Furnace turns on after call for heat has been satisfied:

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.

How to Diagnose & Fix a Blower Fan That Keeps Cycling On/Off After Call for Heat or Air Conditioning Has Stopped

Why does the blower fan keep turning on and off repeatedly after the thermostat has stopped calling for heat?

What do you do if the blower fan goes on for its cycle then shuts off only to go on again, going on and off too many times. What is wrong? - Trevor

How do you know if the [fan limit] switch is going bad because my still works but the fan seems to turn on and off too frequently? - Anonymous

Reply: Why the blower fan may turn on and off after the end of a heating (or cooling) cycle

1. Fan cycling at the end of a call for heat -burner has shut off - residual heat in the heat exchanger turns on the fan - discussed below
   
   at BLOWER FAN CYCLES ON/OFF at the END of a CALL FOR HEAT
2. Fan cycling during a call for heat - while burner continues to run - supply plenum temperature drops too low, discussed below
   
   at BLOWER FAN CYCLES ON/OFF DURING a CALL FOR HEAT
3. Fan cycles when there is no call for A/C nor for heat - Check for the presence of a home ventilation system whose controller may be turning on and off to bring in fresh air. In this case the blower fan may seem to turn on intermittently at no particular pattern but in fact it is b eing controlled by indoor and outdoor temperatures, humidity, and settings on a fresh air ventilation system controller.
   
   See an example at VENTILATION, SUPPLY-ONLY
   
   We discuss this problem at
   
   BLOWER FAN CYCLES ON INTERMITTENTLY when there is no call for heating or cooling on our thermostat switch settings page where a reader posted that question.
4. Check for damaged thermostat or control wiring or control board or fan controller - discussed below
5. Check for damaged fan limit switch - discussed below
6. Check for improper location or installation of the fan limit control & its sensor - discussed below, for example a lost insulating gasket

According to Honeywell, voltage transients or even other conditions around the limit switch that impact the temperatures to which it is exposed can affect the cut-on or cut-off temperature behaviors of the switch, but the company's instructions to not mention apparently excessive on-off cycling of the furnace blower fan traced to the limit switch.

First let's review the normal warm air furnace operating cycle and fan limit switch settings

It is normal for the blower fan to continue to run for a brief time at the end of a heating cycle after the room thermostat set temperature has been reached - that is, after the room thermostat has been satisfied.

But the blower fan should not keep turning on and off at that point (except for the odd case of someone upping the thermostat or blowing cold air on the room thermostat right after it just told the heating equipment to shut down.)

Here we explain what might bne wrong with the furnace or blower equipment. But first let's review normal operation just a bit further: when the HVAC system is in HEATING mode and the thermostat calls for heat, the oil or gas burner turns on and begins to warm the heat exchanger.

When the air in the supply plenum served by the heat exchanger becomes warm enough the blower fan turns on to begin circulating building air through the occupied space.

On most warm air heating systems both the burner and the blower fan unit will run continuously until the temperature at the wall thermostat rises to the thermostat set point. Then the thermostat stops calling for heat and the burner turns off.

The blower fan, however, will continue to run until the heat exchanger and the supply air plenum have cooled down.

That "run-on" period scavenges otherwise wasted heat and sends it to the occupied space, and it prevents warping and possible cracking damage to the heat exchanger by cooling it down.

Finally, when the heat exchanger and supply plenum have cooled, the fan limit switch will shut off the blower fan. The system stops.

What are the Three Temperature Settings on the Fan Limit Control Dial?

Note the three temperature settings on the fan limit switch dial in our photo, and in the sketch higher on this page.

1. FAN-OFF: the lowest temperature setting (blue arrow, the left end of the dial) is the "FAN OFF" temperature.
   
   The FAN OFF setting lets the furnace blower continue to run for an interval after the furnace burner has turned off, but will shut the blower off after the heat exchanger has been cooled down to this temperature (usually 90 °F).
2. FAN-ON: the middle setting on the fan limit switch dial (green arrow) is the "FAN ON" temperature. Ths setting turns the blower fan ON when supply temperature is rising, and is usually set around 130 °F.
3. FAN LIMIT OFF: The rightmost setting on the fan limit switch (red arrow) is the "FAN LIMIT OFF" temperature, a safety setting that turns off the burner if the system temperature becomes too high. This switch is usually factory-set at 200 °F.

The fan limit switch temperature settings and adjustment procedures are explained in detail

at FAN LIMIT CONTROL SETTINGS.

