Furnace will not fire on battery power

Discussion in 'Heating / Cooling Systems' started by Dr. Marneaus, Sep 10, 2017.

  1. Dr. Marneaus

    Dr. Marneaus Station Wagoneer

    Dec 2, 2012
    This is driving me absolutely insane. I have a 1980 JAYCO with a dometic furnace. My furnace blows but will not fire up on battery power. It fires on shore power, fires if I hooke my running truck up to the camper battery with jumper cables, but it just will not fire on plain only battery power. Seems to me like it just doesn't have enough oomph to trip the sail switch.

    -Battery is brand new 24 series deep cell from Costco. I topped off the charge when I got it home.

    -4 gauge wire to a kill switch and a frame ground, then new 10 gauge wire back to my power distribution center. The 'in' wire on the power center is like 14 gauge

    -I took the furnace completely apart and cleaned it. Made sure everything moved freely and air flowed. Sail switch moves with ease by hand. Fan spins without resistance by hand.

    -I cleaned all grounds and connections. I added grounds from the lift frame to the camper frame because I noticed the internal wire grounds to the lift structure.

    - I jumped straight to the battery from the furnace for the 12v in, and bypassed the distribution center. Still won't fire!!!!!

    After all of this it's the same thing, fires on shore or when my running vehicle has jumper cables to the battery, but not on just battery.

    What is going on here? Bad fan motor? Bad sail switch?

    It all works but will NOT work from the battery which is the only way i camp in this thing.
  2. Antipodes

    Antipodes Call me Paul

    Jan 28, 2015
    Christchurch, New Zealand
    Does the fan sound like it is moving faster or moving more air on shore power.
    It does sound like the sail switch is not making! Not sure if you can access it during the start cycle to move it the right way on Battery power.
    The sail switch is just a microswitch, and my bet would be the motor just has a bit of stiction in the bearings (1980 getting on in age)and on Battery power it is just not quite enough air flow to make the switch.
    Have you measured the voltage at the furnace during the various different starts you have done in your above description?
    I would be going for a new motor first, as the switch works. But would depend on what I am seeing with the voltages.
  3. tombiasi

    tombiasi Well-Known Member

    Sep 1, 2012
    Northwestern New Jersey
    I would also like to know your battery voltage with the furnace running.
  4. kitphantom

    kitphantom Well-Known Member

    Dec 26, 2009
    Albuquerque, NM
    We replaced the sail switch in our TT this year, it would refuse to fire at times. The switch looked OK once we removed it, but putting a new one in solved the problem.
  5. Dr. Marneaus

    Dr. Marneaus Station Wagoneer

    Dec 2, 2012
    Hey all, thanks for the responses!

    The fan DOES blow stronger when hooked to the jumper cables or the shore power.

    12.24v when running on either shore or jumped battery power.

    When the thermostat trips, the fan comes on, if i turn the thermostat off, and the fan runs for another 30 seconds or so, it speeds up a bit. If i then turn the thermostat back on to heat, you can hear the fan bog a little.

    This got me thinking, maybe its not the sail switch, maybe its the gas valve drawing too much power?

    My furnace is a pilot light, so its not failing to spark or something.

    If the fan was the main source of resistance, why would the fan speed back up as soon as you tell the furnace to "stop burning" and the gas valve closes? Fan on, burner on, is a slower fan speed than fan on, burner off.

    Who knows, the sail switch could be tripped, but it could be the gas valve that isnt able to do what it needs to do.

    I think from here my approach will be to test the sail switch. If I can trip that remotely with a wire or something, and it still doesn't fire, then there's an issue elsewhere.

    I really think its just old parts, old wiring, etc, all coming together to cause too much resistance somewhere.

    My camper is old and on its last leg, i'm just a little frustrated that I spent good money on a new battery and new wiring only to have it still not work. I'll probably be done with this trailer within the next year anyway :-(
  6. Antipodes

    Antipodes Call me Paul

    Jan 28, 2015
    Christchurch, New Zealand
    Sounds like you are going in the right direction and are fault finding carefully.
    I have a different manufacturer and model to yours, but the fan also sounds like it speeds up when the heat turns off.
    I put this down to other causes as the blower fan also pushes air into the combustion chamber and when the flame is burning the pressure will be a lot higher in the combustion chamber, causing a back pressure on the impeller. When the flame stops, the back pressure reduces substantially, which allows the air to flow better and reducing the load on the fan and motor.

