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.
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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
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.
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.
Note the three temperature settings on the fan limit switch dial in our photo just above, and in the sketch higher on this page.
The fan limit switch temperature settings and adjustment procedures are explained in detail
at FAN LIMIT CONTROL SETTINGS.
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.
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.
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. 
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. 
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. 
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.
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
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.
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.
In our FAQs above, please take another look at "Why does the blower fan keep turning on and off repeatedly after the thermostat has stopped calling for heat?" for some discussion of why the blower might cycle on and off once or even twice after the heating furnace has stopped its usual operating cycle.
And 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.
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
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 should be ok.
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
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
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.
DO NOT try tweaking any screws - again I'm worried about safety.
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.
Rather than drillband 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
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.
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.
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
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.
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
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.
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