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Old 12-01-2020, 05:59 PM   #1
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Atwood Furnace TOO hot!

Hi all, Atwood/ Hydro Flame #8535 35,000 BTU unit in my 26 foot Airstream class A. It works great but the air is too hot, though the 'overheat' element doesn't activate. I'm wondering if this is too much heater for the size; there are just three 4" hoses inside the coach and one 2" hose (to the trunk). none are blocked or restricted, the blast of air from the
hose in the bedroom is so hot you can't leave your hand in front of it for more than a couple seconds, it's frickin' scalding! I'm also concerned about our cats.
Any thoughts?
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Old 12-01-2020, 07:33 PM   #2
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I suspect Airstream over cooked it. (pun intended). We also have a 35K furnace in our 33' Bounder and have kept warm at -7F. 3 4" hoses are not enough air flow for that heater. Go check the air flow specs for your heater. We have 4 4" hoses for the house and 2 2" hoses for the wet bays and the air is very warm.
There is little doubt that your Air stream is better built and better insulated than my 95 Bounder. You may want to consult a respected RV tech in your area about change your furnace to a small BTU unit.

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Old 12-02-2020, 12:28 AM   #3
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Thanks! i expected it to be a 25000 btu piece, maybe I'll vent to the outside for all the people i wont let in during the apocalypse!
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Old 12-02-2020, 01:51 AM   #4
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Have you added any screens or restrictions to the outside intake/ehxaust ports ?

That can also cause higher air exchanger temps.
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Old 12-02-2020, 09:07 AM   #5
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That's a good question but no and the exhaust is clear of restriction, I think it's too much power for just the 3 4" lines. I'm going to try to add another as the front could use a bit more anyway. Thanks! I looked at a bunch of specs for new class a and the smallest i found with 35000 btu was 32 feet and I'm only 26, no slides.
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Old 12-02-2020, 09:24 AM   #6
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Maybe the furnace can be down graded. Like changing the gas orifice. A good furnace tech may be able to help. Possible to change the basement branch to 4"? Adding more outlets should help if you can manage that. I am surprised your sail switch is not turning off the gas due to low air flow. Is there good air flow from existing vents, if not maybe crushed, kinked, etc.
You could add a 4" and let it blow under sofa or cabinet if easier.


May be a good idea to have your high limit and sail switch checked. If these are defective there is potential for fire.


I think that if the furnace is too big for your coach it should short cycle. That is turn on run for a minute or less, then shut off. If the air is too hot from the vents then the furnace is malfunctioning. The furnace will heat any air entering it. If the coach air temp is very high, say 85 degrees, the exit temp at the vents cold be quite hot.
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Old 12-09-2020, 07:54 AM   #7
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thanks for the info, triplewide
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Old 12-09-2020, 08:43 AM   #8
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It's generally a lot of work to get to a furnace and open it up. There's a sail switch that has to close before the gas valve can open. The sail switch assures there is sufficient air flow to prevent the furnace from over heating. Unless it's stuck open or shorted air flow is probably good, but if you're up to it you could open the furnace and check it.

My guess is the furnace is fairly close to the bedroom vent? You can heat with a lot of warm air, or a little hot air. Because of space limitations RVs do the latter. If you've got metal covers plastic ones don't get as hot but don't last as long either.
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Old 12-09-2020, 08:54 AM   #9
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Air in-flow is as important in such cases as air out-flow. Check to see that return air or in-flow is unobstructed.

The furnace installation manual will provide necessary clearances and other requirements.

I wish you good luck and happy trails ahead!
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Old 12-09-2020, 09:30 AM   #10
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Atwood 85-II uses a 130*F High Temp Limit Switch

Atwood 85-III & 85-IV use a 190*F High Temp Limit Switch

Based on your RV year suspect yours uses the 190*F limit switch


Atwood 8535 requires at MINIMUM 48 sq/ft exhaust ducting
Minimum of 4 ducts
4" is 12 sq/ft
**2" duct is only 3 sq/ft

You do NOT have enough ducts.....
Three 4"-----36 sq/ft
One 2" ------ 3 sq/ft
That is 39 sq/ft Total........9 sq/ft LESS then Minimum
You NEED at least ONE more 4" or Three 2"
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Old 12-09-2020, 04:50 PM   #11
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X2 on Old Biscuit's advice.

Just to clarify, the required dimensions are square inches, not square feet.

From Atwood tech manual:

#3 - Minimum Ducting Requirements
The various BTU sizes of 85, 89 and 2-stage series furnaces require a minimum number of square inches of heat ducting.
The most common size of soft ducting is 4˝ diameter. Therefore, since a 4˝ duct is equal to 12 square inches, we can call
out the minimum number of ducts needed for the models of furnaces noted below.
Furnace Model Minimum Minimum
Ducting Ducts
8516, 8520 24 sq. in. 2
8525. 8531 36 sq. in. 3
8535, 8935, 8940, 1522, 2334, 2540 48 sq. in. 4
CAUTION - Ducting Installation
4˝ Flexible Hose
• each 90° bend adds the equivalent of 10 feet of ducting
• ducting should be securely attached to furnace
• each run should be as straight and short as possible
2˝ Flexible Hose
• 2 - 2˝ duct runs do not equal one 4˝ duct. A 2˝ duct is only 3 sq/inches
• 2˝ duct adapters are available
• 2˝ ducts are ideally suited for bathroom and holding tank compartments
Closeable Outlets
• a closeable outlet does not contribute to the minimum of total outlets recommended for a furnace.
Bottom Discharge
• furnace must be completely sealed to floor and plenum with a bottom discharge gasket, with no air gaps.
• if furnace is installed in middle of run, the main duct run must be a minimum of 24 sq/inches.
• if furnace is installed at the end of the run, the main duct must be 48 sq/inches.
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Old 12-09-2020, 09:38 PM   #12
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Quote:
Originally Posted by Talloaks View Post
X2 on Old Biscuit's advice.

Just to clarify, the required dimensions are square inches, not square feet.
Isn't a foot made up of inches???

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Old 12-10-2020, 07:57 AM   #13
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thanks everyone for the great info!
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