Expanding duct work from 4" to 6"

[Lynn]

On top of my furnace are six 4" metal elbow ducts pointing in different direcitons towards the six rooms(no main trunk). I would like to relocate my supply registers in four of these room, from low on the interior wall, to the ceiling at the exterior walls. Would this be a good time to expand the duct work from 4" to 6"? Is there a coupling (w/damper) that exist for this sort of effort?

[imeduc]

Id take the 4" out of the plenum and cut out and put new 6" takeoff in it.

ED

My mistakes dont define me they inform me.

[Lynn]

Is that do-able from above in the attic, or would I have to disassemble the plentum from below?

[imeduc]

just where ever you can get to it is all

ED

My mistakes dont define me they inform me.

[Umby]

If I did increase two of the six 4" holes in the plenum up to 6", would I see and increase in cfm to these two rooms? (assuming the same run lengths)

[imeduc]

you should but you dont say what cfm the blower has now. you could kick it up.

ED

My mistakes dont define me they inform me.

[Umby]

How do I know what the cfm is? All I know about this furnace is that it is a Kenmore, 80%AFUE, 2-stage, non-variable, 75000 btu furnace (up flow). I cant find any more specs on it on the web and the installers left me no specs either. Installed with it is a Kenmore central air unit rated at 2-ton. Is this enough info to calculate cfm???

[imeduc]

A 2 ton Ac need 800cfm for it to work ok

ED

My mistakes dont define me they inform me.

[Umby]

So with six 4" ducts, the maximum cfm I am presently getting is:
6 X 25cfm or 150 cfm? (Assuming 6"=100cfm, 5"=50cfm, 4"=25cfm)

I also have four 6" ducts lower on the plenum going off to the newer section of the house, so that's another 400 cfm for a total of:
550 cfm. Is that correct?
Is this a problem, i.e. the 550 cfm ducts vs. the 800 cfm blower?
Thanks Imeduc for your help
BTW
My equipment is all Kenmore

Model#
Furnace: C8MPT075F14A
Condenser: TCA024AKC
Coil: EPD30F15C

I cannot find any specs on these units except for a "Comfortmaker" which I don't think is Kenmore.

[imeduc]

A 6" round pipe is good for 100CFM of air. You might look on the blower door there and see what speed the blower is set on. for heat most are set on MED and for the AC set on HIGH speed

ED

My mistakes dont define me they inform me.

[Umby]

But are my calculations correct?