Atwood 7900-series Furnace - Inconsistent Lighting

Discussion in 'Heating / Cooling Systems' started by Douggro, Sep 11, 2017.

  1. Douggro

    Douggro Active Member

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    Jul 26, 2017
    Seattle, WA
    I'm going to start chasing this problem in our pup and thought I'd seek input for avenues that I haven't considered from the collective knowledge here.

    Unit: Atwood 7900-series furnace (exact model not yet identified), in a '95 Coleman Utah.
    Problem: Inconsistent lighting of burner. Happens when running from battery or shore power.
    Symptoms: Sometimes the furnace lights right up on the first or second click of the ignitor cycle; other times it may take six or eight sparks for the burner to light. Sometimes it doesn't light at all through several ignition cycles. And the worst case is it will light the burner after loading the combustion chamber with propane and make a hellacious BANG!
    No other LP appliances are running when this happens, though I haven't checked if there's any difference if the galley stove is lit. The external stove is on a separate high-pressure line.
    I have taken to lighting the galley stove after setup and running it for a few minutes to purge the gas line.
    Plan of Action: I'm going to do as much inspection of the wiring (I have the manual and diagram) to make sure that PO's haven't mis-wired anything over the years, as well as inspect and clean the burner (if I can remove it without removing the entire furnace) and the ignitor. Will also check the gas lines (regulator to furnace), valves and solenoids to make sure they're clean and operating properly. I've also learned the correct anticipator setting for the thermostat and will dial that into the correct range to the Atwood specs (0.48 amps).
    Previous History: The PO had replaced the thermostat with an analog White-Rogers unit thinking it was the cause of non-operation, and the furnace wasn't working when we purchased the pup. I found that the time-delay relay was bad and replaced it and the furnace started working but with the problem as noted. I also had to replace the LP regulator due to a leak in the (very old) service selector switch - it's a two-tank setup.

    Any input would be welcome! Thanks!
     
  2. Fless

    Fless Active Member

    Sometimes replacement LP regulators are not adjusted to the proper pressure. Also, have you tried to switch propane bottles, in case of bad gas? [:D]
     
  3. Douggro

    Douggro Active Member

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    Jul 26, 2017
    Seattle, WA
    I haven't checked the pressure with a manometer - something that I plan on doing anyway after I make one. As for the LP, it's been refilled in the interim from a different source and the problem has remained. The stove works fine and the propane side of the fridge does as well, so I'm thinking the LP is OK.
     
  4. Antipodes

    Antipodes Call me Paul

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    Jan 28, 2015
    Christchurch, New Zealand
    A couple of things I can think of.
    The main jet may be blocked sufficiently to only allow a small flow. This will not provide enough fuel to start the process, but after a few seconds the build up will ignite giving the symptoms you have described. If this is the case when it is running does the flames seem small in the window (if you have one) and is the heat output not what is expected.
    Check the ignitor is the correct gap to ensure a good spark is present.
    And possibly but least likely the fan that blows air into the combustion chamber is damaged and not flowing the air through sufficiently. (Least likely as it does run and would not expect it to if this was the case)
     
  5. Boatnman

    Boatnman Active Member

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    Jan 19, 2017
    western Colorado
    We went camping over Labor Day weekend and our Atwood 7916-II started doing the same thing. Sometimes it would go through the heating cycle just fine and other times it would "bang" (firecracker type sound) just once during a heating cycle. I removed the furnace and tore it down but couldn't find any obstructions that would cause the LP to built up. (All of the other LP appliances work fine and our regulator is only a couple of years old.) To be safe, I dropped the furnace off at our local RV dealer to have it completely serviced. I'm not sure how long it will take them to look at it, but I will post the results when I get it back. (We've had one cylinder of bad LP - too much scent additive - so it may be a clogged valve.)
     
  6. Douggro

    Douggro Active Member

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    Jul 26, 2017
    Seattle, WA
    Not sure what the internals of the gas valve are mechanically but it uses a dual-solenoid system to open the valve. If I can get to the burner easily, a thorough cleaning of the burner and jet (if applicable) are in the plan. The ignitor is on my "Top Suspect" list given the age of unit and the fact that it's probably hasn't seen a proper servicing in way too long. I'll check the porcelain and gap spacing as part of the inspection and cleaning.
    Yes, please do. It's somewhat reassuring to know that someone else is having the same problem, but I'm sorry that you're having it too! Likewise, I'll update what I find when I do my work it.
     
  7. theseus

    theseus Centerville, OH

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    Feb 6, 2007
    Centerville, OH
    Funny. I just replaced the Atwood thermostat with a Suburban model because I personally hate the Atwood model. Where the thermostat is located in my PUP, you cant tell if the on or off switch is actually on or off. At least with the Suburban thermostat, which is a duplicate of the White-Rogers, I know if it is off.

    Your problem reminds me of what my refrigerator was doing this summer. The orifice was blocked and when I would turn it on it would have a nice bang then go out. I replaced the orifice and it worked fine. I assume you've cleaned all gas parts in alcohol and blown compressed air through?
     
