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Author Topic: Atwood 8535 IV furnace  (Read 611 times)

Minerkev

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Atwood 8535 IV furnace
« on: December 17, 2017, 05:40:38 PM »
I have a 95 dolphin 533 with an atwood 8535IV furnace
The furnace was working fine then the other day when it came on we smelled gas and shut it off
I've tried it again it lights but smells like propane and the exhaust is sooty
Would appreciate any help

Gary RV_Wizard

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Re: Atwood 8535 IV furnace
« Reply #1 on: December 17, 2017, 06:05:51 PM »
Sooty exhaust generally means that something is partially blocking the external air intake and/or exhaust tubes. Insect nests, mud daubers, etc. or common causes. You will smell propane if combustion is incomplete, which is the reason for the soot. However, the smell is usually outside, or at worst leaking in through windows or other places.

Another possibility in a furnace that old is that a hole has burned or rusted through the combustion chamber. That lets air into the burner and usually disrupts the air/fuel mix and causes poor combustion. That hole will also let exhaust into the interior of the RV, a very seriosu problem because of both fire danger and carbon-monoxide formed when combustion is incomplete.

The furnace needs a thorough check and probably some overdue maintenance.  There are some documents available to help you on this. One is anAtwood Service Manual, which can be obtained (free) at http://bryantrv.com/docs2/docs/hflameservice.pdf
The other is an article I wrote for the RVForum Library titled Furnace Troubleshooting. Get it at http://www.rvforum.net/miscfiles/Furnace_Trouble-2.pdf
Gary
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Gary Brinck
Summers: Black Mountain, NC
Home: Ocala National Forest, FL

Fatgnat

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Re: Atwood 8535 IV furnace
« Reply #2 on: December 26, 2017, 07:23:18 PM »
I have just encountered the same problem. It was suggested to me by a local RV business that the gas pressure might be low especially because it's been cold (24 F) and that this would cause the burner to be inefficient, causing the soot buildup. So I got a manometer and t'd it into the system and found that with the furnace and refrigerator on the pressure was a bit below 9 inches of water column -- the minimum for the furnace being 11 inches! So with the manometer connected I adjusted the regulator to about 11.25 inches under the same conditions. It's now burning cleanly -- though it's warm today. The real test should come tonight. But I think a pressure test would be in order for anyone experiencing this problem.

John From Detroit

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Re: Atwood 8535 IV furnace
« Reply #3 on: December 27, 2017, 10:57:17 AM »
Re Temperture and gas pressure.

There are two gasses.. Some folks, mostly if they fill up in the south, have a nice full tank of BUTANE, which is not much good below freezing as that's the "Vapor Point" or "Boiling point" of Butane

Propane, on the other hand.... The boiling point is somethign like -39 as I reall (I usualy say 40 below your choice of C or F because it is within a degree).

So no, 24 degrees will not lower the tank pressure if it is full of propane.. Neither will -24..
Nothing adds excitement like something that is none of your business
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Fatgnat

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Re: Atwood 8535 IV furnace
« Reply #4 on: December 27, 2017, 01:25:30 PM »
Well, now I have to report that raising the pressure didn't fix the problem. Sooted up again last night. There's no blockage visible but there must be something restricting the airflow. I'm not sure what to do next. I don't know if there's butane in the propane here in Quartzsite but I doubt it's the problem as we haven't had it in previous years.

NY_Dutch

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Re: Atwood 8535 IV furnace
« Reply #5 on: December 27, 2017, 03:35:36 PM »
Despite John's continued spreading of the old "butane myth", HD5 "consumer grade" propane is the most widely sold and distributed grade of propane in the US. HD5 is the highest grade propane available to consumers in the United States and is what propane companies ordinarily sell to their customers. HD5 has a minimum of 90% propane, a maximum of 5% propylene, with the balance made up of other gases such as iso-butane, butane, methane, etc. The odds of finding an unscrupulous dealer that's selling high butane content LPG to consumers are extremely slim. Mexico is a different story...
Dutch
2001 GBM Landau 34' Class A
F53 Chassis, Triton V10, TST TPMS
2011 Toyota RAV4 4WD/Remco pump
ReadyBrute Elite tow bar/Blue Ox base plate

Fatgnat

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Re: Atwood 8535 IV furnace
« Reply #6 on: December 29, 2017, 09:18:02 PM »
Today I removed the burner and found the perforated flame spreader to be partially plugged with soot and many of the holes slightly constricted by rust. Cleaned it up, ran a drill bit through all the holes, blew it out and put it back together. I'm betting that the slight constrictions added up to enough to mess with the air/gas mixture, causing the soot. If that's correct, it should burn cleanly tonight!

