped supply duct (slowing air movement through the heat exchanger) need to be checked as well.
Watch out: If this little-used furnace has not been cleaned since the 1960's that could be the problem - as well as a safety concern.
Photo at left:Therm-O-Disc™ fan limit switch control installed on R.E.'s furnace. Notice that 8-61 - this switch was probably made in August 1961, making it over fifty years old - in my OPINION that's a remarkable track record of reliable service. The AF-series fan controls made by Therm-O-Disc were a fan control that used a bimetallic blade temperature sensing element that extends just a few sixteenths of an inch into the air stream.
Bad Fan Limit Switch - Only after checking these external causes of furnace short cycling off and on would I then try replacing the fan limit switch itself. That's because we don't want to replace the part only to discover that the problem continues (and we wasted our time and money) because we failed to checkout other more fundamental causes than the switch itself.
Therm-O-Disc AF-301 Set Differential Snap-Disc Control Thermostat
For R.E.'s Therm-O-Disc AF-301 fan limit control, the HI LIMIT and FAN ON were either hard-wired in the control or were handled by separate controls not shown in his photograph. With this type of fan limit control we cannot see a rotating disc that one could watch during the furnace cycle. Instead this is a snap-action type control switch. This is a single pole single throw snap-disc control whose temperature sensing disc operates the switch.
If R.E.'s furnace control has been damaged or has simply failed it will need to be replaced. Therm-O-Disc™ is alive and well and can be contacted at www.thermodisc.com or at www.emersonclimate.com. The company does not sell parts direct to the general public.
Thermodisc technologies is a division of Emerson Electric Co., now Emerson Climate Technology, and has been in business since 1947. Emerson Climate Technology encompasses these brands: Browning, Control Products, Copeland, Dixell, Fusite, Intelligent Store, Therm-O-Disc, Vitter, and White-Rodgers a brand originated by two Emerson managers, Chesterfield White and James Rodgers.  In the 1970's Therm-O-Disc, Incorporated was located at 1320 South Main St., Mansfield OH 44907. Our image at left shows the Therm-O-Disc catalog image of the AF-301 Fan Limit control switch.
You will need to purchase replacement parts through your heating equipment and parts supplier. In fact the company is so reluctant to be contacted that the Therm-O-Disc website contact page keeps the company's actual location a secret - referring readers only to their distributors - which is where you need to go anyway.
Replacements for the Therm-O-Disc AF-301 furnace control switch: snap-action thermostats & thermostic control switches
A review of the present Therm-O-Disc line did not come up with an obvious replacement for the Therm-O-Disc AF-301 furnace limit control switch, though the company continues to produce a very large range of temperature sensing controls and switches, including at least twenty different bimetallic controls and limits. We have written the company to ask if there is a replacement part for the AF301 fan limit switch in case an expert examination of the heater determines that that's what is needed.
Some online vendors such as partsguy.com offer what is reported to be a replacement part for this control identified as the "Therm-0-Disc Set Differential and Snap Disc Thermostat Mfg# 3F01-140".
Other switch models include the 3F01-200, 3L01-180 and 3L01-200. Presumably the last three digits are the nominal response temperature for the control. In addition to checking with your local HVAC supplier, check with these possible online parts sources:
121 S. Spruce St. Traverse City MI 49684
Toll Free (800) 597-4575 ~ Voice (231) 946-7644 ~ Fax (231) 946-9332
Chatham Brass Company (this may be only a supplier of water heater controls that can replace other Therm-O-Disc control models) chathambrass.com/
Elmwood Sensors: Website: http://www.aaxico.com/aaxmfr/elmwood.html (now part of the Honeywell Corporation) USA office: AAXICO, 8881 NW 13th Terrace
Miami, FL 33172
Ph: +1 305-592-4633
Fax: +1 305-592-6828
At LIMIT CONTROL, SINGLE we discuss similar devices made by Sensata - Klixon, the company's Snap-Action thermostatic sensors for HVAC/R applications that include the Sensata “Stat-on-Stilts”™
that may be used anywhere a temperature switch
must be mounted remotely from electrical
A photograph of the company's sample thermostatic sensors is at left. Contact: Sensata Technologies
529 Pleasant Street, MS B19
Attleboro, MA 02703-2964
If you cannot find a new Therm-O-Disc Inc. AF301 Fan switch it should be possible to replace it with a different model and type of switch such as the Honeywell unit shown above, provided there is space and location in the supply air plenum that meets the control manufacturers's specifications.
