I am trying to fix a furnace for my neighbor, problem seems very obvious but hit a couple bumps.
Armstrong Ultra SX-80- (Vintage 1996)
Let me explain- Furnace calls for heat, system goes on triggers the exhaust fan, igniter lights, flames come on 30 seconds later the blower is suppose to turn on but does not turn on until the high limit triggers. He has been running the system for the last 2 years with the system in "FAN ON" mode instead of auto.
With all that said my attempt to repair was to replace the main board and since the blower motor was making noise and going bad I replaced that too. Installed all that plus a new 24v transformer. Put the system in Auto, call for heat, same thing, exhaust fan turns on igniter on, fires, hits high limit, flames go out then blower goes on. What am I missing here....
To add to the clues, I was told the gas valve was changed out for some reason a few years back but I could not identify if the existing valve was the exact replacement or not. Is there a possibilty that the gas valve controls the blower on after it calls for heat? The gas valve appears to be functioning properly but I am thinking it may not be the exact replacement losing that function.
Any insite on this?