Common, acceptable extra furnace blower on cycles?

But because on some furnaces the heat exchanger itself and the combustion chamber below it are still hot, that residual heat continues to heat air in the supply plenum (where the fan limit switch is located).

If the temperature rises enough in the supply plenum, that will activate the fan switch once again, causing the fan to turn back on again.

This happens because the fan limit switch turns the blower on when the air temperature in the supply plenum rises above an on set point, both to deliver warm air to the building interior AND to prevent the heat exchanger from overheating.

That is, the fan is blowing to cool down the supply plenum - a step that helps prevent heat cracking or damage to the heat exchanger.

So at some warm air furnace systems the blower might turn on once, even twice after the call for heat has been satisfied.

Typically, because the oil or gas burner is NOT running (because the thermostat is not calling for heat), the blower fan should not run more than a minute or two before stopping.

 

 

Signs of trouble with the HVAC controls or air flow can cause the blower fan to cycle repeatedly / intermittently

Heating systems: If however when the thermostat is not calling for heat the blower fan comes on and runs repeatedly and for longer intervals then the fan limit switch may be defective, or there may be a problem with the airflow rate or temperature through the system and you need a service call.

Heating and/or cooling systems: If your air handler blower fan continues to turn on and off intermittently more than 15 minutes after your heater or air conditioner has last cycle "on", there may be a fresh air venting sysetm installed, or there may be damaged wiring or a bad fan relay or fan control board

Check for an improperly installed fan limit switch

If the fan limit switch was installed askew, or if the sensor element (a long bimetallic spring in a metal protective enclosure) is too long, such that either defect causes the sensor assembly to actually touch an internal steel part of the heating furnace, then the switch will not perform properly.

[Click to enlarge any image]

A "too long" fan limit switch sensor problem may occur if a previous replacement of the fan limit switch installed the wrong model - a unit that did not match the original.

Watch out:  Some fan limit switches are mounted using a rigid bracket that requires tightening a set-screw (red arrow in the edited Honeywell sketch at left] to hold the limit switch in the bracket.

But a simple error of mis-locating the bracket set screw can cause the screw to contact the bimetallic spring inside the switch - a dangerous condition that can cause the switch to fail to shut off properly in response to high temperature. [1]

Also the fan limit switch should be installed in the same location on the furnace as the original switch.

If someone relocated the switch it may be in a too cool or too warm location, or in a location that does not allow the switch to reliably sense supply plenum air temperatures.

The Honeywell L4064B fan limit switch is designed for use in both line voltage and low voltage installations. But for low-voltage installation a brass jumper must be removed (red arrow at left). Honeywell warns:

Failure to remove brass jumper, if limit switch is in low voltage circuit, can cause electrical shock hazard or damage low voltage controls. [1]

Check for a missing heat insulating gasket at the fan limit switch mount

At furnaces set to higher operating temperatures, a heat-insulating gasket is required between the switch mounting contact body and the surface of the furnace.

If plenum surface temperature exceeds 190OF (88OC), heat insulating material or a mounting bracket must be used. [1]

Check out the wall thermostat for heat call cycling

Before replacing the fan limit switch, try checking the wall thermostat too. Some thermostats can develop a contact bounce, or may have a failed heat anticipator circuit, resulting in some cycling of actually calling for heat.

To debug this condition try calling for heat up to a fairly high thermostat setting, say 74 °F. Then when the room temperature has reached 72 °F., disconnect the thermostat wires at either the thermostat or the furnace control. That's essentially the same as the thermostat reaching its set point and ceasing to call for heat.

Now watch the performance of the furnace blower assembly. The furnace burner should stop, but the blower fan should continue to run after the call for heat stops until heat has been extracted from the heat exchanger and supply plenum. If the blower fan cycles on and off as before, then the problem is not the wall thermostat.

Typically the HVAC tech will replace the fan limit switch in these conditions. The switch itself retails for around $100.

 

Furnace Fan turns OFF then Back ON During the Heating Cycle

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 at CASE 1: the furnace BURNER cycles ON-OFF DURING CALL for heat, BLOWER remains ON - high limit rocking, 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. [2] 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:

• Partsguy.com Inc. 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 U.S.A. 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 terminations.

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: sensors@sensata.com 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.

Reader follow-up:

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, Natural Gas 5 amps.

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.

