Has this been done and if not why

[DigitalDreams]

I have a 73 double wide on piers my furnace is dead center of my 24x45 home on and just a few feet of the center joint.

Ducts on furnace side run in line with furnace so downdraft furnace basically blows straight down into duct system on furnace side.

I need to pull furnace to Sheetrock furnace closet.

The furnace is presently connected to other trunkline by insulated flex,had been connected by 8 inch pipe but kept rotting out and problem with flex is I live in country and the mice keep eating there way into the flex tube.

I have had to repair duct connectors at both trunks for this crossover and am curious if replacing round crossover with rectangle aluminum duct which would have somewhat larger capacity matching trunkline sizes would cause to much flow to opposite trunk.

Is there a reason they made the crossovers flex and smaller or was it just the cheap way to connect them in the 70's and fuel was cheap so no one thought about the flow not being as great on the other side.

[Robert]

Hi,

Are you saying the duct connector collars are for 8" pipe ?


I like to use 90* ells and either alum. or galv. hard metal across or stretch flex tight across between the ells.


I always go with size there and as of yet, haven't found under 10" with about same amount of 12".


You also sometimes have to divert air into the crossover by making and installing a diverter shield in trunkline.



Should be ok for that short distance if you do not go too big and reduce it down and back up on other size.


Thanks,
Robert

[DigitalDreams]

Yes the collars of my trunk lines are 8 inch round galvanized,thats what was on there when I bought the place.

The furnace sits right over the crossover point on the north trunk.

I get excellent flow on the north trunk even tho the plenum box on the downdraft furnace blows straight down the crossover, which would make you think I should have better flow on the south trunk line than I do.

I have checked for obstructions in both trunk and crossover found none, but flow is about 1/3 of north trunk in the south trunk,what I meant to ask is if I build a crossover which is rectangular like the trunk ducts between the two trunks,the same dimensions as the trunk lines do you think that will even out flow.

And if it will,should I build a baffle to cut the air flow so I don't suddenly send all the air to the crossover.

Also is a duct fan used in any crossovers towards the opposite trunk from the furnace feed one to create a negative back pressure in the crossover to help the flow to go that way.

[Robert]

Hi,

Can't really answer with certainty because I've never tried it, so would have to go trial and error.

Duct booster fans have been used to draw more air across to other halve.


A baffle may/may not help, again I can't say having never attempted it.


Your ideas sound doable and may help or resolve it altogether, will have to try and see.



Take care and best wishes,
Robert

[flcruising]

Sound like it would be easier to just increase the size of your start collars and flex x-over duct, then just box frame it in to keep out pests.

I can tell you that the x-over duct in my attic is 14" flex. And one half of my house gets no more airflow than the other. Not saying you should go that big, but it sounds like the 8" flex is your bottleneck right from the start.

[Robert]

Before you go to making changes, you need to know or have means to test your actions and know.

Guessing will get you into big problems quickly.


Hard metal is a lot easier than box frames and easier if repairs or changes are ever needed.



Take care and best wishes,
Robert

[DigitalDreams]

Thanks for the come backs i'll try it as soon as the snow melts enough I can get under the place without fearing avalanche from snow I dig away from access door.

[Robert]

Hi,

You're very welcome, happy to help.


Thanks,
Robert

[A/C Jay]

Round metal duct is the best way to go about this, like Robert said. And the stuff is not that expensive.

Increasing to 10" duct will help the problem you're having. It might not completely fix it, but it will help get more air to the other trunk line. You could also overkill it on the duct size, say go with a 12", then install a damper in it. You could then throttle the damper back to where the airflow is somewhat equal on both sides of the house. There are many ways to "skin this cat". Obviously, being in this industry, we have a lot of tools at our disposal (like an air balance hood) to help us get these thing right w/o a lot of R&D. If you do use the overkill route, the make sure to put the damper in a section of the duct that is straight. Dampers don't work good when they're close to elbows or bends.

The metal duct will need to be insulated. But then the critters will tear up the insulation. So what we do is wrap the metal duct with the standard 1.5" (r-6) insulation. Then wrap over that with Chicken Wire. This will keep the critters form tearing up the insulation.

You could also box in flex duct like suggested above. I've never done it, but I can see how that could work too.


Jay