At left is a sample of snap action type thermostatic controls from Elmwood Sensors - cited just above.
I'm wondering if the switch I sent the photo of is the overheat switch
(130º)? I'll look under the housing I took the cover off to see this
switch and see if there's an actual fan limit control switch.
This Therm-o-Disc AF301 switch is installed on a
Gaffers and Sattler
Model S 80 FDF
S/N: 520 H1,
Input 80K BTU/Hour
Bonnet Cap 64K BTU,
FYI, I turned the limit switch down to about 110, and fan stays on
longer and cycles less, so we'll see how that goes.
I only use the
heater when "very" cold for here in San Diego, and typically only to
take the chill off in the morning before sun comes up/warms things up.
So, not on at this point but will be using again as days get colder
still and/or I'm actually home all evening and need the heat.
The Gaffers and Sattler
Model S 80 FDF is a gas fired furnace that we also cite
at THERMOCOUPLES. There we include this historical note illustrating the complexity of mix and match and shift around of brand names and company names made more complex as different product lines from an original corporation often found later homes in different companies.
Gaffers and Sattler was an appliance brand (kitchen ranges, heating equipment, air conditioning) owned by Maytag and actually preceded "Maytag" as a company name. The Gaffers Sattler and Maytag Washing Machine Company was founded in 1893 by businessman Frederick Maytag. G&S cooking ranges were identified as a subsidiary of MagicChef in 1969. Magic Chef found its way back to Maytag in 1986. If you are looking for parts for Gaffers and Sattler equipment check with Maytag. Separately a Gaffers & Sattler appliance company still does business in Murray UT.
I think from looking at a 1970's catalog that the Therm-o-Disc AF301 incorporated a set differential around the setting that is visible. But indeed as there could have separate additional controls. Do look and let me know - send along photos of the whole furnace and close ups of its data tags and model information and of other controls you find and I can comment and research further.
The 130 fig. is usually a fan-on temperature. Typical fan off is 110 F and the upper limit or safety limit is closer to 200. Click to enlarge our Honeywell limit switch to see those numbers.
Furnace short cycling with burner OFF:
I have seen this problem of the fan shutting off and then coming back on quickly before or without the burner itself igniting. This seems to happen when the heat exchanger is still so hot that after the fan stops moving air across it, the residual heat in the exchanger warms the air in the plenum enough that it reaches the "turn-on" temperature and the blower starts up again even though the oil or gas burner itself is not yet "on".
A detailed explanation of furnace cycling after the burner has shut off is at FURNACE FAN CYCLES AFTER HEAT where we offer more diagnostic suggestions.
The underlying cause might be the fan limit switch settings (try setting the FAN OFF to a little lower temperature - drop it 5 degrees)
Check the fan limit switch settings: normally we see
FAN ON at 130 °F (or possibly lower)
FAN OFF at 110 °F (or lower)
HI LIMIT at 200 °F (never higher unless the manufacturer so instructs)
This same furnace-fan-runs-again phenomenon may also occur (before the burner ignites) followed by the burner igniting as well while the fan is running. This second case occurs when both the fan has re-cycled on due to what we just explained combined with the thermostat switching back to calling for heat. This process is more likely to occur in very cold weather or in a very drafty building.
While you're at it, be sure that the air flow through the system isn't being reduced by
a dirty air filter
a dirty blower fan
some other obstruction in the air handling system, both supply and return ducts
Continue reading at FAN LIMIT SWITCH or select a topic from the More Reading links shown below.
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 Jeff Wilcox, "Evaluating Duct Work, How to Evaluate Furnace Duct Work & Cure Short Cycling or Inadequate Ductwork Problems" Vermont Department for Children and Families, Office of Economic Opportunity, - dcf.vermont.gov/sites/dcf/files/pdf/oeo/WAPManual/AppendixI.pdf retrieved 12/5/2013. Geoff Wilcox
Vermont Office of Economic Opportunity
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Reference Material for the original article came from:
1. Saturn Mechanical Systems Field Guide
2. Bacharach (Rudy Leatherman)
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