Furnace Cycling


  #1  
Old 04-06-02, 07:52 AM
chambersdj
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Gas furnace runs a few seconds then shuts off

My 1970's era gas furnace ignites and runs for a few seconds (30-60) then shuts down, only to start up again and run, perhaps for 10-30 seconds, continually recycling this process if the temp is not at the setting on the thermostat. I have a manual fan switch on the furnace so I have been leaving the fan running all the time. This allows the house to heat even though the burners are cycling on and off.

I suspect the thermostat. I had a problem a few years back with it running the furnace for too short a time, but as far as I can tell it is set for about the longest time it can be.

Any ideas?
 
  #2  
Old 04-06-02, 06:18 PM
carlitos 1
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heat running short

Try to replace the thermostate for a digital. it ajust automatic the burning time with your gas control valve
 
  #3  
Old 04-06-02, 07:40 PM
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Several Possibilities

Hello chambersdj. Welcome to our Heating and Cooling forum and the Do-It-Yourself Web Site.

There could be several reasons for the burners to be cycling.
If the burner flames turn on and then turn off, the most likely cause for the problem description is a dirty and or defective flame sensor. The flame sensor is the metal rod element that is heated by either the pilots flame or directly by the burner flames.

One of the most likely causes is a restricted intake air supply or warm air flow thru the ducting and or to and though the registers in the rooms. Check the filter. Replace or clean as needed. Open all warm air registers fully.

May or could be a piece of furniture is too close to the intake air supply and restricting the intake. Could be a defective blower motor that spins too slowly or any of many other air flow problems.

The above conditions will still allow the fan to run but the restricted air flow causes the furnace to overheat which then activates the high limit safety switch which in turns cycles the burners off until the firebox cools down some.

Another possibility could be a defective solenoid within the gas valve itself and or several other componet failures.

Flame Sensor:
This part is located on the burner or near the pilot assembly. If it's defective, it will cause the problem your describing.

The flames sensor element, is the part which a flame heats once a flame is established. Cleaning the flame sensor element may solve the problem. Also check for a good electrical connection of the part.

Not all furnaces have a seperate flame sensor element. If your heating unit is equipped with one, insure the flame sensors element is securely and correctly positioned in the pilot assembly.

Be sure the sensor is being heated by the flames and is properly connected to the computers module or spark module. Be sure that all parts are grounded well and correctly.

The pilot flame sensor is not the same as a thermocouple. It's a true flame sensor that sends a signal the main electronic module that a pilot or burner flame is established.

On some models of furnaces, the pilots flame will be heating this element directly. Cleaning this sensor with fine steel wool or lite weight sanding paper often corrects the problem.

Read the ARCHIVE postings, within this forum, for other postings on this topic and the replies offered.

Regards & Good Luck
Tom_Bart....TCB4U2B2B.....Company Enterprises
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Energy Conservation Consultant & Gas Appliance Problem Diagnostics Technician.
 
  #4  
Old 04-07-02, 10:02 PM
chambersdj
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Heater Cycling

Tom had it right on. And made me feel like a schmuck! Filters were clogged, and the furnace was over-heating causing it to cycle. Checked flame sensor, it was ok. Replaced the filters and giddy-up, no problems.

Thanks Tom!
 
 

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