  8. Douggro

    Douggro Active Member

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    Seattle, WA
    I think the thermostat they installed is about the cheapest White-Rogers model you can get and it has no On/Off switch. I had to install a toggle switch on the return wire as a "fix" after waking up to the fan constantly running in the middle of the night during our trip a few months ago.

    Flushing the lines and a blowout are on my list for the general service I'm going to do. Solvent alcohol, yes? I don't think isopropyl has enough tooth to dissolve some of the gunk that could be in there.
     
  9. Boatnman

    Boatnman Active Member

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    western Colorado
    I checked the ignitor on mine and it had the proper gap. It actually looked good for a 10 year old part - no corrosion or signs of wear. The tip of the burner was discolored (black) compared to the burner surface closer to the valve.

    I have replaced the thermostat with a digital one. (I included a pigtail so I can set the thermostat at an appropriate height.) I don't think the thermostat has anything to do with the issue since as soon as the thermostat click on, the fan starts and the fan then shuts off a couple of minutes after the thermostat clicks off.

    Because of the bad cylinder of LP (which caused the water heater and refrigerator to have issues lighting), during the off-season I disconnect the main LP line from the bottom of the regulator, pointed the loose end down, lowered the front of the PUP and let the line drain. After a couple of weeks, several drops of brown oil had dripped out. I also blew out the line as I reassemble the piping, but I have no way of knowing what is still in the various valves.

    I asked the RV service department to check the entire furnace and to make sure it's safe. I'm very interested to hear what they find.
     
  10. Douggro

    Douggro Active Member

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    Jul 26, 2017
    Seattle, WA
    Yeah, that BANG that it makes is rather disconcerting - especially to the spouse and the neighbors.. Do tell what the service folks say when you get it back.

    It's a fair bet that the ignitor is the original part, now edging 23 years old, and it still remains my lead suspect. I may just order one and replace it on principle as I think they're less than $20. I hate using the "parts cannon" approach fixing things, but intermittent poor sparking from a hairline crack in the porcelain is a likely cause.
     
  11. Boatnman

    Boatnman Active Member

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    Jan 19, 2017
    western Colorado
    With the ignitor being connected to a component on the circuit board, there were just too many options as to why ours was misfiring. The combination of LP, ignition, usually at night, sleeping without hearing aids, and the desire to have everyone in the PUP wake up the next morning, I want to make sure the entire unit is safe. You can see the discoloration in one of the heat exchangers so it was time for ours to be completely serviced. I'll post the results...
     

    Attached Files:

  12. Douggro

    Douggro Active Member

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    Seattle, WA
    :D Yeah, those are some good reasons!
    Yours looks like an 8000-series unit, different than mine but still the same species. I'm going to try to remember to document what I find as I tear into mine and post pics as appropriate.
     
  13. Boatnman

    Boatnman Active Member

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    It's a 7916-II.

    Well, I got it back already. The repair ticket says, "Removed burner assemble and cleaned out housing. Reassembled and tested for proper operation with no further issues found." I spoke with the service tech that did the work and he said they cycled it at least 10 times and it worked fine. I further explained what was happening and he said that didn't happen for them and the only other thing could be gas pressure which they'd have to have the PUP there to test that.

    I'm going to reinstall it and test it. Our regulator is less than 2 years old and all the other propane devices work fine...but, we'll see what happens. I'll post the results.
     
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  14. Douggro

    Douggro Active Member

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    Jul 26, 2017
    Seattle, WA
    Hmm.. well, thanks for the report and I hope that it works without problems for you!
    Digging into mine tomorrow. :)
     
  15. Boatnman

    Boatnman Active Member

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    western Colorado
    They said they removed soot from the burner, but I had already blown that out - very little. The only thing they could have removed is the discoloration I described above. It was Friday afternoon and raining, so the unit will be reinstalled tomorrow...maybe.
     
  16. Douggro

    Douggro Active Member

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    Jul 26, 2017
    Seattle, WA
    Had to go back and look at your picture; realized that you had it somewhat disassembled at the time and didn't recognize it. Discoloration looks like it's on a heat-exchanger type of body, yes?
     
  17. kitphantom

    kitphantom Well-Known Member

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    Dec 26, 2009
    Albuquerque, NM
    Intermittent issues can be frustrating. Ours began to act up last year, with inconsistent lighting - though at least we never had a bang - but the shop couldn't find anything wrong. At least we knew they had tested the gas pressure, etc. that we couldn't. Finally, we did research on the web, and found that the likely candidates were the sail switch and a limiter switch. When I went in to order, the knowledgeable parts guy suggested just ordering the sail switch, since that was usually the culprit. We replaced that, and it has worked fine since.
     
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  18. Boatnman

    Boatnman Active Member

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    Jan 19, 2017
    western Colorado
    I talked to the service manager about the discoloration of the heat exchanger and he said it happened during manufacturing. I'm skeptical. And it's still there - it looks similar to the grille over a camp stove burner after it has been subjected to heat, but it is very slight.