Reply:

The Gaffers and Sattler Model S 80 FDF is a gas fired furnace that we also cite

at THERMOCOUPLE REPAIR / REPLACEMENT. 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 degrees 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 FAN Short Cycling with Burner OFF - at End of a Heat-On Cycle

Typical Extra Fan Cycles Due to Hot Heat Exchanger

Reader Question: fan control comes on every 10 seconds after initial shut down of burner and fan - about 5 times, then it stays off

Why does the fan control come on about every 10 seconds after the initial shut down of the fan it does it about 5 time befor it stays off - Ted Aston 11/6/2012

Reply:

Ted,

Try lowering the FAN ON temperature by 5 degrees or widening the gap between the two lower settings, FAN OFF and FAN ON by about 5 degrees.

Also look for something blocking airflow.

When the fan cycles back on while the burner remains off we figure that the heat exchanger is still so hot that even without the burner being on, the air in the plenum heats back up after the fan cuts off, causing the switch to turn the fan back on.

 

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)

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.

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

 

Change the Fan Limit Control Settings to Stop Fan Cycling?

Reader Question: setting the fan limit switch FAN OFF down 5 degrees seemed to fix extra fan-on cycling - is this OK?

I notice that the fan on the heater came on briefly after the heating cycle concluded. I checked this article and decided to adjust the fan switch shut off temperature from 90 degrees to 85 degrees. So far the problem of an additional, short running fan cycle has not reoccurred. I've not read where this remedy is recommended. - Larry K 1/8/12

Reply:

Thanks for the comment Larry. Indeed in the article above we discuss the concern of blower fan cycling on and off one or more extra times at the end of a heating cycle. If that small shut-off temperature fixed the problem that's a great tip for other readers.

Just be sure that none of the more problematic reasons for fan cycling are the cause (as we describe above) and you may be ok, but

Watch out: I suspect that your fan limit switch is defective or that one of the other causes we discuss is causing those extra fan-on cycles at the end of your heating cycle.

Reader Question: fan cycles back on without burner operating

We have a Lennox G11 furnace, forty years old and still kicking, by cracky!

However, it's recently developed a quirk: Sometimes the blower won't turn off, after the furnace cools down.

I've traced the problem to the L4064 fan-limit control unit. The fan-relay switch sometimes is not springing open like it should, but stays stuck in the closed ("fan-on") position.

When this happens, I've tried jiggling the rocker arm that toggles the relay, but this doesn't help. Nor does tapping the unit lightly with a stick. The only remedy I've found is to rotate the bimetallic shaft and then let it snap back sharply.

1. Would it help to spray some compressed air around inside the unit, near the relay switch, in case there's some gunk that has collected on the contacts?

2. On the narrow side of the unit (not visible in your photos) there's a tiny imbedded screw that I imagine is part of the fan relay component inside the unit. What's the purpose of this screw? Can tweaking it solve the above problem?

Thanks for a great site! - Roy 11/6/2012

Reply to Roy:

This sounds like the same problem Ted discussed just above.

IF a fan limit switch is not turning off the furnace, I would

- inspect the switch and hot air plenum for dirt and debris and clogging

- replace the switch

Watch out: I appreciate your cleverness in snapping the switch around but it's risky - forcing spring-loaded rotating parts risks subtle bending of the internal spring parts that can de-calibrate the device and make it unreliable and even dangerous.

So sure you could try cleaning the sensing portion of the bimetallic spring that projects into the hot air plenum, just use air and maybe a soft brush - don't force anything, for the same reason I gave above.

The switch is not costly - I'd conside replacing it.

Please send me (use the CONTACT US link) a sharp photo or two of the scrw that you are discussing so I can research it.

Watch out: DO NOT try tweaking any screws - again I'm worried about safety.

Reader Follow-Up:

Hi,

You described my problem as the furnace not turning off.  However, the furnace turns off fine.  The fan is what doesn't turn off (occasionally)

Attached are two photos of the fan limiter on my furnace (Lennox G11).

Photo screw.jpg shows the screw I was referring to, the one that's probably imbedded in the fan-control switch and looks like an adjustment screw.  A yellow arrow points to the screw.

Photo limiter.jpg show the fan limiter itself.  This photo also has a yellow arrow, pointing to a mounting screw.  This screw might be the solution to my problem.

You're probably saying, "That's a mounting screw.  How could that possibly cure a fan problem?"  The answer is that the fan control switch has been binding, and the binding may be due to that screw being overtightened.

This occurred to me because the fan-control switch works fine whenever I've removed the fan-limiter bakelite assembly for examination.

