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jbrack's avatar
jbrack
Explorer
May 21, 2019

Atwood Furnace

I'm a little frustrated with my Atwood furnace. I have a 2018 5th wheel that I bought at the RV show in Sacramento. I hit the road to Arizona and almost immediately had trouble with the furnace. For example, if the temp was 60" inside the coach and I set the thermostat on 70", the furnace would come on and cook for about 5 minutes and then the burner would shut off. The fan was still blowing and then in about a minute, the burner would fire back up. It would cycle on and off like this until if reached the desired 70". I took it in to a mechanic in Tucson and they replaced the circuit board. About 10-12 months later, the fan when I wanted heat, the fan would come on but the burner would not light. I took it in again (Camping World in Vacaville, CA) and they replace the Sail Switch. Now it lights right away but is back to that cycling operation from before. I called Dometic who took over Atwood, and they can't help me. They said I just need to take it back to a mechanic. My furnace is accessed through the storage compartment or basement. Each time I have to remove all of my gear and take down the walls so the guy can crawl in there. It's a royal pain the butt. Anyone have a guess why its cycling like this? None of the mechanics seems to know.

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  • dougrainer's avatar
    dougrainer
    Nomad
    May 25, 2019
    BFL13 wrote:
    The limit switch must be working for it to be doing that intermittent burning? It can't be stuck open. Too sensitive maybe?

    Changing the main board fixed the OP's intermittent problem for almost a year so it wasn't the ducts etc. Then it would not light at all after the fan came on (limit switch stuck open now?). New sail switch fixed that but back to intermittent action. (if they didn't also change the limit switch then it must still not be stuck open)

    Has the replacement board from a year ago gone bad? What was that all about with the duff set of boards?--was that furnaces or fridges, can't remember.

    The limit switch might be the most likely culprit, but it is the hardest thing to get at, so IMO it is the last thing to try for a DIY. In my case it was the main board at fault, which is the easiest thing to swap out, but last on the list of things to try according to the pros. Maybe getting at the limit switch is easy for a pro! :)

    Is there no warranty on this furnace in a 2018 RV ?


    You misunderstood what I meant about stuck OPEN. Usually, when you get a furnace that the fan runs but the Gas Valve and Ignitor fail, it is the Hi Limit switch and it is stuck OPEN. When I have this condition, all I have to do is take a long metal punch, insert it by the Hi Limit switch and then pop the punch with a hammer. THAT resets a defective open Hi Limit and then if the furnace fires up, I know to disassemble and replace the Hi limit switch. Sometimes when disassembling to gain access to the Limit you bump the furnace and it resets. You can also go to the 2 white wires feeding the Limit and test for continuity or 12 volts when the furnace fan comes on, but that takes time. Using my punch method gives you a diag in less than 30 seconds. The OP's problem cannot be a defective sail switch. Possible but highly unlikely. Doug
  • BFL13's avatar
    BFL13
    Explorer II
    May 25, 2019
    The limit switch must be working for it to be doing that intermittent burning? It can't be stuck open. Too sensitive maybe?

    Changing the main board fixed the OP's intermittent problem for almost a year so it wasn't the ducts etc. Then it would not light at all after the fan came on (limit switch stuck open now?). New sail switch fixed that but back to intermittent action. (if they didn't also change the limit switch then it must still not be stuck open)

    Has the replacement board from a year ago gone bad? What was that all about with the duff set of boards?--was that furnaces or fridges, can't remember.

    The limit switch might be the most likely culprit, but it is the hardest thing to get at, so IMO it is the last thing to try for a DIY. In my case it was the main board at fault, which is the easiest thing to swap out, but last on the list of things to try according to the pros. Maybe getting at the limit switch is easy for a pro! :)

    Is there no warranty on this furnace in a 2018 RV ?
  • dougrainer's avatar
    dougrainer
    Nomad
    May 25, 2019
    BFL13 wrote:
    Old-Biscuit said in my Atwood: (see link in my post after the OP)

    "Mid...2 White Wires going into motor cover....Sail Switch, when it closes the DC from Sail Switch goes to High Temp Limit Switch and then to circuit bd via connector"

    In the OP you said after they changed the main board it worked for a year until they changed the sail switch.

    So the suspect would be the wiring they did for the sail switch?


