Duo Therm Furnace - lower voltage that triggers blower?

I would think the furnace should operate from 15 down to 10.5 volts.
That is voltage at the furnace. Voltage drop in the wiring is another issue.
Yes check the airflow past the sail switch. Many RVs barely meet the minimum duct size requirement. If you can add a duct I recommend it.

If the sail isn't going up enough than the blower isn't turning fast enough which could be low voltage, bearings going bad, or motor going out. Read the voltages while the furnace is running as that will show the bad contacts. The furnace should work down to a loaded battery voltage of around 11.0 - 11.5V.

JoeChiOhki wrote:
Bet ya a dollar if ya voltage test while on only the battery from the leads for the thermostat to the Time-delay relay, you're going to see a voltage drop ;).

I'll check that too thanks!

Okay after further research it looks like my furnace "sail switch" isn't getting up to enough air flow capacity on battery to trigger the gas main.

I also just learned that my 2008 GMC Sierra has an RVC voltage regulator that lowers the alternator output to like 12.5 - 12.8v to reduce engine load or some such... so my camper battery isn't even getting fully charged! Sigh. Haha. Going to disable that and pump 14v into it like it's meant to get. Probably will solve this entire thing.

Maybe sometimes "talking" to oneself on a forum is good enough for diagnosing things. Haha.

teambeeson wrote:
Wait I take it back... the grounding is fine (though just to the camper frame... hmm...). The battery is brand new deep cycle from Napa so it's not the battery.

Anyway... here's the real problem... it's not the BLOWER that doesn't come on it's the main gas element itself. So the voltage is too low (even at the 12.6v the battery is reading right now) to trigger the main gas heater so the blower is just on with the pilot light, but that's it. Switch over to converter and pow... on goes the main heating element (you can hear the click and the gas starts flowing).

Back to the drawing board.

Bet ya a dollar if ya voltage test while on only the battery from the leads for the thermostat to the Time-delay relay, you're going to see a voltage drop ;).

Wait I take it back... the grounding is fine (though just to the camper frame... hmm...). The battery is brand new deep cycle from Napa so it's not the battery.

Anyway... here's the real problem... it's not the BLOWER that doesn't come on it's the main gas element itself. So the voltage is too low (even at the 12.6v the battery is reading right now) to trigger the main gas heater so the blower is just on with the pilot light, but that's it. Switch over to converter and pow... on goes the main heating element (you can hear the click and the gas starts flowing).

Back to the drawing board.

teambeeson wrote:
The ground... that makes so much sense. I notice when I have my water pump on the voltage plummets so I bet I do have a bad/no ground. I'll check it out. Thanks guys!

Yeah, that right there says you have bad connections to your batteries / bad battery.

The ground... that makes so much sense. I notice when I have my water pump on the voltage plummets so I bet I do have a bad/no ground. I'll check it out. Thanks guys!

--- double post sorry... having a hell of a time posting from work.

Also sounds like you may have a bad ground. Double check that your furnace has a dedicated negative wire and is not using the camper's skin for its return link.

I had this issue with a Suburban DD16, the negative wire had come unhooked and the unit was using the siding as its return route, which it had a bad ground connection.

On battery, the voltage would drop too much for the furnace to run properly, but it would work fine when on the converter.