Good morning,suddenly my 1992 Hrydro Flame furnace model 8535 blows cold air but no ignition for heat. I have seen video on a dirty sail switch being the problem but unable to find the switch on this model, any ideas anyone ? thanks

Just went through this same problem.

Open the cover plate and plastic plate. make sure the gas is off. some wire run along the front find the red one that goes to a relay located on the right side of of the blower motor. before the actual blower housing. Disconnect it.

Turn the thermostat on and all the way up make sure the house batteries are full charge. put your volt meter on one yellow and one red wire on the gas valve.

Touch the red wire to that relay, the blower motor will start, wait like 30 to 60 seconds watching the volt meter, you should hear a clicking that is the igniter, during this clicking the valve will also getting voltage, it must be above 10.5. If you get any voltage at all the sail switch is good. My problem was a faulty igniter board and thermostat..no clicking of the igniter and after replacing the board, low voltage to the valve it runs through t-stat.

Every time you disconnect the red relay wire the system resets so you can experiment as often as it takes to make sure.

good luck!!!

Good writeup and photos here:

https://www.barthmobile.com/eve...513981757#8513981757

Great Steve always has the good stuff!! Also shows the location of the sail switch, that I forgot to mention.

Maybe you have a mouse running in the squirrel cage.

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Link here to the mouse