I have a problem that has just started recently with my furnace. Let me try to do this logically....Day One Boondocking on 12v Battery power. furnace comes on and operates normally-no problemsDay Two Boondocking - Same as aboveDay Three Boondocking Furnace comes on fan runs about 30 seconds and stops - furnace has turned itself off.Propane is good both stove and reefer working properlyBattery is showing at 75 per cent on meter in trailer (note I dont take this figure literally but it is an indication)All lights working ok and pump ok - indication is Batt is still charged enough to work.Last day AM putting in slides Battery has enough power to put both slides in.When I plug into AC or run my generator with the trailer plugged in the furnace works as it should.This all kind of points to the battery but is confusing as the battery still has enough charge to bring in the slides but not run the furnace fan?Is there a voltage switch on the furnace that if it doesnt see enough volts will turn it off?Also this shutoff occurs right about the time the propane should ignite but it doesnt get a chance. But it ignites on que when plugged into shore power or Gen.Anybody with any info or guesses out there? Has any one had this scenario before?

might be a corroded ground wire on the furnace

I would say the furnace fan is running just a bit slower with the result of a little less air flow to operate a sticking or stiff sail switch.

thanks for the responses Grounds are one of the first things I always look at but in this case the furnace works perfectly on AC/Gen??? So that to me would negate the ground idea. My grounds at the battery are very good but I will check the one at the furnace if I can access it. I am not familiar with a "sail" switch. Can you enlighten me on that? But the fan when it comes on seems to be turning at normal rpm's.My understanding of how the furnace works is that the fan initially comes on before the propane is allowed to light to clear any accumulated fumes from the unit.Then an electrically operated switch allows flow of propane another electrically operated switch operates the igniter.But again I don't suspect any of the switches (but I dont know for sure how they are triggered) because they work with the AC. (shore power)

I agree with PaulJ2. Sail switch is not opening up enough. Battery voltage low. Try it on shore power see if it works. Good luck.

I've seen several instances over the years where dirt buildup on the sail switch "sail" has caused cut offs at higher fan speeds than normal, usually in older RV's or ones where a lot of fried foods are prepared. It's an easy fix, although getting to the switch can be a hassle on some furnace models.

Furnace Problem - Shuts off while on Battery | Good Sam Community

14 Replies

PaulJ2
Explorer
Apr 28, 2016
The sail switch is a micro switch operated by a small stiff wire or rod with a paddle that is located in the air stream of the blower, and is moved by that.
However it is near the fan that is usually in the back of the furnace.
time2roll
Nomad
Apr 27, 2016
Remove the furnace and disassemble.

I also suspect a combination of low voltage and inadequate airflow due to minimal duct install or restricted ducting. When running does the burner cycle during the heating cycle?
4X4Dodger
Explorer II
Apr 27, 2016
corvettekent wrote:
I have had this problem too. So how do you get to the sail switch to clean it?

x2 ???
wa8yxm
Explorer III
Apr 27, 2016
That one, on it's face, appears to be easy.

When the T-Stat calls for heat the blower starts.. Ignition is delayed.
When the blower comes up to speed and air flow is great enough a device called a SAIL SWITCH closes, then the control board lights the burner (Ignition happens) I won't go through the entire procedure.

But after two or 3 days of boondocking the batteries are getting a bit.. Run down. and the blower never comes up to speed.. The solution is to run your Generator long enough (During the day of course) to FULLY charge the batteries, This takes, at the minimum, six hours.
DanNJanice
Explorer
Apr 27, 2016
corvettekent wrote:
I have had this problem too. So how do you get to the sail switch to clean it?

On most trailers you would have to pull out the furnace to work on the sail switch. Depending on the location of the furnace this may, or may not, be easy.
corvettekent
Explorer
Apr 27, 2016
I have had this problem too. So how do you get to the sail switch to clean it?
4X4Dodger
Explorer II
Apr 27, 2016
teb1272 wrote:
I am not sure anyone described the sail switch for the OP. The sail switch has to be blown closed by the fan before power is allowed to reach the rest of the furnace. If the sail switch sticks open, even a little bit, the furnace will not operate. A sticky one could fail to make contact with a small drop in fan speed because of slightly lower voltage. I hope this is helpful.

Ok thanks, that was the critical info I was missing that didnt really allow me to close the loop on this.

I am going to have my battery tested. But last time I tested it myself it was good. (once charged and left un connected it stayed at 99% for 24 hours.)

I am on my way to Alaska and I really want my furnace working correctly. It will still be plenty cool there at night in early June and on the way up.
teb1272
Explorer
Apr 27, 2016
I am not sure anyone described the sail switch for the OP. The sail switch has to be blown closed by the fan before power is allowed to reach the rest of the furnace. If the sail switch sticks open, even a little bit, the furnace will not operate. A sticky one could fail to make contact with a small drop in fan speed because of slightly lower voltage. I hope this is helpful.
4X4Dodger
Explorer II
Apr 27, 2016
sclark wrote:
I agree with PaulJ2. Sail switch is not opening up enough. Battery voltage low. Try it on shore power see if it works. Good luck.

What is the "Sail Switch" and where is it on most furnace models?

As for the battery voltage being low. Do you think that if the battery has enough power to operate the slides with no problem shouldnt there be enough voltage for this sail switch? what does this switch operate?

This is the logical hurdle I cant quite get over?

Any thoughts on that?

And yes it works on shore power/gen power

thanks for the answers in advance.
Dutch_12078
Explorer III
Apr 26, 2016
I've seen several instances over the years where dirt buildup on the sail switch "sail" has caused cut offs at higher fan speeds than normal, usually in older RV's or ones where a lot of fried foods are prepared. It's an easy fix, although getting to the switch can be a hassle on some furnace models.

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Furnace Problem - Shuts off while on Battery

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