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.
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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
Common Caauses of furnace short cycling or rocking
Common causes of heating furnace short cycling or rocking are
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.
See INCREASING RETURN AIR for details.
Heating Furnace Short Cycling Diagnostic Tests & Inspection Points
In addition to this short list, please read the diagnostic discussion and examples that follow
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.
At AIR FILTERS for HVAC SYSTEMS we discuss various problem experienced by heating systems due to a dirty or blocked air filter.
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
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]
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.
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:
A photograph of the company's sample thermostatic sensors is at left. Contact: Sensata Technologies 529 Pleasant Street, MS B19 Attleboro, MA 02703-2964 Phone 1-888-438-2214 email: firstname.lastname@example.org www.sensata.com www. Klixon.com
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
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
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.
Check the fan limit switch settings: normally we see
Details are at FAN LIMIT CONTROL SETTINGS.
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.
Continue reading at FAN LIMIT SWITCH or select a topic from the More Reading links shown below.
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