Fatgnat

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Re: Atwood 8535 IV furnace
« Reply #7 on: December 30, 2017, 08:09:35 PM »
But that still didn't do it. Took it apart again today and really worked at getting the soot build-up out of the combustion chamber suing various makeshift tools plus air compressor. Got a whole lot out and re-assembled it. Now it sounds better but it's smoking big time! Smells better, the exhaust pipe is turning grey instead of black, so I'm hoping that it's busy burning off 18 years of build-up caused by operating with low gas pressure. We'll see. It's definitely heating the inside a lot better than it was last night.

Is anybody interested in these reports? I'll quit if not.  :)

kdbgoat

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Re: Atwood 8535 IV furnace
« Reply #8 on: December 31, 2017, 05:52:26 AM »
Is anybody interested in these reports? I'll quit if not.  :)

Yep, reading every one of them. I'm probably not the only one concerned about your safety while running a furnace that's not working correctly either. I just don't have anything to add to what others have said to help you out.
I know you believe you understand what you think I said,
But I am not sure you realize what you heard is not what I meant


2016 Leprechaun 319DS

Heli_av8tor

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Re: Atwood 8535 IV furnace
« Reply #9 on: December 31, 2017, 07:08:19 AM »
I had a mud dauber constriction in my furnace when I bought the coach. The soot on the vent should have tipped me off but I didn't know that then.

I ran an endoscope camera inside to see what I was dealing with and to tell when I had it all broke away from the tube and removed. It was too far up to see any other way.

Hope you are close to a final solution.

Tom
Tom & Theresa
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2014 Honda CR-V Toad, Roadmaster -5 Base and tow bar
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Gary RV_Wizard

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Re: Atwood 8535 IV furnace
« Reply #10 on: December 31, 2017, 10:34:15 AM »
Quote
The odds of finding an unscrupulous dealer that's selling high butane content LPG to consumers are extremely slim.

Mostly true, I think.  Higher butane blends are reportedly common in the Deep South during the summer, where concerns run more to high temperatures than low, but I don't know how common that may actually be.  Also, I understand that the HD10 blend is common in California. HD10 allows for 10% propylene rather than the 5% of HD5. Neither HD5 not HD10 have much butane content, though, no more than 2%.  HD5 is rated for use in internal combustion engines, e.g. fork lifts and such, and must contain at least 90% propane and no more than 5% propylene.  Butane, methane and other gases cannot exceed 5%, and butane specifically cannot exceed 2%. 5% butane & other gases would seem to have very little effect on vaporization at low temperatures.

The so-called "commercial grade" LP is neither HD5 or HD10 and may contain higher amounts of butane. It is used in commercial processes such as refineries and plastics and is readily available from LP producers. From what I can find, typical commercial grade LP is similar to HD10 but may contain up to 5% butane rather than 2%, but there is no standard commercial blend.  I suspect that commercial grade LP is sold and delivered by tank car rather than by the pound at local refillers.
Gary
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Gary Brinck
Summers: Black Mountain, NC
Home: Ocala National Forest, FL

NY_Dutch

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Re: Atwood 8535 IV furnace
« Reply #11 on: December 31, 2017, 07:45:47 PM »
Good point, Gary. I should have mentioned the "dirtier" HD10 sold in California. Higher propylene content has a tendency to make valves, etc, stick, but it still works fine for most RV uses. LPG powered vehicles don't fair as well on HD10, so I'd be a little concerned about running it a LP fueled generator. The higher propylene content doesn't affect the low temperature performance for heating and cooking though.
Dutch
2001 GBM Landau 34' Class A
F53 Chassis, Triton V10, TST TPMS
2011 Toyota RAV4 4WD/Remco pump
ReadyBrute Elite tow bar/Blue Ox base plate

Fatgnat

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Re: Atwood 8535 IV furnace
« Reply #12 on: January 01, 2018, 02:13:47 PM »
Well, the saga continues. Got extended blowgun and another to which I could attach a fairly stiff tube I could push pretty far back in and blew out a lot more soot, some if it in fairly big chunks. Still smoking big time -- watched it in the dark with a flashlight and it was impressively yucky. Another experienced RV mechanic insisted the problem was low voltage to the motor so I checked that and while the system voltage was 12.4 volts the blower motor was only getting 10.8 so I turned it on and jumpered from the batteries to the motor directly and the motor sped up significantly, now getting 12.2 volts, but the burner continued to smoke (and stink!). So that's where I'm at right now. I'd like to eliminate the voltage drops and can visualize doing that most effectively by running a fused power wire to the motor via a relay driven by the present power connection. But There still has to be some other reason for the burner to be so far from burning cleanly. Even if there's still some buildup in the combustion chamber I blew so much out and moved so much air through it that it's hard to believe that it's constricting the burner output. So. Any more thoughts about what could be going on here?