So yesterday, when the fan once again failed to turn off, I decided to test this theory.  I got a screw driver and started to loosen the mounting screw.  I hadn't gotten more than 1/32 of a turn before the fan turned off.  Coincidence?

Normally, the tension on that screw is probably not a factor.  But as you can see in the picture, the fan-limiter bakelite case is damaged.  I had broken off the stub that's now held on only by electrical type and a bolt used as a buttress.  

Since the stub is no longer intact and not longer helping to keep the bakelite case rigid, the case can probably flex a little bit too much.  To compensate, the mounting screw may need to be tightened only gingerly.

I'll let you know in a week or so if loosening that screw has solved the problem.

Thanks.

Reply:

Rather than drill and spray, pulling the whole switch assembly should include the helix, no?

Have you tried replacing the fan limit switch assembly? If you do so, keep the old parts for our study

Reader Follow-Up:

Daniel,

I don't remove the assembly, I simply remove the electrical module, by unscrewing its three mounting screws.  The assembly case and the helical cylinder stay undisturbed and attached to the furnace.

The backside of the electrical module is solid plastic, with holes only for the three mounting screws.  So simply removing the module does not give me any better access to cleaning the inside of the module.

Likewise, removing the entire assembly wouldn't add any benefit.  Nor would there be any benefit to removing the helical cylinder.  (As I mentioned, the helical cylinder isn't causing any problems.)

The price for a new assembly (helical cylinder and electrical module) is around $190.  I haven't found a shop that will sell the electrical module individually,  but I imagine it wouldn't be cheap.

Regards,

Roy

Reader Follow-Up: - Fan Limit Switch Problem Solved

Dan,

Problem solved.

To recap the problem: The furnace blower would occasionally not turn off when the furnace cooled down. I traced the problem to the heat-sensor assembly's plastic case, which contains the fan on/off switch.

The solution was obvious once I realized that pushing the rocker arm is what turns the fan switch off. Originally, I thought that releasing the rocker arm is what turns the fan switch off.

The rocker arm has a roller on the end that contacts a cam on the helical-cylinder heat sensor.

Yesterday I noticed that, to turn off the fan, the rocker arm had to be pushed further than the cam was now pushing it. (The roller doesn't look like it's worn down, so the problem may actually have been due to my previously damaging and/or poorly repairing the assembly's plastic case.)

So I increased the diameter of the roller by winding one turn of electrical tape around it.

The fan now works like a charm.

It's true that, when the furnace gets running, the cam gets pretty warm. But it's still cool enough to press my fingers against, so I'm not worried about the masking tape going up in flames. - Roy L.

Comment & Warning:

Excellent detective work, Roy, I'll be sure our entire conversation appears in the original InspectApedia article as it will surely help other readers. Thanks so much.

Watch out: But a safety warning: even if temperatures are low enough that you're not worried about that tape burning up, often at high temperatures the adhesive on tape becomes soft and gummy - the tape may not stay in place. If the tape comes off, the problem could be dangerous if it jams the workings of what is intended to be a safety control.

A tinkerer might check for a bent rocker arm or a misaligned part, but a heating professional, concerned with safety, would almost certainly replace the entire assembly.

I suggest replacing the control. You might also want to

s ee FAN LIMIT SWITCH TROUBLESHOOTING

Reader followup:

Given your advice about the tape adhesive getting gummy, I'll keep an eye on it, and will think about some other way to take up the slack in the rocker arm. - R.L.

 

Clean the Fan Limit Control to Stop Short or Extra Cycling?

Reader Comment: reader suggests cleaning the fan limit switch

The question above is: Why does my blower motor keep coming back on for short/intermediate/momentary bursts/pulses immediately after the furnace cycles normally? I assume we are talking about L4064's and there equivalents/competitors

The answer was not helpful. The fan/blower limit switch may be "sticking" at the on or "OFF" setting or intermediately one or the other.

It is, after all, an electro-mechanical device, relying on a spiral shaped bimetal coil to turn a disc that has mechanical contact points for the on and off settings for the blower.

That heat sensitive bi-metal spiral attached to a shaft passes through a bushing that simply dries out after hundreds if not thousands of cycles...then starts sticking and jumping between the on and "OFF" settings instead of smoothly transitioning as it did when it was new.

