Furnace Only Lights With Fan Set to "On"


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Furnace is exhibiting strange behavior and I can't figure out what part has failed.

System is set to "Heat", fan set to "Auto", and temperature set to 74.

Room temperature drops to 70 and furnace will not start.

I move the fan switch to "On". a moment later, the fan starts running and I set the switch back to "Auto". The fan continues to run, and, the furnace heats the house up to the set temperature. Then everything shuts off and will not start again, no matter how cold it gets, until I move the fan switch back to "On".

OR

I move the fan switch to "On" and leave it there. The fan motor runs continuously but the furnace cycles on and off, maintaining the set temperature in the house, even if I run it this way for 8 hours.

Maybe it's the blower motor temp switch but I don't understand this behavior. Thoughts as to what is happening are appreciated.

TIA
 
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Through a process of elimination we know 1. the furnace is working and heating the house 2. the heat sensor is functioning because the heater is shutting off when the correct temperature is reached. This would seem to indicate that the problem lies in the control unit. Is it a mechanical or digital unit?
 
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Here is a link to the furnace details: http://herberts.org/furnace/furnace.htm

The thermostat is a Sensi Smart (https://sensi.emerson.com/en-us/products/wifi-thermostat) which has been installed for about 3 years and seems to be working OK.

So, if I've got this right:
  1. Thermostat senses low temp condition and sends signal to furnace
  2. Furnace lights
  3. Blower temp switch starts the blower motor once the exchanger reaches operating temp
  4. Furnace runs with blower until thermostat senses temperature is correct
  5. Thermostat sends signal to furnace and furnace gas cuts off, blower still runs
  6. Blower cuts off once exchanger temp is below operating minimum
Etc: Hi temp limit switch kicks in only if gas is lit, blower motor does not start, and exhaust vent temperature runs too high. It cuts off the gas.
 

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