    Possible, but I would change the Hi Limit switch. THEN that takes the Hi limit switch out of the possible problem. It is almost impossible for a sail switch to close then open in operation(killing the signal to stay running the burner) and then close again. Flame On and Off after running for a few minutes is almost 100 percent the Hi limit switch. Usually the Hi limit on this model, it sticks OPEN and does not allow the furnace to fire. Doug
  • dougrainer's avatar
    dougrainer
    Nomad
    May 24, 2019
    Limit switches have a habit of failing. Since the ductwork appears open, the fix is to replace the Limit switch. Doug
  • BFL13's avatar
    BFL13
    Explorer II
    May 24, 2019
    Old-Biscuit said in my Atwood: (see link in my post after the OP)

    "Mid...2 White Wires going into motor cover....Sail Switch, when it closes the DC from Sail Switch goes to High Temp Limit Switch and then to circuit bd via connector"

    In the OP you said after they changed the main board it worked for a year until they changed the sail switch.

    So the suspect would be the wiring they did for the sail switch?
  • jbrack's avatar
    jbrack
    Explorer
    May 24, 2019
    Old-Biscuit wrote:
    BFL13 wrote:
    People add muffin fans to their fridge vents to solve that problem if needed.

    So to help with this, how about putting a fan behind the big furnace return air vent to suck more cooling air in to where that limit switch is mounted back there?


    High limit switch is a safety device that SHUTS down burner because heat exchanger is overheating.
    Fan cooling area where high limit switch is located does NOT resolve the overheating issue.
    Lack of sufficient air flow THUR heat exchanger is the issue...NOT that the high limit switch is actually doing it's job as intended.

    Find/fix REASON high limit switch temp set point is being met.


    All of my duct work appears normal...the exhaust does not have an obvious obstruction when I look in with a flashlight. Would adding an additional floor vent help? I have a total of 4 vents now.
  • Old-Biscuit's avatar
    Old-Biscuit
    Explorer III
    May 22, 2019
    BFL13 wrote:
    People add muffin fans to their fridge vents to solve that problem if needed.

    So to help with this, how about putting a fan behind the big furnace return air vent to suck more cooling air in to where that limit switch is mounted back there?


    High limit switch is a safety device that SHUTS down burner because heat exchanger is overheating.
    Fan cooling area where high limit switch is located does NOT resolve the overheating issue.
    Lack of sufficient air flow THUR heat exchanger is the issue...NOT that the high limit switch is actually doing it's job as intended.

    Find/fix REASON high limit switch temp set point is being met.
  • Old-Biscuit's avatar
    Old-Biscuit
    Explorer III
    May 22, 2019
    jbrack wrote:
    starcraft69 wrote:
    Old-Biscuit wrote:
    Burner cycling on then off/back on/off is called 'limiting' which is when the airflow has been restricted and heat exchanger high temp limit switch is opening (too hot) then fan continues to run so temp drops...high temp limit switch closes and burner lights back off

    You have something obstructing/blocking either the furnace discharge air flow OR the return air to furnace
    Could be closed/blocked register, could be crushed ductwork, could be how/where stuff is stored


    Fan runs cause high temp limit switch opened.burner keeps firing back off cause thermostat temp set point has not been met....high temp limit switch controlling burner shutting down/relighting



    Listen to this man! This is the correct answer.




    This makes the most sense to me...and has been suggested to me before but none of the mechanics who have looked at the furnace have investigated this. None of my inside vent are blocked or restricted. I can see most of the ductwork from inside the belly but cant see the duct that feeds the one floor vent in the extreme rear of the coach. My next question is...if I continue to operate the furnace in this on/off fluctuating mode, will it cause any damage?


    YES!
    Continued operation where high limit switch is 'controlling' the burner cycle can result in damage to the HEAT EXCHANGER.....cracks/warping---gasket leaks. That then results in CO Leakage into the INSIDE of RV,
    CO is the 'silent killer'.

    Every RV Furnace has a 'minimum ducting' requirements AND Return air requirements.
    4" flex duct = 12 sq/ft (2" duct is NOT 1/2 of 4"---only 2 sq/inch)
    16K/20K btu furnace---minimum 24 sq/inch (2 ducts)
    25K/31K btu furnace---minimum 36 sq/inch (3 ducts)
    35K/40K btu furnace---minimum 48 sq/inch (4 ducts)

    Return Air......respectively
    33 sq/inches, 65 sq/inches & 80 sq/inches



    Furnace overfiring is another means of high limit switch cycling.
    LP System pressure set too high will cause overfiring.
    LP System should be set to 11"WC with at least 50% of propane appliances in service.



    And then there is Thermostat Location
    Does a register blow towards thermostat?
    Is warm air coming thru hole where wires to thermostat exit the wall?
  • BFL13's avatar
    BFL13
    Explorer II
    May 22, 2019
    People add muffin fans to their fridge vents to solve that problem if needed.

    So to help with this, how about putting a fan behind the big furnace return air vent to suck more cooling air in to where that limit switch is mounted back there?

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