The camper I purchased last fall has a forced air furnace and it works fine on AC but when I switch to DC (marine deep cycle battery) it will not light the burner. The difference seems to be that the fan slows down on DC even though the battery is putting out 12.5 volts but when connected to AC it is at 13.5 volts.

There must be some kind of air pressure safety switch that is tripped to make the burner come on. The fan will run and keep running until the battery is dead. This unit has a pilot not electronic ignition.

Where is this switch and can it be adjusted to compensate for the slower speed?

 

I am not a furnace technician, but I am going to sleep at a holliday Inn Express tomorrow.
There should be a little port on the front of that furnace that allows you to adjust the flame on setting with a small screwdriver. This is an air/fuel adjust. Itis a poor substitution of a fix, and may or may not solve your prolbem. I had to adjust it everytime we changed altitude by as little as 1000 ft. You have a dinosaur there, and the local furnace guys will not like working on it. They will refer you to a RV tech, which is more expensive.
sometimes it means changing a circuit board, but on the old ones, you may not be able to get one. There are obsolete suppliers, but finding them is tough.
We went through everything above before we gave up.
I just changed the furnace in ours for about 600 bucks, and it was not easy.
It is bigger, and the vents where not the same, had to do some sheet metal work. But now it has a piezo lighter, and works like a dream.
I was able to mail order everything.

 

If you look in the path of the fan, there should be a little switch with a paddle on it. When the fan runs, the air current moves the switch, and lets the furnace light. If the fan is slowing down on D/C, I would check the battery health, and also check that the switch is moving freely. It should move with very little effort.

Hope this helps

Kris

 

I do not think the " sail" switch can be adjusted. That is the switch that allows
the 'main' lp valve to open.

For a manual pilot : It should stay lighted, Does it.. when the furnace fails to start?

You probably have the ac ok / DC not good diagnosed , Fan speed not
fast enough, Does the fan make noise, may need a little lube.
Or is it not running on Battery

And, check the grounds in the furnace, there should be a central place where
all the grounds come together, usually slip ons. They get loose,
Grounds act funny,

 

Thanks guys for the input.
We are 4600 feet right now at home and I am having these issues. It will be worse this weekend because we are going to be camping above 5200 feet.
My old 18FT trailer had a convection furnace that worked no matter what altitude we were at and used a lot less battery than this thing does.

The pilot stays on all the time. I have started the furnace on AC but when I switch over to DC you can hear the fan slow down a little bit...probably just enough to let the sail switch drop out.

The battery is reading 12.55 volts without a load. Isn't this OK?

I will check the grounds and the sail switch.

 

what kind of furnace is it? i thought all rv furnaces were 12 volt, all of the ones i have had over the years were....

 

I think he's saying it is 12 volt, just that it runs fine when the converter is supplying slightly higher voltage, and doesn't run when the converter is shut off and it's running on battery.

What brand and model furnace do you have? Often if you google, you can download a PDF for your specific furnace.

I think I'd start by taking a voltage reading at the furnace while the thing is trying to run off the batteries. If there's a weak connection, there may be significantly less voltage at the furnace than what you read at the battery.

After that, I think I'd give the thing a thorough once over cleaning and inspection (by the directions if you can find them) paying particular attention to making sure the sail switch isn't dirty or sticking, and that there isn't dirt or bug nest in the fan or one of the other air passages.

Also, I presume the thing has a thermocouple of some sort to shut the gas down if the flame goes out? If so, those have been known to fail or to act up when dirty. I wonder if it were boarderline, if that could cause it to shut down when voltage is a bit lower?

If you can manually hold the sail switch out and the thing still shuts the gas off when running on battery, I guess that would be my 2nd guess...

Good luck with it.

 

KBennett
My old 5er had the same problem and it was the rod connecting the sail to the switch. I took the heater core out and cleaned everything with air then silicon sprayed the rod and bushing. It has been about 5 yrs since and works fine.
The battery power didn't put out enough juice to push the sail.

 

johnh-sorry I wasn't clear. The unit is powered by 12VDC. When I say AC I mean that the camper's built in battery charger is plugged into AC, this increases the battery voltage.

It's a Duo-Therm furnace. I found the manual for it. The circuit is very simple, no circuit board. Just a relay, sail switch, limit switch(I think for over-temp), and the gas valve coil.

I went out to troubleshoot the unit and it's working fine now but don't know why. The battery has not been on the charger in 24 hours and the voltage is still 12.5. Maybe the gravel road I drove down vibrated something. I can't see the sail switch from the front so I think it is buried in the back where the fan is.

I think I have a solution in case it keeps giving me trouble.
The company I work for sells speed sensors for sensing motor speed. Maybe I will put a sensor on the fan blade and as long as it is spinning above a certain RPM then I will close a contact across the sail switch input. This way I can eliminate the mechanical issues.

Thanks again for the input.

 

my problem with the sail sw turned out to be the ground spade lug,
It was loosing its crip on the central ground post, when I tryed to tighten it, it broke.
new lug, fixed problem...

but your problem sounds like sail sw or fan motor ( dirty,bearings, lube )

 

12.5 volts is a little low. You should see about 13.6 vdc.

Lar.

 

Make sure you have BOTH propane tanks hooked up. The selector valve will only start with both tanks hooked up, then when it has pressure you can remove one tank.