Useful Thoughts and Actions Taken

Symptoms

The gas main burner would turn on and the fan limit switch torsion switch would rapidly rotate turning on the air handler fan motor then it would appear that the heat exchanger would cool down as you would observe the fan limit switch rotate quickly towards the cool side. This would end up turning off the motor.  The air handler fan motor would cycle on and off  in short cycles.  

Fix

I bought a new fan limit switch and replaced the existing unit. Now the furnace acts as I would have expected it to.  My unit uses the 8 inch long Honeywell part number:  L4064B 2236 L4064 

Notes

• Fan limit switch unit price is very gougy from retail sources like furnace fixit shops.  Bush Heating wanted 150 US dollars.  I ordered one from ebay for 64 dollars.

The fan limit swich bimetallic torsion bar must have failed in my 

Fan Limit Switch

1. Fan does not turn on until heat exchanger has reached set termperature
2. If the motor stops working the gas is turned off if the heat exchanger reaches the limit temperature

Control Circuit Diagram:

Sunbeam Seneca SL-XXX

Research Links

• Test and Replace the Fan Limit Switch on a Furnace
• Gas and Oil Controls:Automatic Pilot Safety Valve – Explains how the mechanics of the pilot light and main gas valve

How the Pilot Light Safety System Works