[yuri]

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.

 

[waitingtohear]

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.

 

[waitingtohear]

Here is second photo:

https://drive.google.com/file/d/1uVE0_zB3GAtA-VO8Zdkv6eWr2vVL1eGd/view?usp=sharing

 

[waitingtohear]

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?

 

[waitingtohear]

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

 

[waitingtohear]

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.

 

[waitingtohear]

I am staying away from flex duct and using galvanized with mastic glue to do the best job and keep the air flow stable over long term. I don't trust flex tubes to hold tight over time.

 

[yuri]

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: