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Old 02-02-2012, 08:55 AM   #1
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1983 34' Excella
Minter , Georgia
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Hot furnace exhaust

Hello, all!

We spent a few nights recently "yard camping" in our 34' 1984 Excella. The furnace and the stove both work very well, but I do have one question for you far more experienced folks.

The exhaust from the furnace is VERY hot. I mean, hot like you could cook with it. Is this normal? I know that you're going to lose some heat to the great outdoors, but I did not expect to lose this much. The air coming out of the vents in the Airstream is warm, but not nearly as hot as the exterior vent. I can't help but feel like I'm wasting half the heat being generated.

BTW - We do have a CO / smoke detector. I've read something about propane detectors. Should there be one already installed in my trailer, or is it something I should look into purchasing?

Thanks!

jkelley73
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Old 02-02-2012, 08:59 AM   #2
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It should be hot. The hot mixture of gasses from the flames passes over a heat exchanger and then out the outside vent. It is the hot air from the (other side of) heat exchanger that heats the internior of the rig.

Lynn
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Old 02-02-2012, 09:03 AM   #3
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It is very hot coming out of my fairly new to me 02 Safari. It was also hot on new SOB's that I have owned in the past. I believe it is normal for the furnace exhaust to be hotter than the warm air blowing inside the trailer. Some trailer exhaust plates have the warning "HOT" stamped on the outside of the plate.

A concern I have is that the entrance door on my AS covers the furnace exhaust when the entrance door is open. I worry that someone will leave the entrance door open covering the furnace exhaust and damage the door if the furnace was to come on while the door was covering the furnace exhaust.
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Old 02-02-2012, 09:33 AM   #4
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There are a lot of variables involving oxygen content, but a propane flame in a furnace of that size will burn at up to around 2000F. You certainly would not want air that hot feeding directly into the RV. Nor should it be allowed to blow into a surface like a door (unless, of course, you're wanting to destroy the door).

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Old 02-02-2012, 10:12 AM   #5
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Yes the furnace exhaust is rather hot, I make sure not to leave the door open in front of it while it is running (such as when carrying things in or out of the trailer). This heat just shows how inefficient the furnace is and how much propane you are wasting running it. I wonder if they make models with higher efficiencies? Similar to hot air systems at home where they only need a plastic vent pipe as the exhaust is only slightly warm.
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Old 02-02-2012, 10:46 AM   #6
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There could be an entire thread on the stuff that Airstreamers have toasted with their furnace exhaust...
dog crates
patio chairs
etc.
I'm sure even a pink flamingo or two have been roasted.
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Old 02-02-2012, 11:54 AM   #7
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Yes, the furnace outlet air is fairly hot. That is more or less "normal". The only thing I would check is the quality of the flame to be sure that there is enough combustion air. Since most RV furnaces have a sealed from the inside combustion chamber, they use a two wheel blower system, one for the combustion air, the other for the air that circulates through the trailer. If the combustion air is partly blocked with things like mud dauber nests, the volume of air through that part of the furnace may be restricted. If so, you will see a very "lazy" yellow flame inside the chamber and not the "hard" bright blue flame which is normal. So, look at your flame through the inspection port, and if you see a poor yellow flame, your furnace needs service.

Second: Don't confuse temperature with heat. A wooden kitchen match will make a very hot flame, but has very little heat in it, about 1 Btu when fully burned down. Just because something is hot, does not mean that it is losing a lot of heat. No RV furnace is much more efficient than 70%, that is only 70% of the energy you buy in the propane is turned into usable heat in the coach. 30% goes out the exhaust vent. A conventional home furnace has a similar to slightly higher efficiency, and might go to 75%. You just don't see or feel the heat going out the chimney.

In the home market, they do produce furnaces that do have higher efficiency, up to 95%, and can use plastic vent tubes. They do that by running the output through a second heat exchanger, and condensing the water in the combustion gas out, and using that heat to increase the efficeincy of the unit overall. It is a process that is quite expensive to build, and takes a lot of space, two things that RV manufactures will not pay for. A $2000 furnace in an RV to save some propane and make the furnace 20 to 25% more efficient will not sell.

For a while the RV AC manufactures made high efficiency rooftop AC units. They cost about $50 to $100 more than a regular one, but a 13,500 btuh unit only took 10 amps rather than the normal 13 to 14 amps. The RV manufactures would not pay the extra to put them on their new rigs, and the replacement market generally would not buy them because campground electricity is "free", so who care about the efficiency has always been the thinking. (sad). I have not seen the high efficiency units listed in a while, maybe they are still there, but I think they have been discontinued.

