Bench test power hookup for furnace using 12v automotive battery

[Ray-IN]

In October 2014 Dometic Group AB acquired Atwood Investment Holdings LLC (and its subsidiary Aircommand) as part of a global transaction. He has said that the part shows as not available.

Dometic is now owned by Lippert Industries, but operates as a stand-alone company.

 

[Kirk]

Dometic is now owned by Lippert Industries

Not true. Dometic Group is a Swedish company that manufactures a variety of products, notably for the outdoor, recreational vehicle, marine, and hospitality industries in the areas of Food & Beverage, Climate, Power & Control, and other applications. It operates in the Americas, EMEA and Asia Pacific. If you visit the Lippert brands webpage, you will see that Dometic is nowhere on it. You may be thinking of Furrrion, who is listed there.

 

[calstar]

I received the electrode(see pics),ordered from the page Kirk posted and it works on the 7900 series. The replacement is on the right in the pic below, different configuration and I straightened the spade connecter a little so the ignition wire is easier to connect. Click on the second pic to expand and you'll see what it's meant to replace.

I'd really like to say the furnace is 100% and ready to reinstall but there is one hicup, the furnace runs great on the bench, but not with the combustion wheel cover on. I put it on and took it off several times, when its off the furnace will ignite and run as long I want it to. Put the cover on and it ignites(usually not on the first attempt) but will only run for a few minutes or so, sometimes only 10 seconds. The pic below shows it running without the cover(cover is laying on the bench to the right), seems fine, try it with the cover on and not so fine. Combustion wheel is #47, the cover #53, #2 is the burner air tube which I assume is the source of combustion air to the burner, its free of obstructions. My only thought is the cover is somehow restricting the amount of air entering the air tube. When the furnace is running without the cover it seems I can detect a slight pulsing of the flame, not a continuous steady "roar", but I don't have the experience to know if this is atypical. Suggestions, thoughts, ideas? thanks

New parts installed: limit switch, sail switch, circuit controller board, electrode

 

[Lou Schneider]

Make sure your motor is rotating in the correct direction, centrifugal force will force air into the combustion chamber either way with the cover off but it has to be turning in the right direction to work with the cover on.

 

[Henry J Fate]

Also, make sure all the fins on the squirrel cage are nice and clean down to the metal both sides and the edges are clean. Carefully clean it without disturbing the fins.

During the failed test (with cover on) I assume the sail switch is functioning properly? If it is, probably a flame verification problem. The ignitor/electrode needs proper flame and proper exposure to it. This may be changing with the cap installed.

Another consideration is that you do not have a factory ignitor. This is something to keep in mind. May or may not be of an issue. Aligning the ingnitor/electrode into the flame is critical for flame verification.

 

[Henry J Fate]

The other thing to consider is your power source. If you are using a battery to bench test, keep it charged. Voltage should be over 12 volts when the heater is running.

 

[Kirk]

I am wondering about the screenshot of the parts breakdown that you posted? It is somewhat different than what I have. Not a lot but it left me wondering if we are looking at the same service manuals. Also, have you checked motor direction of rotation, as Lou has suggested? Reversing the leads on a DC motor will reverse it's rotation direction.

 

[calstar]

Make sure your motor is rotating in the correct direction, centrifugal force will force air into the combustion chamber either way with the cover off but it has to be turning in the right direction to work with the cover on.

Interesting, I'll remove the cover and check the rotation

Also, make sure all the fins on the squirrel cage are nice and clean down to the metal both sides and the edges are clean. Carefully clean it without disturbing the fins.

During the failed test (with cover on) I assume the sail switch is functioning properly? If it is, probably a flame verification problem. The ignitor/electrode needs proper flame and proper exposure to it. This may be changing with the cap installed.

Another consideration is that you do not have a factory ignitor. This is something to keep in mind. May or may not be of an issue. Aligning the ingnitor/electrode into the flame is critical for flame verification.

