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Furnace diagnostic guide for heater cycling on/off:
This article describes what to check if the warm air heating blower fan cycles on and off after the call for heat has stopped, that is after the room thermostat has been satisfied.
Some conditions that cause unexpected furnace fan cycling on and off may be dangerous, risking overheating of the furnace heat exchanger which in turn risks cracks and even carbon monoxide leaks. So it is worth checking out this problem promptly.
This article series describes how to diagnose & fix just about any problem with forced air heating & cooling systems.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
How to Diagnose & Fix a Blower Fan that Cycles On/Off During the Heating Cycle
Discussed here: what to check if the forced warm air furnace blower keeps cycling on and off during the heating cycle - while the thermostat is continuously calling for heat.
Synonyms: Forced warm air heat short cycling - furnace short cycling or rocking, or shutting off.
Where to buy replacement furnace limit switches, parts or repair parts.
Watch out: a return air inlet close to a gas heating furnace like the one shown in our photo may be unsafe, drawing combustion products or fatal carbon monoxide into the building air supply, shown in our photo at the start of this article.
CASE #1 (Common) Furnace BURNER Short-Cycling On and Off During a Call for Heat - High Limit Rocking
In this common situation the furnace heat source or BURNER goes OFF then back ON DURING the HEAT CYCLE while the blower FAN KEEPS RUNNING - that is while the thermostat is continuing to call for heat.
In this situation the temperature inside the supply plenum is becoming TOO HOT - that means that the plenum is at risk overheating - that can cause cracks or damage.
For safety the burner is turned off by the fan limit switch when the plenum temperature reaches or the HI LIMIT set on the limit control switch. The burner will stay off until things cool down.
In general when this problem occurs you should look for an air flow blockage, dirty air filter, dirty squirrel cage blower fan, loose fan belt, etc.
Formal Definition of Furnace Short Cycling (also called Shutting Off or Rocking on the High Limit)
Short Cycling is when the burner shuts off before a call for heat is met (thermostat setting
is not reached).
The burner is shut off by the fan limit switch as a safety measure. The air handler
will keep running.
The temperature in the plenum will go down and the burner will
come on again, only to shut off again quickly.
This rocking back and forth on the HI LIMIT
setting is bad for the furnace.
The heat exchanger is seeing temperatures it was not
designed for, and may crack. It is also very inefficient and does not allow proper heat to be distributed to the living space.
Watch out: a cracked leaky heating furnace plenum risks leaking fatal carbon monoxide into the building's occupied space. Someone could die.
Causes of Furnace BURNER Short Cycling or High Limit Rocking
Common causes of heating furnace short cycling or rocking are listed here alphabetically. The most-common defects include a dirty air filter, damaged ductwork, and inadequate return air.
Dirty air filter: a dirty air filter clogs the flow of air through the system, causing the furnace to overheat and go off on high limit
Dirty air handler blower fan: dirt accumulated on the blades of a typical air handler squirrel cage blower fan assembly can reduce air flow through the system by 30-50% or even more in some cases
Fan limit switch defective: a defective or consumer-damaged fan limit switch may fail to operate normally and in fact may be unsafe - a switch that permits the furnace to overheat can crack the heat exchanger, causing both costly damage and potentially fatal carbon monoxide leaks into building air.
FAN LIMIT SWITCH describes how this control is installed, tested, diagnosed & adjusted.
Warnings include avoiding bending or forcing the control which then pushes it out of calibration.
Furnace pressure switch tubing leak: a furnace that is cycling on-off can be caused by a leak in pressure switch tubing.
that a furnace that cycles on and off may be caused by pressure switch tubing that has a leak.
Improper fan limit switch settings or a damaged or defective control
Furnace defects or air handler defects (wrong blower speed) that prevents the furnace from producing enough heat rise across the heat exchanger.
For a furnace with adequate duct
work this heat rise should fall in the range of 45 degrees to 70 degrees. For belt-driven air handler fans, a too-loose or slipping or damaged drive belt can slow the blower and thus reduce air flow through the system.
Inadequate return air / ducts: omission of proper return air ducts to deliver building air to the furnace return plenum; some systems take some or all return air from a cold basement or crawl space - expensive and potentially very dangerous if the return air can draw flue gases into building air.
Watch out: most modern air handlers include a safety interlock on the air handler door - the switch will turn off the furnace if you open the door with equipment running. This is because it's easy to lose a finger or be even more seriously injured by opening the air handler when equipment is running.
Oil fired heater BTU output estimating tip: If there is no data tag on the furnace or you can't find the oil burner output BTUH you can guess it as follows:
Multiply the oil burner nozzle size (firing rate in gallons per hour) x 138,200 (BTUs per gallon of No. 2 heating oil) x an efficiency estimate (use .75 for older furnaces and .80 for newer furnaces if you don't have a recent efficiency measurement).
Furnace over-sized for the heating load? - usually this just gives too-short on-cycles, an inefficient way to heat the home; in a cooling system an over-sized A/C unit will have short on cycles that lead to failure to adequately dehumidify the building.
Thermostat wiring or device defects: check for loose thermostat wires, debris-clogging of a thermostat's bimetallic spring or thermocouple (and clean them)
Bad thermocouple or dirty thermistor on gas fired equipment: Short cycling or unexplained on-off cycling of heating equipment has also been traced (by one reader) to a dirty or failing thermocouple (possibly the reader meant a dirty thermistor).