    Your voltages seem a bit low to me.
    This is the voltage I would expect on just battery with a full charge!
    When on shore power the voltage should be reading around 12.8 to 13.5 Volts.
    Your car battery with jumper leads maybe as you state? But if the car is running it should be higher and in line with the converter voltage.
    You did not state what voltage you have with just the camper battery running?

    Run the system with your jumper leads and then remove them so it is only running on your camper battery. What happens?
    If the heat shuts off immediately then you know the fan is slowing down and not maintaining the sail switch tension.
  7. neighbormike

    neighbormike Well-Known Member

    Oct 6, 2012
    Just out of curiosity, Dr., did you have this same problem with the previous battery?? Also, have checked to see how many amps the fan is pulling?
  8. Dr. Marneaus

    Dr. Marneaus Station Wagoneer

    Dec 2, 2012
    I had the same issue with the previous battery but t was unmaintained and 7 years old. I have not checked the amp draw.
  9. Dr. Marneaus

    Dr. Marneaus Station Wagoneer

    Dec 2, 2012
    okay, here's my thoughts. Essentially all the distribution center is is a fuse/breaker, a 3 position kill switch that has off/battery/shore, and it serves as a converter when hooked to shore power while still providing AC the the one outlet in the camper. I have a big redundant killswitch right at the battery.

    I just ran from the 'out' side of the killswitch directly to the furnace and left the main power lead that comes FROM the distribution center. This powers all D.C. Accessories directly from the battery, even when the distribution center is 'off'

    So I can leave the distribution center off and simply run everything on the main killswitch, and just killswitch it when I pack it all up. Same as turning off the distribution center.

    This only complicates things if I ever hook to shore power. It would be sending un regulated D.C. Power to the battery and over charge it. Simple solution is to just open my kill switch before ever plugging into shore power. Problem solved.

    Ps I do have a fuse equal in amperage to the one inside the distribution center in line before the battery supply hits the furnace
  10. tfischer

    tfischer Well-Known Member

    For what it's worth, when our T-stat kicks off the fan goes ever so slightly faster too (raises in pitch slightly). Ours works fine.
  11. Dr. Marneaus

    Dr. Marneaus Station Wagoneer

    Dec 2, 2012
    to be honest, if the camper was worth more or in better condition, or was planned to last longer than another year or two, max, I would probably look into a better fix for it. However, its not going to be around for much longer due to alot of things.

    roof wood is rotted out and leaks (in the processof resealing), the body is coming apart at the seams no matter how much i screw it back together and brace it and pump it full of liquid nails, the torsion axle sits lower on one side and needs replaced, brakes dont work, and I may be going in on a newer camper with my parents as a sort of family trailer in the next year or so.

    I think this fix will get us by for now, and i have a functional furnace now :)
  12. Heilman_5

    Heilman_5 New Member

    Sep 25, 2017
    The termocupling?

    The pilot heats it up and it allows gas in, if it fails no gas will flow

    Just a thought
  13. tenttrailer

    tenttrailer Art & Joyce - Columbus, O

    Jul 18, 2013
    Thornville, OH
    I would be looking at the voltage at the furnace when running on your battery. I know in the past when I had issues with a furnace I found a service manual and there was a defined voltage that if below it the furance would not fire. I'm thinking it was somewhere in the high 11 volts.

    I also agree the voltage you are reading 12.4 when on shore power is low. I see like 13.3 to 14.x. on all my campers. I would think you should be seeing 12.8 plus when jumpered off you TV. I think you have a bad connection somewhere that you have a voltage drop or a bad meter?? Check the voltage across you TV battery posts. Check the output of you converter it is the common 12v bus for the converter and the battery. I would check it 3 different ways. Running off shore power, running on the camper battery and running off the battery with the jumpered TV battery.

    When you jumpered from the furnace to the battery did you also jumper the ground wire?? If not try jumper both wires. But make sure the camper battery is charged up to at least 12.7 volts before doing it.
    Last edited: Oct 2, 2017
  14. Douggro

    Douggro Active Member

    Jul 26, 2017
    Seattle, WA
    Is it a fair bet that the furnace is an Atwood instead of a Dometic? Not sure that Dometic ever made furnaces...
    If there's a time-delay relay in the circuit, it's normal to see a voltage drop when the relay kicks in since it's using an internal heater to control the delay. On the Atwood furnaces, voltage is sent from the relay to the limit switch, then to the sail switch and finally to the gas valve. Limit switches are normally in a closed state and you should see nothing more than ~0.5V loss from the relay through the switches to the gas valve.

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