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furnace fan limit switch control switch (C) Daniel FriedmanFurnace Blower Fan Or Burner Cycles On/Off During the Heating Cycle
Forced air heating furnace control diagnosis & repair for furnace short cycling

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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

Unsafe return air input at furnace © D Friedman at InspectApedia.com 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.

See UNSAFE DUCT OPENINGS.

[Click to enlarge any image]

Why does the furnace keep turning on and off repeatedly during the heating cycle (the thermostat is calling for heat)?

Our furnace seems to turn off and then back on repeatedly during the heating cycle.

I've read that normally on a call for heat the burner should come on, then the blower, and both should keep running until the call for heat is satisfied.

What's going on? - Anonymous

Reply: Clarify: Three Different Furnace Short Cycling Problems

Thanks to reader Tim (2019/04/12) we have first to distinguish three different furnace burner or blower-fan on-off cycling problems:

CASE 1: the furnace BURNER cycles ON-OFF during a call for heat, BLOWER remains ON - high limit rocking

CASE 2: the furnace BLOWER FAN cycles ON-OFF while BURNER remains ON - blower short-cycling

CASE 3: the furnace BLOWER FAN cycles ON-OFF AFTER a call for heat, BURNER remains OFF - residual heat purge problem

But first let's check the fan limit switch settings: normally we see

Details about these three settings are at FAN LIMIT CONTROL SETTINGS.

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. [1]

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.

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.

If there 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). [1]

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

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

EXAMPLE of CASE-1 LIMIT ROCKING with the burner staying on and the fan going on and off

 

 

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:

Details about this problem are

at FURNACE FAN CYCLES AFTER HEAT

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Continue reading at FAN LIMIT SWITCH or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see FURNACE FAN CYCLES DURING HEAT FAQs - questions and answers posted originally on this page.

Or see these

HVAC Blower Fan Articles

Suggested citation for this web page

FURNACE FAN CYCLES DURING HEAT at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

INDEX to RELATED ARTICLES: ARTICLE INDEX to HEATING FURNACES

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