Duct duct goose - Fiberglass RV


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Old 05-06-2014, 09:06 PM   #1
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Duct duct goose

I have been looking at the duct requirements for some ducted suburban and atwood furnaces. I see that they all have very specific requirements for their air return. Why is it that the furnace recycles the air from inside the trailer it is heating? Wouldn't it be better to use outside air as instead of recirculating air from inside? Using outside air would improve air quality inside. Since cold air tends to be dry, it could help deal with high humidity problems too. Thoughts?
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Old 05-06-2014, 09:12 PM   #2
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Almost all heaters in the world reheat room air, be it in an RV, House or Department store. Plain and simple, it takes a lot less energy to reheat air than to heat cold air.
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Old 05-06-2014, 09:27 PM   #3
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Is it just an issue of "fuel economy?". That fuel economy goes out the window if you have to open windows up to allow stale air and humidity out.
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Old 05-06-2014, 09:38 PM   #4
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In a little egg, I think that the advantages might outweigh the disadvantage of using more propane.
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Old 05-06-2014, 10:14 PM   #5
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Well one thing to keep in mind is most of our trailers are probable not anywhere near as air tight as most new homes are..... so you will be getting more fresh air intake than you would in your home whether you want it or not ;-)
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Old 05-06-2014, 10:44 PM   #6
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We always keep the roof vent cracked in cold weather. Takes more propane, but the fresh air and lack of condensation are worth it.

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Old 05-06-2014, 10:53 PM   #7
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Originally Posted by John Linck View Post
We always keep the roof vent cracked in cold weather. Takes more propane, but the fresh air and lack of condensation are worth it.

John
Yup its a rare occasion that I am a place were I need to run the furnace and in its a dry enough climate that I don't need to also have a window open to avoid condensation.
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Old 05-06-2014, 10:55 PM   #8
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Originally Posted by glamourpets View Post
I have been looking at the duct requirements for some ducted suburban and atwood furnaces.
I see you have a 13 foot Boler- what's the necessity for a ducted furnace? Frankly, I can't even visualize how ducting would be accomplished in a trailer that small.

As I understand them, ducted (circulating) furnaces are for much bigger spaces, and as others have said the reason for return-air is efficiency. No reason to heat new air when preheated is available.

BTW:

Combustion air is a different matter, and is without exception drawn from the outside for these furnaces
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Old 05-07-2014, 12:28 PM   #9
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Originally Posted by Francesca Knowles View Post
I see you have a 13 foot Boler- what's the necessity for a ducted furnace? Frankly, I can't even visualize how ducting would be accomplished in a trailer that small.

As I understand them, ducted (circulating) furnaces are for much bigger spaces, and as others have said the reason for return-air is efficiency. No reason to heat new air when preheated is available.

BTW:

Combustion air is a different matter, and is without exception drawn from the outside for these furnaces

I agree. A ducted furnace in a 13' trailer would be overkill. Most ducted furnaces seem to be at least 15000BTUs or more. You would need to be in Alaska with the windows open to benefit from this much heat. I'm asking a general question for a different project.

The one place I have seen air taken from outside is the automobile. Some cars have a switch to allow you to recirculate the air or to heat cold air from outside. Perhaps this is simply a different situation of no relation?

Derek
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Old 05-07-2014, 01:45 PM   #10
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ASHRAE (American Society of Hearing, Refrigeration, and Air Conditioning Engineers) of which I was once a member during my working days recommends a 10% makeup intake of outside air for housing; note this is not a code requirement. A 15,000 BTU furnace is relatively small, one of the smaller sizes carried by the RV furnace makers, in fact. Ducted furnaces from Suburban exceed 40,000 BTU - in a small trailer you could probably make toast by hanging a piece of bread on the front grill........

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Old 05-07-2014, 06:33 PM   #11
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Originally Posted by tractors1 View Post
ASHRAE (American Society of Hearing, Refrigeration, and Air Conditioning Engineers) of which I was once a member during my working days recommends a 10% makeup intake of outside air for housing; note this is not a code requirement. A 15,000 BTU furnace is relatively small, one of the smaller sizes carried by the RV furnace makers, in fact. Ducted furnaces from Suburban exceed 40,000 BTU - in a small trailer you could probably make toast by hanging a piece of bread on the front grill........

Charlie Y
15000BTU is the small end of the ducted atwood and suburban furnaces. You certainly could go bigger. Maybe a 100000BTU Webasto would be a good option? Webasto is a slightly different kettle of fish, but some of their units are small enough to fit in a trailer yet big enough to heat a house.

On a more serious note, the 10% statistic is interesting. The question becomes, how long does it take for all of the air in the trailer to cycle through the heater (assuming it runs continuously)? Its all going to be relative because if you have an excessively oversized furnace in a tiny trailer, that timeframe is going to be shorter than it should be. At 10% how long would it take to replace all the air in a trailer?

Derek
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Old 05-07-2014, 06:51 PM   #12
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Quote:
Originally Posted by tractors1 View Post
ASHRAE (American Society of Hearing, Refrigeration, and Air Conditioning Engineers) of which I was once a member during my working days recommends a 10% makeup intake of outside air for housing; note this is not a code requirement. A 15,000 BTU furnace is relatively small, one of the smaller sizes carried by the RV furnace makers, in fact. Ducted furnaces from Suburban exceed 40,000 BTU - in a small trailer you could probably make toast by hanging a piece of bread on the front grill........

Charlie Y
Living in a cold climate state ,I have changed the linkage on the fresh air dampers of roof top units to their minimum position in the winter .At 20 below zero even taking in 10% fresh air results in high fuel costs .if you really want to get rid of moisture ,bring in fresh air ,and conserve energy, an air to air exchanger would help but not very practical in a trailer
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