I'll pull the cage out and use compressed on it. Sail switch is functioning. The factory ignitor is no longer available, I could only find after market. Its not possible to see the alignment of the electrode on the burner head/flame on this model, they are both hidden with respect to one another when installed

The other thing to consider is your power source. If you are using a battery to bench test, keep it charged. Voltage should be over 12 volts when the heater is running.

Battery is fully charged, I pull it out of my wife's van when testing and its our daily driver

I am wondering about the screenshot of the parts breakdown that you posted? It is somewhat different than what I have. Not a lot but it left me wondering if we are looking at the same service manuals. Also, have you checked motor direction of rotation, as Lou has suggested? Reversing the leads on a DC motor will reverse its rotation direction.

Might be a different manual, what I posted is in pic below. I will check the rotation. I did not at any time remove the leads to the motor, what was done by previous owners is unknown, but I will check the leads when I remove it for cleaning. Interesting my burner assembly is one of the two lower ones as it has a deflector, I think the longer one, so....?

 

[Henry J Fate]

Interesting, I'll remove the cover and check the rotation

I'll pull the cage out and use compressed on it. Sail switch is functioning. The factory ignitor is no longer available, I could only find after market. Its not possible to see the alignment of the electrode on the burner head/flame on this model, they are both hidden with respect to one another when installed

Battery is fully charged, I pull it out of my wife's van when testing and its our daily driver

Might be a different manual, what I posted is in pic below. I will check the rotation. I did not at any time remove the leads to the motor, what was done by previous owners is unknown, but I will check the leads when I remove it for cleaning. Interesting my burner assembly is one of the two lower ones as it has a deflector, I think the longer one, so....?

Do you have the model number ? I know its a 7900 series but there are several. Also, Did you get a part number for the ignitor or the ignitor assembly ?

 

[calstar]

The paper sticker that gives the model number is not on the furnace, someone was in there before and must have removed it, they did write 7920 in place of the sticker, that's all I have to go on. The ignitor(electrode?) part # is 37517 which is an obsolete part, a cross reference part # was also not available, I looked and Amazon(as suggested above) and bought one that looked similar, at least it does fire up now. All of the model #s are confusing(see pic), don't know if I have a 7920 or a 7920II. I believe the "20" designates the btu rating.

 

[Henry J Fate]

Dometic will give you part numbers even though the parts have been discontinued. It may not matter whether it is a 7920 or 7920 ll. The part number for the ignitor is probably the same and Dometic can confirm that.

You may be able to purchase the complete assembly. Electrode, grounding rod, and bracket which could be advantageous.

If the furnace does fire up and stay running, the new electrode is working. You could reinstall the original control board to see if that makes any difference. The original board should be good.

Use a soft nylon brush to clean up the squirrel cage. Chase it with air. You can spray cleaning fluid on it but don't let is get to the motor or bearings. The cage is balanced. Be gentle.

You either have an air flow problem or the electrode isn't sitting where it should. Possibly a combination of both.

 

[calstar]

It'll be a few days before I work one again as we're going out of town, I will definitely try all the suggestions given. One question regarding the sail switch, if the squirrel cage/blower is rotating the wrong direction how would the switch's contacts close, the air direction would be forcing it to stay open. thanks

 

[calstar]

Dometic will give you part numbers even though the parts have been discontinued. It may not matter whether it is a 7920 or 7920 ll. The part number for the ignitor is probably the same and Dometic can confirm that....

If the furnace does fire up and stay running, the new electrode is working. You could reinstall the original control board to see if that makes any difference. The original board should be good.

Use a soft nylon brush to clean up the squirrel cage. Chase it with air. You can spray cleaning fluid on it but don't let is get to the motor or bearings. The cage is balanced. Be gentle.

You either have an air flow problem or the electrode isn't sitting where it should. Possibly a combination of both.

I called Dometic, very helpful, the part numbers they had were the same I found, 37517, 36998(no longer made/obsolete) and, 31380 available but not through them, I found the part on Amazon but nowhere else. I ordered it and it looks exactly like the one I ordered by visual comparison which runs the furnace fine until I put the cover on. This seems to point to the current electrode(knockoff) being ok for this application. How can one tell the electrode is in the correct positon, they are separate pieces, when one is taken out there is no way check the position of one to the other, when both are out they're just separate parts.

