Page 1 of 1

Crossover duct sizing.....

Posted: Tue Aug 16, 2016 10:24 pm
by Casper2016
I'm a recent owner of a DW with an airflow issue on one side of my house (opposite of furnace).

I replaced the 12" flex duct with 2, 90 els and new R8 insulated 12" flex and supported with straps. I haven't found any major holes, collapsed duct, or blockage on the main supply duct. Problem still exists (low airflow opposite side of furnace). So I'm trying to figure out if the flex (crossover duct) should be reduced in size to compensate/equal the approximate size of the metal supply ducts (12x5").....? If my theory is right, I would need approximately 9" round duct. Does this make sense? I don't know if this side of the house has ever had ample airflow and the previous owners just dealt with it. I'm thinking I need more velocity. Any feedback would be appreciated!


Re: Crossover duct sizing.....

Posted: Wed Aug 17, 2016 6:48 pm
by Greg
Sizing ducts correctly is a science. I would try contacting a hvac professional and see if they can get things correctly sized.


Re: Crossover duct sizing.....

Posted: Wed Aug 17, 2016 9:29 pm
by HouseMedic
I would temporary close off the vents on the good side and see if it changes. It seems that crossovers never seem to work right for some reason. Like Greg said, duct work is a science and can be very difficult to figure out even with a computer program.


Re: Crossover duct sizing.....

Posted: Thu Aug 18, 2016 11:26 pm
by Casper2016
Thanks for the replies guys. For the moment I think I'm going to do some more research with a snake camera. Seal up all my registers with mastic. If all else fails, trial and error with a 9" round duct. If that doesn't work, I may call for an HVAC company to fix the issue. Just like saving money on labor since I'm capable. If only I had to knowledge to know exactly how this was meant to be designed.... I'll post my results when I get it fixed.

Re: Crossover duct sizing.....

Posted: Tue Aug 23, 2016 4:57 pm
by Greg
Unfortunately the way it was designed and the way it will work correctly are usually two different designs. If there is a way that you can use metal duct over flex plastic I would go that way. Many times they just connect plastic flex duct and lay it on the ground so critters can chew through it.


Re: Crossover duct sizing.....

Posted: Sun Nov 24, 2019 11:50 am
by waitingtohear
I happen to be going through same issues as you are right now.

After inspecting the ducting I found out that someone has jerry-rigged it.

They didn't even have the right duct parts in proper alignment by size or by matching up via which has a crimped edge. They didn't know how to connect them so they actually took two pieces that were never meant to connect together and used a bunch of duct tape to get around the issue.

I also notice my furnace outlet on very bottom for the crossover ducting is 8 inch diameter in size. So I "assume" the duct for the entire crossover should stay at whatever the size of duct outlet is from the furnace box.

However, is that not always the case? And for a small 6 foot run I would think decreasing the diameter would actually cause wrong pressure and make the furnace work harder over time and fail faster.

I have heard there are times where you "do" want to reduce the duct sizing over a run, but it's normally at the farther end points from the furnace. In other words the farther you get from the furnace, the more probability your furnace vent run may need to be reduced to compensate for the lower pressure rate that happens as you get further from the furnace blower motor.

So part of my reply is to help you, but also to get help from you as well. Perhaps we can share notes as we progress with our diagnosing and testing.

I'm about to replace this 6 ft. length of crossover ducting with proper galvanized pieces instead of the backward parts and cheap flex tube they used. Really bad setup for air pressure needs.

If 8" diameter doesn't' work I can reduce it down to 6" and see if it gets better results, but you have to keep an eye on how often the heater kicks in also. You may find it's working harder to keep the house warm if the duct sizing is off.

Hope we can help each other. Perhaps we can even share phone numbers and have a real chat about this. I too, am a handyman and I don't think it's worth spending hundreds on a tech to come out and most likely tell me what I probably already can figure out on my own in my spare time...

Here is a link to my current findings where they put in wrong duct connector I also have my own questions that come with this photo. ... sp=sharing

David T. : )

Re: Crossover duct sizing.....

Posted: Sun Nov 24, 2019 11:51 am
by waitingtohear
oh geez, I just noticed how old this post was.

I assume you have now fixed you issues.

Maybe you can tell me what ended up working for you in the end....

Re: Crossover duct sizing.....

Posted: Sat Dec 07, 2019 11:33 am
by Mark440
I see this is a very old thread...but I am hoping my experience can help others.

As Greg mentioned - my flex duct (one 12" run to each half of a DW) - was pretty much just left laying on the ground. And - yep, critters had a field day. And, after they chewed up the flex - they made their way through the 35ft long "plenums" that run the entire length of the house - one for each half. Come winter time - my electric bill was hitting $600.

Luckily - I am a veteran - and luckily there are programs out there to help vets get stuff done on their homes when needed - especially when the work exceeds the capabilities.

What i noticed about my house was the marriage seam and walls + vaulted ceiling at that seam. What came of that was a clear shot from one end of this place to the other to hang ducting - FROM THE CEILING! Took my measurements, scribbled down some ideas - and sough advice from some pros. I set up an account on and posted my questions in the HVAC section. The help that came back was phenomenal !

In some cases - the exchange units - with electric heat - installed in mobile homes is NOT reconfigurable. Usually, these are a dedicated down draft. With that in mind, I had to figure out how to take the output of the blower and redirect it so that i could route my ducts along the ceiling. this meant making a full U-turn under the house and coming back up through the floor in a manner that aligned with the proposed route at the ceiling. It did take some doing - but i got there. Next - sizing.

The chatroom guys were spot on in helping with the sizing. It took some doing to work through the rated output vs total length vs cfm/room....and it was aggravatingly tiresome cuz there are no clear cut answers. In the end, I had a sheet metal shop make the ducting pieces (15"W x 15"H x 60"L). I used the threaded 1/4" rod for hanging electrical - and made a suspension system so that the ducting would not be just laying on the new soffit ceilings.

It took a while to get all of this done - and the end result has made an incredible difference. Summer electric bill (@74°) was $140. So far this winter (@72°) - $180. Every inch of the new system is 26 gauge metal duct so critters are no longer any concern.