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mobile home furnace. Does duct connection on bottom dictate diameter of ducting

1087 Views 8 Replies 2 Participants Last post by  yuri
I have a 1973 double wide mobile home.

I have a Bryant 80 series gas furnace.

The past owner seems to have messed up and jerry-rigged the ducting. And I can't seem to find any manual etc. that answers my question. So I'm going to try my luck here:

Mobile home furnaces have two heat duct connections from the furnace.

One side of home can connect to the furnace heat, via the same side of home that the furnace is located on.

The other side of mobile home requires about a 6 ft. length of round ducting to reach the location of furnace and connect to the very bottom of furnace which has an 8 inch diameter duct connection (where the heat is pushed out by the furnace blower), and is accessible by going under the home.

My overall question is: does the diameter of duct connection on furnace (which is 8 inches on mine) dictate what the entire run of round tubing under the home should also be, OR are there situations where you would need to reduce or enlarge the duct diameter after it leaves the furnace chamber.

My confusion on this is because I "assume" that whatever the proper duct sizing should be used on a furnace is dictated by what the furnace manufacturer has used on their outlet duct connection. But I also hear that depending on how long the duct run has to be, sometimes you need to reduce it's size as the length goes on. But seeing as how I'm dealing just with a 6 ft. run, I again "assume" I should stick with the 8" diameter coming out of furnace for best airflow pressure and CFM consistancy, and to keep the furnace from working harder over time which can kill its lifespan faster if ducting is wrong size.

I have attached a link to photo which not only shows my connection situation but also some weird things that seem "jerry-rigged" by past owners of home.

https://drive.google.com/file/d/1daQB3wjWYY79q9hsZTeub0G0HtaYI7WB/view?usp=sharing
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The only time you would need 8" pipe is if the entire home had all its branches/pipes coming off that one pipe.

In a regular home you have a plenum and it may be 16" wide and the take off is 8" so you have a 8"x16" square duct ( just using round numbers as some are smaller or larger ).

Then it travels 6 feet and tapers down to 8" x 12" or 14" and later 8"x 10". The takeoffs to the rooms are usually 5" or 6".

I have seen 2-3 story houses with 6" pipe running 20-30 feet up and around joists then going to the upper floors so 6" is as large as we usually use.

Stay with the 6" pipe BUT seal every joint tightly with silver foil duct tape from HDepot so you does not leak air out.
 

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Discussion Starter · #3 · (Edited)
Hey Baltar!

Keep in mind, this is not a "regular home" but a mobile home. So this is about the necessary "crossover" ducting which is something not normally part of a stick home setup for duct work...

I am now including a link to a photo that shows the one run of rectangular duct work that has cut outs to place venting grilles into.

It seems to be about 16" to 24" in width? and is one piece that runs along the entire subfloor.

Does that change your viewpoint on whats up?

Also, you didn't mention your opinion on first photo that shows the jerry-rigged duct and the photo shows a screw on the outgoing piece under the furnace. What's up with this mess? wrong piece used correct?

p.s. Galactica from 80's is still one of my top 10.
 

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Discussion Starter · #5 ·
I re-read your wording more closely.

It seems you are saying 6" is the norm, when we talk about branching off an 8" inch feed.

But this is not quite a branching off. It's for a "cross over" to other side of home as a separate heat feed from furnace.

Meaning, There happens to be 6 feet of distance from furnace to the connection of the rectangular plenum that runs in the floor and has a 50 ft. length.

I am thinking it may need to stick with the 8" so it matches what furnace starts out with since you may need more heat being pushed through so by the time it connects to this rectangular plenum in floor it's going to still have enough force on other end which is a 50 foot run...

Otherwise, if I decrease immediately to 6" duct for crossover, I'm really feeding the main duct work in floor with a 6" connection from the furnace output.

I've come up with only 2 possible answers on how to connect a cross-over properly:

Furnace bottom has 8" diameter (crimped) outlet , then use "6" inch diameter duct for the 6 ft. length that connects into homes sub-floor using rectangular plenum which runs for 50 ft.

Or

Furnace bottom has 8" diameter (crimped) outlet , then use "8" inch diameter duct for the 6 ft. length that connects into homes sub-floor using rectangular plenum which runs for 50 ft.

My thinking is that you should not "decrease" the size of duct coming out of the furnace outlet on bottom of unit. Only "after" you hit the homes plenum can you have a layout that relies on 6" ducts branching off. But in this case there are no branches whatsoever.

Any of my thoughts logical here, or is there still a benefit to using a smaller feed between the furnace outlet and the inlet connection to sub-floor plenum?
 

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Discussion Starter · #6 ·
oh geez.

I may have been using the wrong meaning when I use the description of :crossover duct: a duct connecting the plenums of the two sides of a double-wide mobile home.

I guess the tube that connects to furnace on bottom side to the 50 ft. run of rectangular ducting in sub-floor on one half side of mobile home may just be called:

plenum: the piece of duct work connecting the furnace to the supply ducts; it has a fairly constant pressure throughout its length.

I finally just drew up a diagram to clarify what I have on underneath mobile home...

link is:

https://drive.google.com/file/d/1nofoQZPTA5Bm6hXNGz18RJ-a51nTqmY_/view?usp=sharing
 

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Discussion Starter · #7 ·
Now I find that "crossover duct" is the correct wording but is just a different setup and location than a regular stick home.

For double-wide mobile homes, a crossover duct (generally an R-4 flex duct) is used to carry the conditioned air from one side to the other. There is usually a large discrepancy in air flow between the two sides.
 

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I would be tempted to use 8" for that crossover piece and it may help a little bit.

Yeah Battle Star Galactica was a great series and I thought he was a devious cool character. My eyebrows are not that bushy but on a bad day I frown and look like him.:wink2:
 
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