Intermittent Attwood 7916-II Furnace
 

Our 2007 TowLite's Attwood 7916-II furnace failed a while back, because of bad bearings, so we replaced the motor. It worked fine for ~10 hrs of actual operation, but is now failing intermittently. For example, when we set the thermostat and bed down at night, it later clicks on when the temp goes down, and the unit comes on, and all is good. This cycle repeats 3, 4 or 5 times, but then it stops, and we freeze for the rest of the night.

The next AM I remove the bed that's over the furnace compartment, remove the compartment lid, remove the furnace cover and find that the toggle switch (breaker button) is now "OFF." So, I flip it to "RESET" and the unit then fires up, good as new, until the next night, when it fails in the middle of the cold night and we freeze, because I will NOT tear my bed apart to go after that toggle switch at 3 AM. I'd rather freeze. My wife is not happy.

The compartment and furnace unit are very clean, and there's nothing blocking the intake or exhaust vent openings, so I don't know where to start. I've read over various posts on this forum about sail switches, and I've found on-line a bunch of inexpensive parts available for this unit, but I have no idea how to start troubleshooting an intermittent problem. I've attached photos of the furnace cover, furnace with uncovered furnace with black toggle switch, thermostat and intake vent, which probably are of no help, but there they are...

I'm appealing here for your best guesses on what to replace first, then second, then third,....!

Thanks in advance!

Lou - I feel for you. Intermittent problems can be infuriating.

I'm not clear about something from your post. The second night the furnace stopped working, was the "reset" switch again off? If so, that would indicate to me an electrical problem that is seen as a heavy draw. Now what would cause that heavy draw could be the motor or bearings again. There's not much else in there that's powered by electricity. It's also possible the reset switch itself is bad, and has become too sensitive to the starting current draw when the furnace fan starts.

If you move the fan by hand, do you feel any resistance? That would indicate a bad unit, which seems improbable since you just replaced it, but still .... Just looking at the thermostat, I don't see any way to set it to "fan only", which would be a way to help troubleshoot.

I suppose if the furnace failed to light, there may be a safety feature that would trip the reset switch, but I've never had any problems with mine, so I can't give good advice here. Perhaps someone else might have some thoughts. I do know that something as minor as a few spiderwebs can interfere with successful lighting.

Good luck!

- Jack

Yes, I flip the switch to "RESET" the next day before bed, and same story in the night: it flips to "OFF" after a few cycles. I like your idea that it needs a new toggle switch, because it may have become worn (and more sensitive) to current surges during its previous life with a motor having bad bearings. It's easy to try too.

I can look for spider webs on the sail switch too.

Thanks Jack.

Lou - I think if gunk like spiderwebs were causing the problem, they'd be around or in the burner outlets, or the inlets for where air is added to the propane feed, not the sail switch. That was the cause of an issue with a patio heater I had once and it was a real problem to correct. It takes very little to upset the proper air-fuel mixture ratio.

But, if you can see the burner flame when the furnace is on, I think it should be a blue color, with very little yellow. This is a guess, however, since I've never looked at mine. My patio heater's flame had turned from a vigorous clean blue flame to a "lazy", smokey, yellow one. Maybe someone would confirm this for us. If the flame is blue though, its mixture is good and the problem just about has to be in the blower circuit.

- Jack

Get a new igniter. I have the same furnace in another rv. Did the same thing and was tough to diagnose until I inspected the igniter under a magnifying glass and saw a hairline crack that was allowing the igniter to ground out intermittently and fail to spark...thus throwing the furnace into fault mode for failure to light fault

Hi Tom, and welcome! That sounds like good advice. It's certainly something to keep in mind when furnace problems crop up. Lou, if you're still having the problem, check out the igniter.

But, I don't see how this would trip the "Reset" switch - that symptom brings me back to the blower.

- Jack

Hi Jack, thanks for the welcome. I had put in a new sail switch and control board to solve my problem. Then I found the igniter issue. Problem solved. If you are going to "shotgun" parts by replacing like I did, start with the igniter. It is by far the easiest and cheapest to replace. I wasted about $100 before I changed the igniter. And when it fails the furnace will attempt several tries at ignition. I think it was three times on mine. Then when it senses "Non detect" on the actual burner flame it goes in failure mode requiring a manual reset. Just saying, check the easy and cheap stuff first. But for expensive and not so easy I'm heading out in the driveway now to replace all four lift cables. 1985 FunChaser 21L

Tom, thank you!! That's excellent information! I had no idea the igniter could cause the Reset switch to flip. With this in mind, Lou - since you replaced your blower, I now think the igniter is the next thing to look at.

- Jack

Well folks, sorry for my very delayed response, but I found the problem after planning a major heater intervention and buying a bunch of heater parts ($175). The first thing I did was replace the black toggle switch ($5), and that was it!! This switch is effectively a circuit breaker on a mini scale. It was bad. But why? Because when the RV service guy replaced the failed fan motor, he didn't also replace the $5 breaker switch that had been worn from being regularly tripped by the failing motor. I should have known that, because the same thing has happened in a particularly overused 15A circuit in my home. Circuit breakers are not immortal (like our children). They get old and need replacing when they've been worn down by repeated tripping/resetting cycles (like me). Thanks all for your great thoughts and help!

Thank YOU, Lou, for letting us know how your problem was resolved. It may save someone a lot of aggravation and expense in the future. Hope you were able to return the parts you did not need!

-Jack

Furnice trouble shooting.
 

Thanks for the follow up to your problem. I hate the hard to turn on Atwood thermostat. Dh replaced ours with an electronic one from Walmart. Push button on display panel is a breeze to set and use.

There is an excellent Youtube site that covers all kinds of rv repairs. It's My RV Works. It has become my go to site. Here is one for Atwood furnace trouble shooting: https://www.youtube.com/watch?v=AB69oqtSiwc&t=423s You can subscribe to it and it has lots of inf. You can even pull up manuals for lots of RV appliances.