Poor electrical connections: a loose or corroded electrical wire bringing power to the burner OR at the limit switch or fan control board can cause intermittent burner operation
The amount of supply and return ductwork a hot air heating system has can affects the actual efficiency of the heating system, as well as the life expectancy of the furnace itself.
isn’t enough duct work to deliver the heat the furnace produces, obviously clients are
cold, operating costs go up, efficiency goes down, and eventually a premature crack in
the heat exchanger will result (the furnace can’t get rid of the heat and it stresses out
the metal in the heat exchanger). 
Watch out: while the Vermont government publication that we quote above explains that short cycling can crack the furnace heat exchanger, we need to understand that that means: a cracked heat exchanger is unsafe because it may leak combustion gases or fatal carbon monoxide into a building. Besides leading to an expensive furnace repair or replacement, a cracked-heat exchanger is dangerous.
Watch out: This return air opening boosts the total air output of the system but it is an improper and unsafe design.
In addition to this return air problem given above check this short list, then take a look at the furnace short cycling diagnostic discussion and examples that follow.
CASE #2: Furnace BLOWER FAN Short-Cycling On and Off DURING a Call for Heat - BLOWER Short Cycling
The furnace air handler blower FAN goes OFF and then back ON during the heating cycle while the thermostat continues all the time to call for heat, and the BURNER KEEPS RUNNING.
In this uncommon furnace cycling case, most likely the blower fan is moving so much air or at such a high velocity or is seeing return air that is so cold that the temperature in the supply plenum falls BELOW the FAN -ON temperature.
In that case the FAN LIMIT SWITCH TURNS OFF the FAN to avoid blowing cold air onto the building occupants. This is the same feature that keeps the fan from turning on before the plenum temperature is sufficiently high: a situation that occurs at the start of a call for heat.
Causes of Furnace BLOWER FAN Short-Cycling During a Call for Heat
That happens most-often when
The fan speed is too high, perhaps on a control board the speed is set too high for the system design or the wrong pulley size has been installed on a belt-driven blower fan
The furnace burner is not adjusted properly - weak flame, inadequate combustion air (dangerous), dirty gas orifices, oil burner in need of service, burnt-out electrical heating elements - depending on furnace type
The heat exchanger is dirty or blocked - not enough heat transfer out into building air
Very cold air entering the system - blower on: poor return air design picking up more fresh cold air and not enough return air from the building can cool down the plenum below the FAN ON temperature.
This may be due to improper location (even an unsafe one) of a main or additional cold-air return.
I've seen systems that were picking up 100% new cold air in a crawl space - no conditioned building air was returning to the furnace - that 's the most-expensive heating design possible and could be unsafe.
Poor electrical connections: a loose or corroded electrical wire bringing power to the blower fan motor OR at the limit switch or fan control board can cause intermittent fan operation
A defective fan limit control switch
Watch out: a number of writers (previously including this page) confuse BLOWER cycling on and off during a call for heat - being discussed as CASE #2 here, and BURNER cycling on and off during heat - discussed at CASE 1
Read people's explanations with care: the overheating in the plenum that causes CASE 1 will shut off the burner, not the fan.
Diagnose Case 2: Blower Short Cycling
Watch the dial on the furnace FAN LIMIT CONTROL switch during a heating cycle. If you see the dial rotate to the FAN ON point you should then hear the blower fan start up.
Keep watching. If the dial rotates back towards cooler temperatures a bit that's normal, but the blower should keep running until the thermostat says it has been satisfied.
IF instead the dial continues to rotate towards lower temperatures until it drops below the FAN ON setting,
THEN something is causing the plenum temperature to get so low that the limit switch should TURN THE FAN OFF - that's to avoid blowing cold air on occupants.
An example of burner-on short cycling or high limit rocking is discussed in the reader Q&A given later on this page at
CASE 3: Furnace BLOWER Fan Short Cycles On and Off at the END of a CALL FOR HEAT - BURNER REMAINS OFF - Residual Heat Purge problem
The furnace blower fan cycles on and off AFTER the call for heat has been satisfied and AFTER the BURNER has TURNED OFF.
Usually this is a problem of residual heat in the furnace heat exchanger rising into and re-heating the plenum air, causing the fan limit switch to turn the fan back on to try to cool down the system by extending the heat exchanger temperature purge cycle time.
Some causes of this trouble include:
Fan speed too high: at the end of heat cycle during purge cycle so much air moves through the heat exchanger and supply plenum that the air therein is cooled enough to turn off the blower; but residual heat in the heat exchanger re-heats the plenum air and turns the fan back on
Very cold air entering the system - blower off: a similar but less common cold air problem can occur just after the furnace burner has shut down, during the heat purge interval.
Abnormally cold incoming air temperature enters the heat exchanger while the blower is still running but the burner has shut down - this is the heat purge cycle that occurs after the thermostat has stopped calling for heat.
The incoming cold air (with burner off) may cool down the supply plenum so rapidly that the fan limit switch, working properly, turns the blower OFF before enough heat has been extracted from the steel or cast iron of 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 in an effort to cool down the system to a safe "turn off" temperature.
In this case the blower will remain off but the fan turns on and off one or more times after the thermostat has stopped calling for heat.
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.
Improper fan limit switch settings or a damaged or defective control
 Jeff Wilcox, "Evaluating Duct Work, How to Evaluate Furnace Duct Work & Cure Short Cycling or Inadequate Ductwork Problems" Vermont Department for Children and Families, Office of Economic Opportunity, - dcf.vermont.gov/sites/dcf/files/pdf/oeo/WAPManual/AppendixI.pdf retrieved 12/5/2013. Geoff Wilcox
Vermont Office of Economic Opportunity
Weatherization Assistance Program
Reference Material for the original article came from:
1. Saturn Mechanical Systems Field Guide
2. Bacharach (Rudy Leatherman)
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