Do you need a new $100 fan limit switch? Maybe! But first try removing, inspecting, cleaning and applying an appropriate hign temperature lubricant. Unless the spiral bi-metal coil is broken or the on/off/limit contacts are completely worn down...you may be surprised at how much life is still left in that old limit control switch. - Kevin Sharpe

Reply: we do not recommend disassembly nor modification of heating system safety limit switches or similar controls

Kevin, you are correct that a typical Honeywell fan limit switch retails for around $100. But your advice that people try disassembling, cleaning, lubricating the switch makes me nervous.

Honeywell's own installation instructions (see our references below) mention nothing whatsoever about such steps, but the company does warn more than once that care must be taken to avoid bending internal springs and parts of the switch.

Watch out: If these sensitive parts are modified in any way, say by bending or even by a blob of grease left by someone who may be less meticulous, the switch loses calibration and could fail to shut off the system at high temperatures - an unsafe condition that could also damage the heat exchanger.

If you discover other reasons for unexpected heater blower assembly cycling on and off after the end of a heat-on cycle, do add those comments below. Thanks - Ed.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2022-01-31 by Inspectapedia Com Moderator

@Trace,

Focusing on your observation that the blower fan runs a bit after the burner has shut down,

When the fan turns back on and runs for a time after the end of a heat-on cycle it's purging additional heat that radiated into the plenum from the combustion chamber and/or heat exchanger. It's cooling the heat exchanger and avoiding wasting that heat - sending it into the occupied space.

In general in heat mode a furnace blower should not turn on to blow cold air onto occupants. The fan limit control turns the burner on when the plenum is warm enough and off when cool, and will shut off the burner itself if the system is overheating.

So there's also the possibility that the system in question is overheating - such as from a dirty furnace or a duct obstruction or a dirty heat exchanger. If the air filter is new then the system probably needs service.

Watch out: if your burner ON time is short - just 1-4 minutes, that's probably abnormal and could point to a dirty air filter and overheating heat exchanger. That condition will shut down the burner early.

On 2022-01-31 by Trace

This has happened a few times but not consistently. When it has, it has been mostly in the morning on the first heat call.

We use a wood burning fireplace insert as our primary heat source and many times the furnace does not run all night. The oil furnace will light and burn normally for a few minutes. The burner will shut down with no error code or lock out.

Approximately 1 minute after shut down the the blower fan will turn on and run for a few minutes. After this the furnace will run normally till I get the wood burner cleaned and lit at which time the thermostat is turned down and the furnace wont be used for the rest of the day. Thank you.

On 2022-01-28 by Inspectapedia Com Moderator

@Mary,

Watch the temperature dial on the fan limit control switch in your furnace blower compartment.

From your description it sounds as if your furnace may be overheating - if the burner turns OFF and you see that the fan limit controller is at the highest (emergency shutoff) temperature on the dial, that's what's going on.

Causes can be as simple as blocked air return or a very dirty air filter.

To see what the "Fan Limit Switch" looks like

see FAN LIMIT SWITCH

If you don't find something simple like a dirty furnace filter or something visually obvious like a disconnected or crushed air duct, you need to call your heating service company for repair: remember that if the furnace burner is turning OFF unexpectedly as you describe it may be due to a safety control intended to prevent a more serious event like damage to the furnace itself (from overheating) or even a building fire.

Keep us posted: what you find will help other readers.

On 2022-01-28 by Mary

Need help. Furnace heat turns on after one minute fan kicks in, after another minute heat turns off and fan continues for another 3 minutes. Then there is a break of about one minute and the process is repeated. This seems excessive too me and shouldn't the heat stay on longer?

On 2021-11-29 by Inspectapedia Com Moderator

@Lee,

I'm starting with the easiest guess and easiest thing to check first.

"Thermostat goes blank" tells me that most likely the thermostat is losing power - that could be a failing low voltage transformer or a loose or damaged thermostat wire.

If you remove the thermostat completely and join the two heat wires of the thermostat that's the same as what happens if room temp drops and the TT calls for heat. DO that and let me know if the furnace runs longer and normally until you disconnect the wires.

The next step would be to disconnect the wires at their other end - at the heater and using a VOM or DMM check for wire shorting or opening.

On 2021-11-29 by Lee

You seem the best on the web to answer my concern.

I bought a new house 2 years ago with a carrier 60btu gas furnance. It originally had a core thermostat but that kept clutching after 1 year.

Builder changed for ecobee3lite thermostat. Problem not, one year later, either thermostat or furnace is problem.Thermostat call for heat as usual and furnace comes on runs for approx 4 minutes, thermostat goes blank and fan runs for approx 8 more minutes.Thermostat comes back on after that 4 minutes to calibrate.

