How electrically friendly are you?
This site Service Documents and Manuals
has the service manual for your 8500 Hydroflame furnace. Scroll to the bottom and see the one called this "A newer Hydroflame manual than the above- 2004 version, less theory but more troubleshooting- includes thermostats."
Page 10 top right has the wiring diagram for the 8500 series that should align with your unit.
Assuming you do have LP gas, (stove top works etc) what you are describing seems to maybe point to the saftey circuit or the board itself.
This will be a process of elimination.
The igniter, if it is being told to fire, you can hear it if you listen close enough. It is a low tone high frequency buzz. You can hear this buzz normally before the "whoosh" when the gas valve opens and it lights.
Point, if you hear the buzz and no fire, then you are at least telling it to fire and there is a gas valve issue.
If there is no buzz, the board is not be sending the signal to fire.
There are 2 more normal devices to stop or prevent the ignition sequence, the sail switch is not making OR the high limit switch is not closed. They are wired in series and must make the connection between the two to allow the control board to send an ignition signal.
So is there any buzz after the blower runs for a while after the purge?
From here there is need to start trouble shooting with a volt meter and I do not know how handy you are with one. Let us know and I can drill in deeper. Does it have an ohm meter on it as well?
The need is to sort out if the voltage path exists through the sail switch and the hi limit when the blower is running.
The hi limit switch is buried in the heat exchanger. it has the 2 white wires coming off it. Looks like this. You will not get to it short of taking the whole thing apart but we can test to and from it, so no need to take apart yet.
The sail switch is buried in the fan housing on this newer model. It has a blue and white wire that comes out from behind this blower blade. Again you are not going to get to that switch easy short of a total disassemble of the furnace, so we need to test power going to it and power coming out of it when the blower is running
What you could try first is seeing if the board connection is good. Shut off all power to the furnace. It is all 12 volts, both the battery and the power converter.
Very gently pull off the white wire connector and look for dirt or corrosion on the connector.
You can see it here. That white wire is the return from the hi limit switch
Following the wiring diagram, 12 volts to the furnace feeds the blue wire that goes to the T stat and then comes back from the T stat to the one side of the sail switch and to the control board. That 12 volts to the control board turns on the blower and sends it to one side of the sail switch.
If the 12 volts makes it through the sail switch when the blower is running, it leaves as a white wire into the hi limit switch. It come out of the high limit on a white wire as 12 volts and goes into the board as the white wire to send the signal to start the igniter and the gas valve.
Basically you have to rule out the sail switch and the hi limit is working right and the signal is making it to the control board. If it is, and you have no buzz then the control board may have issues. Since the unit was making heat when this whole problem started, it points to the gas valve and the igniter was working at one point and now does not seem to be.
Hope this helps