Happy New Year everybody!

Gary RV_Wizard

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Re: Atwood 8535 IV furnace
« Reply #13 on: January 01, 2018, 02:52:32 PM »
Soot is a symptom rather than the original problem. It is a product of incomplete combustion, which means the furnace has not been burning properly for some time. When the furnace is operating right, it will not produce more than a smudge coating on the burner chamber, even after years of use.   Removing accumulated soot may get it sort-of working again, but something else caused that soot problem to begin with.

One possibility that has not been explored yet is a bad gasket. Since there are two fans on one motor shaft, there is a gasket in the housing divider that isolates combustion air from circulation air. If that gasket gets damaged or out of place, air flows from one side to the other.  Usually you can smell exhaust inside the RV when that happens, but maybe not always.   These furnaces are extremely sensitive to the combustion air flow, both air in and exhaust out.

A crack or pinhole in the chamber remains a possibility.  Any sign of soot on the outside anywhere?   Or maybe a partially plugged gas valve (though that should be too lean rather than too rich).
Gary
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Gary Brinck
Summers: Black Mountain, NC
Home: Ocala National Forest, FL

Fatgnat

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Re: Atwood 8535 IV furnace
« Reply #14 on: January 01, 2018, 04:08:56 PM »
I hadn't thought about that gasket, Gary. I just removed the squirrel cage so I could look at it and it doesn't look damaged -- but there maybe a little leakage around it because years ago when the motor started squealing and I got tired of taking it apart to re-lube the bearings I took two pieces of 1/8 " copper tubing and bent it up so one end of each is suspended over the motor shaft right next to the bearing at each end of the motor (flared the other ends and positioned them just inside the cover plate, each running downhill to the bearings) so I can lube them with a drop of oil anytime without doing more than removing the cover. At the burner end, however, that tube has to go between the gasket and the motor housing so there's s small gap on each side of that tube which could allow some air past. The back plate of the squirrel cage is on the bearing side, though, so any leakage has to be around the back of it. I can clean that up and plug the gap with silicone rubber and stop any leakage there. Do you think that's likely to be enough to cause such a significant imbalance? ( You should see and smell the smoke it puts out!)

I have an endoscope which I just tried to look into the chamber with but couldn't see much. There are some strange bumps in there but I couldn't see where or what they are and they don't look like enough to restrict the flow.

Nat.

Fatgnat

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Re: Atwood 8535 IV furnace
« Reply #15 on: January 01, 2018, 04:13:42 PM »
No sign of soot anywhere but out of the exhaust and no smell inside so long as the cover plate is on and the windows on that side closed. I removed the jet and it's clean as is the tube inside.

Fatgnat

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Re: Atwood 8535 IV furnace
« Reply #16 on: January 02, 2018, 12:03:46 PM »
Worse than ever last night. That's after cleaning up around that copper tube and filling around it with silicon rubber. So that didn't help and last night the door (which is right next to the furnace) was opened briefly to let the dog in while the furnace was running -- and now there are tiny bits of soot scattered around inside which make black streaks then wiped. I have run out of ideas for what to do. There has to be an explanation for what's happening but I can't find it and am on the point of looking for a replacement furnace. Looks like around $600 for a duplicate right now.

Any thoughts anyone?

Nat.

'99 SeaView.

Fatgnat

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Re: Atwood 8535 IV furnace
« Reply #17 on: January 07, 2018, 02:47:25 PM »
Well, if anyone is still following this saga here's the latest: It was suggested that mud daubers might have built a nest inside the combustion chamber which hardened like concrete. So I removed and disassembled the furnace and tapped the combustion chamber all over with a wooden mallet hard enough to slightly flex the metal but not dent it. A lot of chunks of soot came out and when I couldn't get any more out I put it all back together. Turning it on I saw no smoke in daylight but in the evening with a flashlight I still saw some smoke. The night wasn't very cold so it didn't run much so I'm not sure if it's OK or not -- we'll see.

New question: does anyone know what the minimum acceptable voltage at the motor is? System voltage is 12.4, motor voltage 10.8. I haven't yet zeroed in on the location of that voltage drop. Jumpering in from the batteries directly raises the motor speed some but not a lot.

 

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