Sorry for the mini lecture on all of this, it happens to be a topic of interest to me, and energy is always a concern we should have.
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Old 02-02-2012, 11:55 AM   #8
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Quote:
Originally Posted by jkelley73 View Post
The exhaust from the furnace is VERY hot. I mean, hot like you could cook with it. Is this normal? I know that you're going to lose some heat to the great outdoors, but I did not expect to lose this much. The air coming out of the vents in the Airstream is warm, but not nearly as hot as the exterior vent. I can't help but feel like I'm wasting half the heat being generated.
Hello Jkelley.

RV appliances are not as efficient as their residential equivalents. Whether this is because of inevitable engineering tradeoffs given the size and weight constraints or is a result of outdated designs being kept for reasons of cost and/or management constipation is for you to decide.

Atwood's web site quotes efficiencies of around 80% for their furnaces. I would guess that's about right, and it's close to the limit for non-condensing designs. I haven't measured the exhaust temperature but I would guess that it's around 250-300 degrees with an ambient temperature around 50 degrees. Anything lower than that would result in water condensation when the furnace is operated in low ambient temperatures, and require a design that accommodates both the separation and removal of the condensate and the corrosive effects it would have on the heat exchanger.

Quote:
BTW - We do have a CO / smoke detector. I've read something about propane detectors. Should there be one already installed in my trailer, or is it something I should look into purchasing?
New trailers are equipped with propane detectors.

I personally have concluded that propane detectors in RVs provide little if any safety benefit. Fires and explosions in RVs, resulting from propane leaks, are vanishingly rare, so it's not clear to me that propane detectors solve any problem that actually exists. Except for the range, the propane system is installed in locations vented to the exterior of the trailer.

In marine applications propane fires and explosions are a much more serious problem so the detectors are much more useful there. The difference has to do with potential for accumulation of propane below decks where it has no place to dissipate and where it can lead to deadly explosions.

Propane detectors are prone to false alarms. They draw around 100 mA, enough to pose a problematic battery drain while parked or while boondocking. The detectors have a limited lifespan. I have removed mine.
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Old 02-02-2012, 12:21 PM   #9
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Quote:
Originally Posted by Ridgerunner3 View Post
A concern I have is that the entrance door on my AS covers the furnace exhaust when the entrance door is open. I worry that someone will leave the entrance door open covering the furnace exhaust and damage the door if the furnace was to come on while the door was covering the furnace exhaust.
Yes, it leaves a tough to remove soot stain in a perfect geometric pattern on the door. Don't ask how I know.
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Old 02-02-2012, 02:58 PM   #10
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You do know you have two furnaces in your 34 foot Airstream.
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Old 02-02-2012, 04:33 PM   #11
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Originally Posted by 6PackCharlie View Post
You do know you have two furnaces in your 34 foot Airstream.
Uhhhhh...Huh?

Back to topic, as others have said, that heat out the exhaust is normal.

I am also a big believer in smoke and CO detectors. The propane ones, not so much. Propane is heaver than air, and will find its way out and dissipate unless you have a large leak inside in which case the smell would be noticeable. Trailers, including ours, were made without them for years and survived.

And welcome to the forums. Photos, we need photos. You can see my '83 34' Excella at http://www.airforums.com/photos/brow...d=17830&page=3
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Old 02-03-2012, 06:28 AM   #12
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Quote:
Originally Posted by 6PackCharlie View Post
You do know you have two furnaces in your 34 foot Airstream.
Some 34's were built with two furnaces as a "cold weather" option. I know this only because my parents have one (they had no idea that was an option when they bought it, but since they've wintered in the trailer several times, it has come in VERY handy for them). Not all 34's have two furnaces.

(unrelated note: it looks like you and I have the same trailer! )
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Old 02-03-2012, 08:25 AM   #13
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Quote:
Originally Posted by Skater View Post
Some 34's were built with two furnaces as a "cold weather" option. I know this only because my parents have one (they had no idea that was an option when they bought it, but since they've wintered in the trailer several times, it has come in VERY handy for them). Not all 34's have two furnaces.

(unrelated note: it looks like you and I have the same trailer! )
I was aware that some had two furnaces as an option. My issue was with the statement to the OP that "you (do) have two..."

Myself, my trailer could use something more useful here in Arkansas, like a second AC.
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Old 02-03-2012, 09:25 AM   #14
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Originally Posted by Alumaholic View Post
There could be an entire thread on the stuff that Airstreamers have toasted with their furnace exhaust...
dog crates
patio chairs
etc.
I'm sure even a pink flamingo or two have been roasted.
Entry dorz..

Bob
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