Furnace failure

Question

I’m having trouble with my Suburban SF-30 fq furnace.  When I tried to turn it on for the first time this year it would not fire.  The fan would run, I could hear the usual relays clicking on, but it would not ignite.  I tried cycling it on and off multiple times with no success.  The next day I tried again after banging on the side of the camper and it fired up first time and ran continuously until the camper was up to temperature.  I left the camper overnight with the thermostat on and it was cold the next day and I could not get the furnace to fire again.  I pulled the furnace out and checked all the components.  The gas valve opens, there is a spark, the fan runs normally.  I hooked the furnace up to propane while it was still out of the camper and it works perfectly.  When I put it back into the camper, it still won’t work.  I checked the power to the furnace from the camper itself and found that I am getting good voltage both with the camper on AC and DC power.  If I leave the gas line open on the camper and turn on the tanks, I can smell that gas is getting to the furnace in the camper.  What else could possibly be wrong?  What else can I check?  Thanks.

itinerant1 · Answer

Probably have debri of some sort in the sail switch. Pull the furnace and check/ clean it. Fairly easy diy job. Some youboob videos show how.

opnspaces · Answer

I'm thinking sail switch or possibly an obstruction in the intake or ducting. What was the power source when you had the furnace out? Were you using the same wires just extended from the RV? If the power source or wiring was different it might be spinning the fan at different speeds.

If you really want to get involved with it try pulling the furnace and measure the voltage at the furnace when you power it up. Then install it and measure the voltage again right at the furnace and see if it's different/lower.

It could be that the sail switch is going bad and has some resistance to movement. When the furnace is out there is no resistance to heater intake or exhaust so the fan can spin a bit faster and lift the sail switch. When you install the furnace there is a bit more resistance to airflow which causes the sail switch to not lift quite high enough.

flute_man · Answer

I would verify that none of the registers are blocked. The sail switch needs to remain closed. Also verify that the voltage going into the furnace is proper.

dougrainer · Answer

When installed in the RV, does the gas valve click and Ignitor click? Or it only does when you remove the furnace for bench test?  You need a voltage measurement AT the Blower motor 12 volt wire at the motor pigtail. When installed in an RV, there is more CASE pressure than when on the bench. IF there is a drop in voltage, combined with the increase in case pressure, that may drop the motor RPM's just enough to not fully engage the sail switch. Also, long shot, a bad motor and cannot overcome that case pressure to keep the RPM's at speed. Doug

c_b_ · Answer

I also have a SF-30F Suburban Furnace.It did the same thing that yours is doing.Had it checked out. Everything tested good.Then the RV mechanic took this part off the furnace. Took the orifice out. tapped the tube on the counter and rust particles fell out. There was one plugging the orifice  Looking in the tube we can see it is rusting on the inside.Ordered part two weeks ago. Now it is on backorder till after 12/15/19.C.B.

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Furnace failure

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