This also happens when I bump temp up 2 or more degrees to warm house. It will cycle on till a certain temp then off again for another cycle till it gets to temp ran up.It also does this in automatic mode.

Shouldn't furnace stay on till desired temp reached. I've read a lot of literature but not for this exact problem. Is this the new thermostat or furnace problem. I'm at my wits end. Thanks,Lee

On 2021-11-16 by Inspectapedia Com Moderator

@Aaron T.,

Watch out: first priority: I see what looks like burn marks on the side of your furnace.

If so then the heater may hav been overheating, or might have a combustion chamber defect, and might be damaged internally and may be unsafe; you should be sure to have it inspected, tested, adjusted by a trained service technician.

Watch out: I also see that there have been leaks down your chimney, running out below the flue vent connector thimble: a condition that tells us that there is a chimney problem such as a missing or improper chimney cap, top seal, or other chimney leaks, AND possible draft defects,

AND

there is risk that water leaks down the chimney, through the flue vent connector, and into the heater itself, causing rust or cracking damage that can make the heater unsafe. Same advice as above.

Watch out: finally, among the casual observations we can make from your single photo and thus necessarily incomplete, I see what looks like a simple single air return at the lower bottom back of the heater. If this is a forced warm air furnace intended to provide central heating to a home, that single air return is usually the least efficient, most-costly way to heat a building.

That said, moving on to your question about normal fan limit and blower operation, you're not entirely correct in your summary.

While it's correct that from a period during which the furnace has been off for some length of time, the furnace ought not turn on until there is first a call by the thermostat and second the burner has run enough for the heat exchanger and plenum to reach the FAN-ON temperature,

there other cases that will cause a blower fan to cycle ON AFTER a call for heat - explained above on this page,

and

there are still other cases that will cause a blower fan to cycle OFF then BACK ON repeatedly DURING a call for heat, as explained at

FURNACE FAN CYCLES DURING HEAT

On 2021-11-16 by Aaron T.

@Inspectapedia Com Moderator, thank you for the suggestions. Maybe I am not understanding the proper function of the limit switch.

I'm under the assumption that both these requirements must be met for the blower to engage under normal conditions - the limit switch must read 130 degrees or higher aka the On fan limit is met, and the thermostat must be calling for heat. If both of these are not met the blower should not turn on? Am I wrong in this assumption?

Also, I do not have a heat exchanger. The blower pushes air across the wood furnace housing and into the duct work above. Fan is clean, air filter is clean.

On 2021-11-15 by Inspectapedia Com Moderator

@Aaron T.,

I don't completely understand the problem description, but if you're saying that the blower runs when there is no call for heat, particularly if that happens after there was a call for heat and the furnace is still hot, it's probably one of the conditions described above on this page.

Look at
- dirty heat exchanger
- dirty blower fan
- dirty air filter
and less likely a bad burner control or wiring

On 2021-11-15 by Aaron T.

I have a Royall wood/coal furnace with a honeywell limit switch. Recently the blower acts as if the thermostat is constantly calling for heat and will turn on when the limit switch reaches the "On switch temperature limit" till the limit switch cools to the "Off switch Temperature Limit" even after the thermostat temperature has been met.

I have turned the thermostat off and even completely removed the thermostat from the wall and the blower continues to turn on. Thinking the problem was in the limit switch I replaced it. Unfortunately i'm still experiencing the same problem. Any suggestions?

On 2021-11-13 by Inspectapedia Com Moderator

@Andrew Haninger,

See your initial question and our reply in the Reader Q&A section on this page:

FAN LIMIT SWITCH FAQs #3

On 2021-11-13 by Andrew Haninger

Hello. I posted here not too long ago and saw a reply, but my post seems to have disappeared.

I have a mid-80s Bryant gas furnace (not sure of the model, but it is obviously old). I believe the blower fan is newer (replaced in the past 7 years or so?)

The problem is that the fan comes on once or twice after a satisfied call for heat. So far, I can predict whether it will come on a second time based on how long it comes on the first time. If it runs for about two seconds and then shuts off, it will come back on, run for about a second, and then shut off until the next call for heat. If it only comes back on for about a second, it will not come on again until the next call for heat. This suggests to me that the fan control switch is faulty. Trouble is, I cannot locate the switch for the life of me! There is a large sealed metal box in front of the fan, but I'm not positive if it's function. It's bigger than the fan control switches I've seen in this article.

