Furnace voltage drop and blower pulsing

Discussion in 'Heating / Cooling Systems' started by CarlSP, Sep 9, 2020.

  1. CarlSP

    CarlSP New Member

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    When testing out my furance (Suburban DD17dsi) ahead of cooler camping this fall, the fan was coming on but the igniter was not clicking on. When on shore power it was working fine and lighting, but on battery power the fan was noticeably slower and pulsing a bit faster and slower. I also noticed that when the front screen/cover was removed, it would start on battery, but with the cover back on it wouldn't. I tracked down that the cover was slowing air movement enough that the sail switch was not being tripped and that was preventing the igntor/gas valve turning on. I am seeing over 1V drop at my charge controller, about half way between my battery and the furnace when the fan turns on. I also notice the voltage wavering approx 0.2V in sync with the blower speed pulsing (when the fan speed increases, the voltage reading increases). It is 14ga wire from battery to the furnace - maybe 8ft away. Is this likely a wire gauge issue, or is the blower motor going? Or something else?
     
  2. NMroamer

    NMroamer Well-Known Member

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    Or maybe your battery is weak.o_O
     
  3. Sneezer

    Sneezer Well-Known Member

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    How old is the battery? What is the charge on it?
     
    neighbormike, generok and kitphantom like this.
  4. CarlSP

    CarlSP New Member

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    Battery came with the trailer this year. It's from 2017 but has been holding a charge reasonably well and didn't think this should be the problem, as the furnace only pulls around 3amps. I have another battery i know is good i will try swapping in.
     
  5. decypher

    decypher Member

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    If the unit works on shore power through the converter - its logical to assume the parts are working fine. First step should be to replace the battery to confirm thats not the core issue. Older battery + 14awg voltage drop could be your culprit.
     
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  6. CarlSP

    CarlSP New Member

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    Had some time this weekend so tested with multiple batteries and same issue. I went inside and connected a battery directly to the furnace to eliminate the wiring as the potential issue. Fan RPM are still fluctuating a fair bit.

    I also put an ammeter on when starting the furnace and noticed it started at 6 amps and then seemed to hover around 4 amps after a little while. The manual says the fan motor should be pulling 2.9 amp. Does this sound normal? I know motors spike at start up, but the 4 amp running current seems high to me? The current was pulsing inverse to the fan rpm (when the fan sped up, amp draw decreased). I think one of 2 issues:
    1. fan bearing is going. when the friction increases the RPM would drop and current would increase
    2. something else is pulling extra power intermittently. When it pulls extra current, the fan gets less current and slows down.

    Anyone have any insight? My next step will be to remove the whole furnace and try connecting the fan directly to power to see how it acts on its own, but was hoping to avoid disconnecting the propane line if possible.
     
  7. Sjm9911

    Sjm9911 Well-Known Member

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    It is 21 years old, try lubing it up. Old luberacant gets hard after a while. They dan should be really the only draw, especially if you just hooked the battery up to it directly. So i dont see anything sapping the power away.
     
  8. Econ

    Econ Well-Known Member

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    An Atwood 7916ii pulls 3.4amp. For comparison basis.

    On mine the furnace runs off the battery. Assume the battery voltage is between 12 and 12.7VDC. When plugged to shore power it recharges the battery. The furnace thinks the battery is at the bulk charging rate of 13.7 to 14.4 VDC. That speeds the motor up just enough to satisfy the sail switch. Is the air return inlet to the blower blocked? Rat's nest?

    I just rebuilt the lower crank on a 50 year old bike. Grease dries up after 50 years but it still turns, just not as easy.

    If you pull it make sure your use 2 wrenches when you loosen the flare nut
     
    Last edited: Sep 14, 2020 at 2:07 PM

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