Air flow problem? The air is pushed about 6" through a tube to the combustion chamber, both are perfectly clear/clean so.... I will change out the currently working electrode with the new(31380) but I doubt a difference in furnace function.

I asked above before regarding the sail switch, if the squirrel cage/blower is rotating the wrong direction how would the switch's contacts close, the air direction would be forcing it to stay open and the furnace wouldn't run, right?

Dometic rep also told me they had taken over Atwood, this may or may not clear up that question as raised earlier.

When I exchange the electrode(again) I will clean the blower. If it's not running after that I'll probably give up, I don't do much cold weather camping anyway. If anybody has any miracle fixes I'm all ears.

thanks. Brian

 

[Ray-IN]

Not true. Dometic Group is a Swedish company that manufactures a variety of products, notably for the outdoor, recreational vehicle, marine, and hospitality industries in the areas of Food & Beverage, Climate, Power & Control, and other applications. It operates in the Americas, EMEA and Asia Pacific. If you visit the Lippert brands webpage, you will see that Dometic is nowhere on it. You may be thinking of Furrrion, who is listed there.

Thanks for the catch and correction. LIppert Industries owns so many brands I made a mistake.
Dometic owns Fantastic Fan and I keep trying to remember that, perhaps that was my downfall.

 

[Kirk]

When I exchange the electrode(again) I will clean the blower. If it's not running after that I'll probably give up, I don't do much cold weather camping anyway.

Any news?

 

[Gary RV_Wizard]

I asked above before regarding the sail switch, if the squirrel cage/blower is rotating the wrong direction how would the switch's contacts close, the air direction would be forcing it to stay open and the furnace wouldn't run, right?

Most likely.

Both the sail switch & the limit switch can be bypassed easily so you can test to see which (if either) is involved.

I'm not real familiar with the 7900 series furnaces. Is the sail switch on the air path for combustion air or circulation air? On the 8500 & 8900 it's on the circulation air path. The burner works fine even without adequate circulation air flow, but it overheats after awhile.

 

[calstar]

Opened the furnace up again, cleaned the cage(already very clean, probably didn't need it) put it back together, ran it through multiple cycles on the bench and it works fine with the cover on.....don't know what I did but I'll take the results! Sail switch is on circulation air path.

thanks to all for your helpful input

 

[Henry J Fate]

There is a "combustion wheel" which provides air flow in the combustion chamber. It should be driven by the same motor that the cage is running from. That should be clean and the blades should be undisturbed sitting uniformly in their designed position.

I would continue to test to be sure the problem is solved. Before testing, always take a voltage reading from the battery you are using and note it. Make sure the battery has settled before testing. Over night is good. Variances in voltage will change the speed of the fan motor. A sensor that is not mounted in an optimum location could respond positively or negatively to changes in combustion air flow.

 

[calstar]

There is a "combustion wheel" which provides air flow in the combustion chamber. It should be driven by the same motor that the cage is running from. That should be clean and the blades should be undisturbed sitting uniformly in their designed position.

I would continue to test to be sure the problem is solved. Before testing, always take a voltage reading from the battery you are using and note it. Make sure the battery has settled before testing. Over night is good. Variances in voltage will change the speed of the fan motor. A sensor that is not mounted in an optimum location could respond positively or negatively to changes in combustion air flow.

Combustion wheel is clean as can be, blades aligned. Used the battery from wife's van, it hadn't been driven on the days I used it so it was settled down. I did more bench testing before installing the furnace, all good, also did multiple tests after installation, that too worked as it should. Happy camper!

 

[Henry J Fate]

That sounds good. Still an unknown reason for the failure but hopefully it will continue to provide dependable heat. If it does fail again, keep an eye on the 12 volt supply. Fan speeds and air volume will change as voltage drops. Good luck.