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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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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 cycle
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.
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.
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.
Signs of trouble with the furnace controls or air flow that cause the blower fan to cycle repeatedly - things to check:
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.
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. 
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:
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.
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 degF. Then when the room temperature has reached 72 degF., 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.
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
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. 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.
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
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.
Reader Question: fan cycles back on without burner operating
We have a Lennox G11 furnace, forty years old and still kicking, by cracky!
Reply to Roy:
This sounds like the same problem Ted discussed just above.
Rather than drillband spray, pulling the whole switch assembly should include the helix, no?
Reader Follow-Up: - Fan Limit Switch Problem Solved
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.
I suggest replacing the control. You might also want to see 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.
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
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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