Suburban furnace SF-30F serial#940804195

[scottmac]

long story short- furnace only "fires" gas when plugged in to 'shore' power or 2400 yamaha generator, t-stat calls for heat,blower kicks on, few seconds and gas fires to burners =A OK.. on 12v(fully charged deep cell) when t-stat calls, blower motor runs and run but no fire, if I remove front cover (by limit switch) it fires off ??(blowing hot air into room and not down ducts of course) if I turn temp down and furnace stops and cools(like it should),so I replace cover of furnace and same thing happens, will not fire gas. I bent the cover somewhat so that it still attached to the box with screws but the 'bend' in the sheetmetal at the top lets some forced air out thru 1/2" gap....fires everytime perfectly...I am thinking maybe the sailswitch could be in play,(but I thought the sail had to do with firing off the blower) but as you can tell from the model, that this furnace has no exterior access, so it all comes out the front to access the sail switch and other parts..ughh. can someone shed some light on this situation for me?? thanks so much

[dougrainer]

With everything in place. Bang on the furnace housing when the furnace first comes on. THAT will cause a marginal Sail switch to usually close and fire. IF that fixes it, replace the sail switch, and/or check the mounting of the "sail" of the sail switch. You really need to post the EXACT digital dc voltage of the power at the furnace when running on 12 volts. You may have good voltage at the RV ,but is it the same at the furnace? The difference between shore and battery is simple---the shore power converter is putting out a higher dc voltage (maybe only 1 to 1.2 volts) but THAT is enough to turn the Fan motor at a faster RPM. If the sail switch is defective or is sticking, that faster RPM is overcoming that problem. Also, CLEAR and OPEN heat ducts are required. There is a Case(Static) pressure on furnaces. If that pressure is to high (inadequate number of heat ducts or partially blocked ducts) that will cause the motor impellor to not supply enough force to close a sail switch, especially on battery. That may be why you remove the inside cover and it fires on battery. Doug

[Old-Biscuit]

T-stat calls for heat
Fan blower comes on to purge combustion chamber......when motor comes up to 75% rated speed the sail switch makes up and allows 12V DC current to/thru high temp limit switch and then to power module which then sends current to gas valve and spark electrode to fire main flame.

The sail switch is a safety device......if it doesn't make up then not enough air flow for proper air/fuel ratio and good combustion.
It is just a small make/break switch with a paddle on it that sticks into the blower chamber. Air flow makes the paddle 'sail' making up the switch contacts.

If your furnace works just fine on 110V AC power (shore or gen) and has problems when not on 110V AC..then your fully charged deep cell battery is NOT providing good enough 12V DC power for motor to come up to full speed. (On 110V AC power the converter is supplying the 12V DC circuit).

Bending the front cover out is probably directing air flow at the sail switch allowing it to make contact when just on battery.

In other words the only difference is what is supplying the needed 12V DC power......shore or battery.
Check the battery connections (at battery and at terminals on converter...positive and negative and frame ground.) Problem is with the DC power from battery. Furnace is a DC hog.