Fan Continuously blows, no heat

Tempstar DC90 Ultra High Efficiency Natural Gas furnace
Model: NUGM100EHB1
Serial: L9615 34624
Year: 1996
Location: Burlington, Wisconsin

Turned thermostat down to 58 three days ago and left. Came back turned thermostat to 70 and no heat. Fan continuously runs even if thermostat is shut off. Changed thermostat, no difference. Reset Circuit breaker. Turned gas valve off. Waited 5 min. turned back on, nothing. Whenever power is supplied blower and exhaust blower continuously run but no flame. From reading the other forums, it appears this is a safety feature if something isn't working properly. Any ideas on what to check with a multi-meter.
Thanx,
Jeff Troyanek

 

" Fan continuously runs even if thermostat is shut off." The fan you refer to is the one that circulates the air thru the ducts?? Most modern furnaces are designed to run the circulating fan full time if one of the limit switches opens. Look on the wiring diagram for the limit switches. You could have 1 or more, they are usually wired in series. [sometimes there's one in the blower compartment] Check each one and see if any are open. Reset or bypass temporarily any you find open and try for a startup. Watch closely in case the circulating fan doesn't start and you have one bypassed. Let us know what you find.

 

Thanks for the tip. The limit switch in the blower compartment was open. Problem soved.

 

Found the problem, found 2 limit switches in the upper (burner/heat exchanger) cabinet wired in series; the flame roll-out sensor and the fan limit sensor. I found the problem to be with my flame roll-out sensor (upper sensor next to the burners), mine has a reset button on it and it had tripped from over-heating. I reset the sensor and the (induction) fan went off, then the furnace cycled normally. Thanks for pointing me in the right direction!

ADDITIONAL QUESTION; Why is it that when the burners ignite the flame is rolling back out of the heat exchanger enough to trip the sensor? I removed the inspection cover from the burner compartment and watched the furnace cycle & ignite a few times. When the burner ignites the flame does flash back behind the burners for a second or so. The induction fan is running several seconds before the burner ignites, I would think that the induction fan would create enough of a draft past the burners that all combustion would be in front of the burners (inside the heat exchanger)