I've done mostly visual inspection by removing the furnace and blower housing covers and the problem went away for a few days.

I've replaced the filter and recently switched to filters with a lower MERV rating with the idea that it would improve air flow and maybe be closer to what the system was designed for.

I've had a tech out about a year ago who did a visual inspection, but didn't have any other recommendations.

It could be a dirty blower, but I'm a bit shy about taking it all apart in case I couldn't get it back together. Visually it looks okay. I'm in Ohio where a working furnace is a necessity in the winter, so I don't want to break it if it's still basically working.

On 2021-01-05 by danjoefriedman (mod)

Kenny

Usually the blower turning back on tells us that there was more residual heat in the supply plenum than we want - the fan stops, the residual heat heats the air in the plenum, and the switch turns the fan back on.

That can be due to a dirty part-clogged heat exchanger or due to any other air flow restriction such as a dirty blower fan or even a dirty air filter. `The reduce air flow isn't extracting enough heat (and it's increasing your heating cost).

On 2021-01-05 by Kenny

I have a 1982 Bryant furnace which will be replaced in the spring. It works normally until it shuts off, then the blower motor will turn back on and then shut off again. Any ideas?

We've had 3 repairmen here that cleaned the burner and thermostat, however it was cleaned earlier this fall. We know it's time for a new furnace, but we just purchased this house and want to get the rest of the winter out of it.

On 2021-01-02 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

On 2021-01-02 by Scott

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 2020-11-01 by (mod) - L4064W Tradeline controller on an old gas furnace

Yes, Winston, particularly as once having manually rotated the face plate or dial inside the limit control you've bent the internal spring out of calibration.

The result is a possiblity of malfunction of the control that, like a chain of events in "for want of a nail a shoe was lost" story, ultimately the furnace can become heat damaged, risking fatal carbbn monoxide poisoning of building occupants.

On 2020-11-01 by Winston Smith

I have a L4064W Tradeline controller on an old gas furnace. When I push the button on the controller in to turn the fan on manually, it will stay on when I pull the button out. I have to manually rotate the scaleplate to turn the fan off. Otherwise, it functions correctly. Should I replace the controller?

On 2020-04-18 - by (mod) -

IF the smart thermostat isn't connecting with the app on your smartphone you probably need to go through the initializing procedure again at the thermostat: power it off, back on, connect to your network, etc.

On 2020-04-18 by Kathleen Oboyle

Gas furnace. Thermostat set to 68, room temperature is 68. Furnace comes on for a few seconds, than shuts off. Smart thermostat no longer communicates with app...

On 2020-01-15 - by (mod) -

Scott

If you mean that the blower fan cycles on and off DURING a call for heat, please see the diagnostics at

FURNACE FAN CYCLES DURING HEAT

On 2020-01-15 by Scott

Why does my oil burning furnace blower come on twice on a cycle ?
How can i fix this problem ?

On 2020-01-02 - by (mod) -

that sounds to me like a bad relay

On 2019-12-31 by Anonymous

My name is Trident and I have a Payne furnace model 395B036040 is making clicking sounds intermittently after I turn on the thermostat and stop blowing out the flame.
First, I thought this might be the flame sensor gone bad since the furnace is 30 years old.

Bought new flame sensor and replace the old one, however, problem still existed. Second, I noticed that the relay of the furnace inducer control board flashing lights while the furnace is making clicking sounds, Not sure what is wrong with the unit, please help.
Thanks!

On 2019-12-14 - by (mod) -

Chuck

It would be helpful to know the brand and model of equipment involved as then we could check the IO manual. It sounds as if either there is a fuel or burner problem OR a control board or relay failure.

On 2019-12-14 by chuck

my issue is the exhaust fan blower on my gas furnace just keeps cycling when the thermostat is on the unit never fires the burners as the vent blower goes on and off every 15 seconds or so

On 2019-11-24 - by (mod) -

Mike

Thank you for the interesting question. It's interesting because you're describing two different problems that might even have two different causes.

For a furnace that Cycles on and off during the call for heat,

In the CONTINUE READING links you'll find the link for

FURNACE FAN CYCLES DURING HEAT

On the other hand for a fan that continues to run longer than you think it should after the burner on the furnace has turned off you should review the Diagnostics above on the current page. Take a look at those two information sources and ask for the questions as needed. Be glad to hear from you.