    The other discolored area was the tip of the burner. About 1/3 of the tip end of the spear shaped burner was black with what looked like soot. I used an air compressor to blow out the assembly, but didn't wipe off the soot (just forgot to in my eagerness to reassemble).

    The sail switch will keep the gas valve from opening if it isn't receiving enough air flow. The limit switch closes the gas valve when the combustion box reaches a certain temperature. The tech guy said they were both good and that they also tested the circuit board and it was fine. I guess I need to go out and install it and test it...maybe I'll let it run all night and check the shrubs next to it in the morning for scorch marks...o_O
     
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  19. Douggro

    Douggro Active Member

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    Jul 26, 2017
    Seattle, WA
    I'll be doing a post a bit later with pictures of what I found in my tear-down earlier today. In the interim, let's just say that I ordered several parts today for maintenance purposes.. :mad:
     
  20. Douggro

    Douggro Active Member

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    Jul 26, 2017
    Seattle, WA
    So first-off, I discovered that the furnace is a model 8012-II, not a 7900-series. The 7900's and 8012 are very similar and share many common parts, but still different. I for-sure thought I had a 7912... :confused: My guess is that this was a replacement furnace based on what I found of the quality of the installation. Shoddy workmanship. And one "modification" that I'll detail later that has me shaking my head. I have no idea what, if any, service or maintenance the PO's had done. But based on what I found, it was damn little.

    Disassembly:
    Pulled the screws that mount the circuit board frame; the circuit board was caked with dust, similar to what I found caked in the 3-way fridge compartment when I was working on it:
    IMG_0455.JPG

    Then I dove into removing items from the front of the furnace to get the burner assembly out. Started with the ignitor unit and immediately noticed that its gasket had deteriorated and showed signs of burn-through / sooting:
    IMG_0457.JPG
    {the sooting was more visible than shown in this pic}

    The ignitor itself looked in decent shape, save the soot at the base and the missing porcelain cap on the end of the insulator body. The gap was the proper 0.125" and the electrodes are in good condition with no pitting or corrosion.
    IMG_0456.JPG
    You can see where the porcelain cap has gone missing from the main ignitor lead where it comes out of the insulator.

    Then it was on to getting the burner out. Disconnected the gas line and removed the six screws holding it in; those bottom screws are a bummer to get at! But patience prevailed and it came out. The gasket(s) however did not survive. And the burner looked like this:
    IMG_0458.JPG
    With the exception of the darker area near the tip of the burner, the burner plate was severely corroded and scaled with rust. The holes all seemed to be clear, but the rust/scale was pretty thick.

    The inside of the burner box had lumps of soot in it:
    IMG_0459.JPG

    I took a bronze brush and carb cleaner to the burner plate and had zero confidence with re-using the burner. I decided to order a new burner (part #30268), the ignitor electrode (part #36998), and the electrode gasket (part #32172) and burner box gaskets (2@, part #35890). A local dealer will have them for me on Monday, and the prices were very much inline with what I found them through online vendors for. Total cost of parts, less than $50.

    Checking inside the combustion chamber (fire box) revealed a pile of clumpy soot and ash:
    IMG_0462.JPG
    Pulled out the ShopVac to clear it out and in a fitting bit of irony, the "magic smoke" in the motor of the ShopVac chose this moment to escape. (I've had that ShopVac since before I met the missus. Guess it didn't owe me anything by this time. :) ) It had enough life left to pull most of that crap out of the chamber before dying, and I cleared the rest with a nylon brush.

    I also checked the resistance of the gas valve solenoids and they both were in the proper 30-50 Ohm range. I pulled the orifice jet from the valve unit after cleaning it with a bronze brush and it was clean inside with no debris visible nor was there any oily residue inside. I reinstalled it with a tiny amount of pipe thread sealant. The gas valve itself seems to be a non-serviceable unit, so I didn't tear into it to check the valve body. Can't rule this out as a problem source, but the part is $120 and I don't want to go there yet.

    As for the quality of the installation, as I mentioned it was terrible. It appears that the opening was poorly modified to take the unit which resulted in the sheet metal being distorted in certain locations that made fitment of the other components difficult. But the most puzzling was that the gas valve unit had been rotated a few degrees CCW and the plate that the burner mounts to re-drilled. It seems to have been done because of the position of the gas line coming into the furnace box. It's not very clear in this photo but I've put the red arrow where you can see the original screw hole adjoining the protruding screw holding the burner to the burner box and valve assembly:
    IMG_0453.JPG
    You can also see how the valve unit is rotated by comparing its edges to that of the burner box. I was able to get the burner back into the original holes and reposition the gas line to fit properly fairly easily.

    So at this point, I'm going to wait for getting the new parts installed before doing further testing of the LP system. Very hard to make effective troubleshooting evaluations when the components of the furnace were in such bad shape.
     

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