(Moderator - please move if in wrong section, but I couldn't find the best section. Thanks.)


I've been reading up more on duct work because of recent issues in return & supply ducts from the previous owners/installers.  (See some of my other posts).  The existing duct work was mated to a new geothermal air handler, currently set at 1100 cfm max blower speed.
House is 2100 sq feet of conditioned space, 2 floors, 2 zones.

My new HVAC guy wasn't going to redo all my ducts from the old central a/c, and basically, now, they do work, but I'd like to try and optimize them myself.  I've read a lot about duct work and plenums and cfm per room per floor, return sizing, etc.  I even have a copy of ACCA Manual D Residential Duct Systems, and it's certainly informative.

My current questions concern both supply and return.

Suppy - the air handler is in the basement, the main trunk runs horizontally across the basement, with the handler in the middle.  A third line runs 90 degrees perpendicular and then into a vertical stack to the upstairs.  Upstairs is about 900 sq feet, downstairs is 1200 sq feet. 
The air handler blows straight up into the trunk through a T.  Illustration.  Manual D indicates that the resistance created by such a configuration is far more than if it was more elbowed, possibly with turning vanes.  Question: both work, but is the turning vane/elbow much better?  None of my runs are more than about 40 feet total, from return vent to furthest supply vent.  And what would be the best way to connect the vertical stack for upstairs?  Currently, the horizontal ducts that supplies the stack is only 11x11, which seems narrow for up to 1200 cfm with Z2 on and Z1 off.

Return - obviously, the return should equal or slightly exceed the supply, so my goal would be to get 1100-1200 cfm back to the unit.  I have 4 return vents, all near the center of the house, each measures 4x10, runs using floor joists as duct space for a few feet, then connects to a large return trunk of duct board.  My initial calculations/observations suggest this isn't enough without the 4x10s having excessive velocity.

I"m hesitant to ask my geothermal HVAC guy for many reasons, notably I've already paid him a whole bunch of money, and I'm fairly handy as far as tweaking a system.  I'm not looking to reconfigure the whole duct layout (I'd probably find someone with more experience for that), but tweaking it, adding an extra return, trying to streamline some of the duct work, I'm pretty sure I can do on my own.

assume 2.5ton system (if 1100cfm max) I would expect 14by30 or equal return size min
11 by 11 is to small to handle the supply air on that single zone and bull noseing the tee in the attic is pouring salt on that wound

It's actually a 4 ton system (!) with the blowers scaled back to 1100 because of excessive duct noise! I think it goes up to 1700 cfm at least. That's what I thought about the 11x11. Waaayyyy toooo small. I've read about zoning controls that scale the blower back 25% when a single zone is active, which seems like a good idea, and also about bypass ducts that are pressure controlled (In manual D) and provide an "air escape" in case the zone can't handle the full pressure....a waste of power, I know, but better than trying to cram 1100 cfm into a tiny space. Basement space is unfortunately limited....due to ceiling height...

I'm open to other ideas, and can post photos if desired of the space and layout.

Based on 350cfm per ton you should have 1400cfm minimum. If you cut 1100cfm by 25% your Geo unit won't last very long. Unless your duct can the deliver proper amount of air EER and COP will take a big hit. $$$$$$
This should be addressed before the install, not after.

This has been discussed at length here , my stance is ,and was . a partially controlled dump zone . or adjusting the stop position on the mod motors so it allows enough blow by of the closed zones . both have good sides and bad . but regardless of zone/stage controll we provide dumps on all our zoned system . The only thing I dont like about a bypass line to the return is in heat you are increasing your return air temp and decreasing eff , a big deal around here but often a nessasary evil . with your blowers scaled back that much you have allready lost allot of eff .

I vote for adjusting the stops on the zone damper motors to let enough air into the closed zone that you have to , in order to be moveing the right cfm across that coil ( cheap fix but not bad ). or redo ductwork

I'm all for redoing the initial take-offs from the air handler....I just haven't found much info on how best to do it, given the tight space. I'm thinking a big, wide, angled box with all 3 supply trunks connected,sort of an upside-down tripod arrangement (2 downstairs trunks, 1 upstairs). I would try to make sure every trunk could handle at least 1100/1200 cfm independently. Problem with that is that it interferes with the return stack, which is also a bit dodgy. I almost wish the unit was installed sideways....it would make it much easier to redo the initial duct work.

I'm pretty sure I could easily free up some space by angling the return trunk more. Rather than coming along the basement ceiling, then making a 90 degree angle down, followed by a 90 degree angle to the left into the bottom of the air handler, I could pull the return trunk down at a 45 degree angle, from the ceiling near the handler. It would come horizontal a bit, angle down 45 degrees about 9 feet, then angle 45 degrees to the left into the bottom of the air handler. It seems a much more straight efficient path, and although it takes up more space, it's an underused basement area anyway. Then there would be more space directly above and on both sides of the air handler for supply duct arrangements.

I know the upstairs trunk and riser are too small....I'm thinking, since the new upstairs return stack & ductwork are right next to the supply trunk and stack the entire way, of changing the return stack and making it a supply stack. I'd put another damper in, in parallel to the existing zone 2 damper, redo the ducts by the air handler so that both the old supply and the new return are connected to the output of the air handler. When Z2 goes active, both ducts carry air upstairs.
As far as return path, it never had one before now with the old central a/c. The staircase is open, and there are several return ducts on the lower level (although not enough in my opinion). That, or add a third 'riser' for a new supply or a new return from upstairs.

Is this becoming too big a project and I should get a professional in?? Do you think he'd charge $1000 or something?

More duct measurements:
Main horizontal trunk, connects with a butt-T joint to the air handler. Air comes up the middle of the T from the handler (about a 6 inch distance total from the top of the handler to the start of the horizontal trunk). Left side of the T is 9x23 duct, feeding 1000 sq feet of house. Right side is 6x12, feeding 350 sq feet. A third mini-trunk for upstairs comes off at the middle where the 9x23 and 6x12 connect, at right angles to them both, sized at 10x10. This mini-trunk goes through 3 elbows all in a row (to get around an iron beam), along a horizontal 10x10 (previously I said 11x11), then into a riser that is 9x15. At the top of of the riser, it elbows horizontally again, and feeds 4 8" flexible ducts, 2 for each bedroom and a 6" flexible duct for the upstairs bath.

4 tons is too big. Duct work is too small.
If you must have independant zone control, follow geo fans advice. If you want to get the most efficiency out of your system, balance the system without electromechanical dampers (replace with manual main line dampers and tweek them between summer and winter).
Good Luck,
Joe