Finally a Furnace Diagnosis

[brroberts]

I’ve got enough time with the furnace to pinpoint the issue. It’s voltage. 12.8 volts or more and the furnace is fine. 12.7 down to 12.4 it runs poorly. Less than that and it’s no go. 2 6V GC batteries don’t stay at 12.8 volts over night when it’s cold. Not sure what my remedy will be yet.

[Escape_19]

Originally Posted by brroberts

Not sure what my remedy will be yet.

Get yourself another blanket!

[alanmalk]

Originally Posted by Escape_19

Get yourself another blanket!

An electric blanket!
Ours takes 150 watts per side (his/hers).

[MVA]

Originally Posted by brroberts

I’ve got enough time with the furnace to pinpoint the issue. It’s voltage. 12.8 volts or more and the furnace is fine. 12.7 down to 12.4 it runs poorly. Less than that and it’s no go. 2 6V GC batteries don’t stay at 12.8 volts over night when it’s cold. Not sure what my remedy will be yet.

Interesting observation.

When you say "runs poorly" does the furnace not stay lite? When does it "fail" in the furnace lighting cycle?

Edit - see https://www.escapeforum.org/forums/f...tml#post434903 for an unfortunate furnace operation in cold temperatures. Similar issue?

[Perry Butler]

Originally Posted by brroberts

I’ve got enough time with the furnace to pinpoint the issue. It’s voltage. 12.8 volts or more and the furnace is fine. 12.7 down to 12.4 it runs poorly. Less than that and it’s no go. 2 6V GC batteries don’t stay at 12.8 volts over night when it’s cold. Not sure what my remedy will be yet.

In 2019, our batteries failed but our furnace still ran at 11.2v. It didn't sound happy though, more like a hemorrhaging whale. Still works fine today though. You could try a voltage regulator to raise the voltage.

Enjoy,

Perry

[Eggscape]

Good observation but…the fan may spin at an acceptable speed on a lower voltage if there was less restrictions on air flow or the sail was bent in the perfect position. So there may still be some variables.

[brroberts]

As voltage drops, sail switch short cycles in and out until voltage is low enough it’s a no go. As the sail switch cycles, so does the burner as it should. I might try the hard duct vs ETI’s flex duct. I might try heavier wiring if It looks like I can get it past the bathroom. Not sure yet. I’m still traveling, and the rest of the trip I have electrical hookups, so no below freezing emergency.

[Eggscape]

If you have the 3 way fridge you have a heavy duty 12 volt line at the rear. If you are like us we never use it on 12 volts.

[brroberts]

The fridge line might be an option. 12v I do use on fridge occasionally, but never at the same time as furnace. Nice thought.

[rubicon327]

Originally Posted by brroberts

I might try heavier wiring if It looks like I can get it past the bathroom.

I can’t imagine that being your issue. The newer Atwood/Dometic furnaces only pull 2.4A.

[rbryan4]

Originally Posted by rubicon327

I can’t imagine that being your issue. The newer Atwood/Dometic furnaces only pull 2.4A.

I don't think it's as much the draw he's concerned about as the voltage loss. Thicker wire might help?

[alanmalk]

Originally Posted by rbryan4

I don't think it's as much the draw he's concerned about as the voltage loss. Thicker wire might help?

Not really...
Using the tried and true formula - E=IxR, or in English, Voltage(E or in this case voltage loss) is equal to the resistance (R or thickness of wire) times the current ( I or "draw").

Bottom line. Low draw or thick wire equals low voltage loss. They are both exactly equivalent so dropping the resistance by 50% is exactly equivalent to dropping the draw by 50%.

In the real world, dropping the draw if possible is a whole lot cheaper than dropping the resistance. And if you have to pull a wire around the bathroom, a whole lot easier too. 2 or 3 Amps of draw is a small number if the length of wire is - for example - 20 feet (round trip, battery to furnace and back). But (and I do hate "buts"), the initial startup of many electric motors can be many times the running current, so for the initial 1 or 2 seconds the voltage drop may be considerable. It takes careful measurements to determine what is happening in those initial seconds.

[Eggscape]

In theory the same model voltage regulator that many people are using to protect the fan from high voltage could be used to provide a stable voltage for your furnace.

It is called a step up or step down voltage regulator. This means that if the voltage falls a bit it will maintain the voltage you have set. I have never had the occasion to use one but that is what the documents say.

[brroberts]

I do appreciate the ideas. I’ll certainly entertain other ideas as well. I will be home in December and see what I can discover.

Obviously current and voltage are related, and also voltage falls as a lead acid battery discharges. The amount of air pressure from the fan probably decreases at some voltage as well. Too me it’s strange that at 12.7VDC it’s already experiencing trouble. If it would even be reliable to 12.2V, it would be sufficient for my needs.

[Shiny]

This sounds frustrating.

Are you saying the "normal" voltage drop is causing your furnace blower motor to slow down enough for the sail switch to stay "off"?

I have no knowledge here but this feels like the motor may be suspect? I would expect the motor rpm to follow the current more than the voltage but either way, a drop of half a volt doesn't feel like it should effectively shut down the blower. We have never had this happen when the batteries dropped a lot so I'm curious.

You mentioned changing a duct to increase air velocity. Is it possible the sail switch is simply not working correctly (is binding or has insufficient "margin" when it actuates)?

ugghhh.. hope you figure it out soon

Mike

[Shiny]

Here's a link to a possible method of measuring the blower rpm using a cell-phone app:

https://youtu.be/xutm6oMTBw8

If it works, it would be a relatively easy way to directly see the effect your battery voltage has on the blower.

[brroberts]

Swapping sail switches and boards has not helped.