On 2019-11-24 by Mike

I am having two issues with my thermo pride oil furnace. First issue is that the fan does not stay running once it reaches temperture but cycles between off on settings,it's a honeywell limit switch. My second problem is that the unit continues to run even when there is no call for heat,it has a honeywell r7284 oil primary control. It will only stop if Ichose no on tt configuration.

On 2019-10-08 by (mod) - How do you wire a fan control limit switch from a wood burning unit to my propane furnace

Tonia

Apologies but I don't understand enough about your installation to offer advice with any confidence that it's safe. Normally I'd expect a separate wood burning hot air heating system to have its own limit control on the blower unit fan as well as of course additional temperature controls and safety controls on the wood fueled heater.\

However that's no reason to throw up our hands in dismay and just walk off or climb over the border-wall. Let's start by identifying the brand and model of your wood fired heater; then we can make sure you have the manufacturer's instructions and installation guidelines (and tech support number).

On 2019-10-07 by Tonia

How do you wire a fan control limit switch from a wood burning unit to my propane furnace

On 2019-02-27 by (mod) - no heat after gas was shut off

Connie

It might be that there's a pilot light that needs to be real it on your heater. That's the place I would start

Sometimes you need to hold the pilot light button down for a while to purge air from the gas line after gas has been shut off.

On 2019-02-26 by Connie

Furnace was working perfectly. Due to evacuation in my area gas was shut off. When they came to turn gas back on furnace doesn't work. Can shutting the gas off and turning it back on damage the main valve in my furnace?

On 2019-01-19 by (mod) - I have an electric furnace that is bugging me

Dan

Can you ID the brand and model and thus find the manual? We need first to check the dip switch or pin settings on the blower control board.

On 2019-01-16 by Dan W. Johnston

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 2018-12-26 by (mod) -

RK

Thank you so much for the very helpful and interesting post about the heat of a pilot light being sufficient to cause Purge cycling of the Furnace blower fan. That's not a condition I had heard of before. It's important for us to keep that note with this article as it will certainly help other readers.

Could you tell me the brand and model of your furnace and perhaps if you're inclined you could post a picture of the pilot assembly. I'd like to research this further.

On 2018-12-25 by R Kaz

My furnace blower would kick on and off after its heat run cycle. I turned the pilot flame down slightly and that solved my issue. The pilot puts out eniugh heat at higher levels to engage blower relay on some furnaces.

On 2018-12-11 by (mod) - My furnance is doing warm and cold cycles constantly.

Summer

What you describe is not normal and not proper operation of a heating furnace.

It is possible that the problem is as simple as a dirty air filter or dirty blower fan in the air handler.

Check to see if an air filter is improperly installed or is of a type that obstructs the air flow more than the furnace can handle.

On 2018-11-28 by Summer

My furnance is doing warm and cold cycles constantly. I told my landlord this and he keeps telling me it's normal. Before he supposedly fixed the problem the furnace ran fine. Almost going on 2 weeks without proper heat. Can someone please tell me what could be wrong

...

Continue reading at FAN LIMIT SWITCH - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see FURNACE FAN CYCLES AFTER HEAT-FAQs - Questions & answers posted originally at this page

Or see these

Recommended Articles

• AIR HANDLER / BLOWER UNITS - home
  • BLOWER DOOR SAFETY SWITCH
  • BLOWER FAN ASSEMBLY CLEANING
  • BLOWER FAN CONTINUOUS OPERATION
  • BLOWER FAN CYCLES ON INTERMITTENTLY
  • BLOWER FAN OPERATION & TESTING - home
  • BLOWER FAN SPEED SETTINGS - how to set the air handler fan speed
  • BLOWER FAN WONT START
  • BLOWER FAN WONT STOP - LIMIT SWITCH
  • BLOWER FAN WONT STOP - THERMOSTAT SWITCH
  • FURNACE BLOWS COLD AIR
  • FURNACE FAN CYCLES AFTER HEAT
  • FURNACE FAN CYCLES DURING HEAT
  • FURNACE FAN STOPS EARLY
• FAN, AIR HANDLER BLOWER UNIT
• FAN LIMIT SWITCH TROUBLESHOOTING
• FAN MOTOR START CAPACITORS
• FAN ON AUTO MAN THERMOSTAT SWITCH
• THERMOSTAT SWITCHES, EXTERNAL - switches may be turning on the fan
• UNDERSIZED RETURN DUCTS - not enough return air

Suggested citation for this web page

FURNACE FAN CYCLES AFTER HEAT 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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