We have a Hydro Flame 7900 II furnace in our camper. What would happen if the furnace was needed while the battery ran low or became discharged (no AC connection)? Would the furnace overheat w/o the fan speed? Would it just shut off?Thanks!!

I don't know about that furnace model, but for us when our battery was low, the furnace just clicked. We were newbies and this was our second trailer (on our fifth now).

Depending on the state of charge of the battery, chances are it would not light at all. The fan may run, . . . slowly. The burner won't light without sufficient air flow through the box.Chum lee

I would expect the furnace to run to at least 11.0 battery volts and shut down between 10.5 and 9.0 battery volts. Maybe lower but not much.BTW anything below 11.5 volts is hard on the battery and while I personally would still keep the furnace running... do try to get the battery charged asap next day.

EEWally wrote:What would happen if the furnace was needed while the battery ran low or became discharged (no AC connection)?Your question is moot because you shouldn't ever be drawing down your battery anywhere near as low so as to cause the furnace to not ignite. Generally, you want to draw the battery down no more that ~ 50% capacity or ~ 12 volts on a regular basis, drawing down to ~ 30% or ~ 11.75 volts occasionally is considered acceptable as long as it's not done often and you re-charge as soon afterward as is practical. These voltages aren't anywhere near the low threshold voltage for your furnace OR your fridge to not operate.

The fan speed will continue to get slower and slower to the point that the flame is lost do to 'no air movement', and the gas supply solenoid valve is shut.

Running Furnace on Low Battery? | Good Sam Community

MEXICOWANDERER
Explorer
Oct 13, 2015
On a sub freezing night at 10.5 volts you might want to say "adios" to your flooded battery. An hour or two of overdisharging a battery for heat is a little too rich for my blood.
joebedford
Nomad II
Oct 13, 2015
In the case of my furnace with low battery, the fan blew around very cold air. Fortunately I was able to start my truck to get enough power into the batteries to start the generator.
YC_1
Nomad
Oct 13, 2015
And when the sail switch cuts off the gas the motor can still continue to run steadily blowing cold air until the battery is really run down worse. The fan may not cycle off is what I am saying.
AllegroD
Nomad
Oct 13, 2015
I think THIS is your manual.
Manual, page 21 wrote:
VOLTAGE
Voltage should be between 10.5 and 13.5 VDC at the
furnace during operation. The power at the furnace
needs to be checked with each of the following power
sources when applicable: generator, battery, and
converter. Low voltage can cause the furnace to
overheat and cycle. High voltage can cause
unbalanced combustion, and excessive motor wear.
Note: To increase motor life the furnace should be
wired directly to the battery.
red31
Explorer
Oct 13, 2015
EEWally wrote:
Would the furnace overheat w/o the fan speed? Would it just shut off?

as suggested the sail switch will eventually open if the fan slows too much and cut off the furnace. There is also an overheat limit switch.
Roger10378
Explorer II
Oct 13, 2015
On mine the furnace was still running when the voltage got low enough for the gas detector to alarm. At that point you are not going to sleep so I disconnected the battery and put another blanket on.
lasttruck
Explorer
Oct 13, 2015
Blower motor needs to be turning at 75% of its set speed in order for the sail switch to close, allowing the gas solenoid to open, allowing for burner operation.
D_E_Bishop
Explorer
Oct 13, 2015
There is a Sail switch in the output line that turns power off if fan turns to slow. So yes it will turn off when batteries become discharged.
dougrainer
Nomad
Oct 13, 2015
Low 12 volt will cause the Furnace fan motor to NOT run to correct RPM's. When that happens, there is a "Sail Switch" that MUST be closed for current to flow to the control systems. When the fan motor eventually slows due to low DC voltage, the Sail Switch will open and the furnace burner will shut off. Doug
boogie_4wheel
Explorer
Oct 13, 2015
The fan speed will continue to get slower and slower to the point that the flame is lost do to 'no air movement', and the gas supply solenoid valve is shut.