Hydro Flame 8500 series furnace

[Camper44]

Hello all,
I have a hydro flame 8500 series furnace, the thermostat has an on-off switch,and a 3 position fan switch, low,auto and high,the furnace will not light on the auto setting but if I switch it to the low setting it will fire up, then shut down after reaching temperature. I turn the power off and fan switch to auto,then have to repeat the process to fire back up again when it chills back down again, any input would be appreciated. Thank you

[Old-Biscuit]

T-stat 'fan' setting should have NOTHING to do with furnace operation
Auto,low,high would be settings for A/C Unit fan

Furnace fan is controlled via furnace system
DC from a FUSED source goes to furnace...then backfeds to t-stat
When t-stat closes (temp setting lower then room temp) the DC then energizes a 'timed delay relay' OR a Blower Circuit on circuit board (depends on year model)
When energized FAN is powered by the DC from fused source.
Fan runs/triggers Sail Switch which then allows DC thru sail & limit switches to circuit board for gas valve & spark electtrode for ignition.

8500 series should have an LED light on circuit board which would indicate a Fault Code

Could be relay Or blower circuit, could be sail switch sticking, could be loose connections


This is wiring diagram using a 'relay'



This is a wiring diagram using blower circuit on circuit board



Got a voltmeter ----use it and trace the DC

[mchero]

Why the double post?