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Old 01-05-2020, 11:11 AM   #1
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Suburban furnace sf-35q

I have a suburban furnace. It comes on for 20 seconds and then shuts off. No hot air. I've replaced the circuit board and replaced the thermostat. Have contanuaty through the flow switch. Any ideas of else i can check. Also have a full propane tank.

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Old 01-05-2020, 11:53 AM   #2
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Do you hear sparking when the furnace first comes on?
(You may have to listen outside near the exhaust)

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Old 01-05-2020, 12:08 PM   #3
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You have what is known as a "sail" and a "sail switch" in your furnace and it sounds as if the sail itself may be stuck or the switch may be bad. The sail is basically a small piece of light sheet metal which moves, as a boat sail, when the furnace fan moves air and the movement of the sail makes the sail switch tell the circuit board that the furnace fan is moving air. When the furnace initially lights up then it will satisfy the thermocouple telling the circuit board that the burner is on and producing an appropriate amount of heated air. Within seconds the fan comes on to blow the air into the heating ductwork. This is when the sail is moved to activate the sail switch itself. With the gas and electrical power turned OFF you will need to look into the interior of your furnace to find the sail. Once you find the sail you will readily find the sail switch. You may even have to remove the furnace from you RV to work on it. IF the sail is hard to move you can use just a drop or two of oil at the pivot points to lubricate the sail. IF the sail appears to move freely then you will need to check the sail switch with an Ohm meter to assure that it is working properly. This type situation usually happens to furnaces that have sat for some length or time, such as months, in high humidity atmospheres causing the sails pivot points to sort of lightly rust over causing too much friction at the pivot points.
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Old 01-05-2020, 01:02 PM   #4
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Suburban furnace

I do not hear clicking sounds. Sould i be getting a spark. When i take the wire off the igniter. Like spark plug does when you put it close enough to it ? The sail switch. The switch thats right beside the fan ? If so that has contanuaty.
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Old 01-06-2020, 01:31 PM   #5
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Suburban furnace sf-35q

I to have SF35Q on your control board are you showing any error lights if so count the blinks and look up the error code on the Suburban web site, if your board has the error code light some do not. If your sure the sail switch is functioning correctly, after this is confirmed the control board also checks the high limit switch before opening the gas valve, that is when you should hear clicking at the igniter. There are some older models that check the limit switch before checking the sail switch.

Does your furnace go into lockdown mode after 3 attempts to start. If not then there is something else occurring beyond normal start up issues.
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Old 01-06-2020, 03:15 PM   #6
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If the sail switch is stuck closed it will prevent the furnace from lighting just like if it's stuck open. When it first calls for heat the control board checks to see that the sail switch is open, and transitions to closed when the blower starts.

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