4 ton GeoMax 2 was retrofitted to existing duct work.  Installer added an 2nd floor return.

The downstairs returns- there are 5 separate "intakes" 2 in the front room, one in the middle room, 2 in the sun room.  Each of these is about 14x8, and goes into floor joist bays as duct work.  The distance from each of these 5 returns to the HVAC blower is about 8 feet (basically a star-pattern, each return is between 6 and 8 feet away from the HVAC)

The upstairs return is a single vent, 12 x 5, in a closet at the top of the stairs, with the closet having louvered doors to the landing.  Upstairs doors are always open and are undercut anyway.  The distance from the upstairs "intake" vent to the HVAC unit is 18 feet.

When the unit is running, I can feel absolutely NO air movement going into the upstairs intake, and shredded tissue held close to the intake vent moves only ever so slightly.  Fan speed on the unit is max 1100 cfm (it's dialed down because of noise)

Is this doing ANY good for me, or should it have been larger?  Do people ever zone their returns?

Return air will proportionally take the easiest paths, which may be down the stairs and into the other return ducts. But why do you care?

On another subject, I suspect your low air flow is reducing efficiency.

Enlarging a return that is already experiencing very low air flow / velocity will do no good.

Jonr is correct - return air originates via the easiest path which is often a big grille near the unit.

I do a lot of zoning, but never returns. Returns that don't seem to experience high airflow still perform a useful function by preventing overpressure and exfiltration from returned rooms / zones.

Dampering returns is theoretically do-able, but I would never consider it unless a client had a non-negotiable requirement to maintain radically different tenps in adjacent zones.

Ok, you guys sussed me out. The SUPPLY trunk for upstairs (one that gets really noisy at about 800 cfm when only the upstairs is calling) is only 10x10 and runs RIGHT NEXT to the return.

I was thinking of making the return part of the supply, thus the supply duct would be about 10x20, and there would be no return except down the stairs naturally. It would help with noise and sufficient air flow, but I've always heard the rule about "make sure you have an upstairs return". It's so difficult retrofitting.

It's not difficult retrofitting. With practice most of us have ideas for overcoming many challanges. Your guy might be up for a little more practice.
"make sure you have return upstairs" is not my mantra but it is not a bad one.
Is there a way that is not irreversable to experiment with your plan?
j

one that gets really noisy at about 800 cfm when only the upstairs is calling

Sounds like a poor zoned design (probably not uncommon). You want the system to always run at the manufacturer recommended cfm and always without excessive noise. Combining your ducts would help. Also, don't let any of the dampers close completely. This will also increase total flow/reduce individual velocity.

jonr- currently, because of the noise issues, the unit is set to a max blower speed of about 1100 cfm (I think it can go up to 1500 or so).
Zone 1 downstairs has plenty of ducts & supply trunks and they are all a good size. When only Zone 1 is on, I have the Zone 2 upstairs damper close completely.
Zone 2 upstairs has a single 10x10 supply trunk for about 850/900 sq feet upstairs, and a single 10x10 damper. I have it set now that when Z2 comes on, all the Z1 dampers don't close completely - it's hard to estimate how much they stay open, but I'm guessing about 1/4 open still.

That did cut back on some of the noise when I did that last year. I'm thinking about combining the upstairs trunk (10x10) with the upstairs return (about the same size). It's easy 'coz they are next to each other, made of duct-board, and already there (!) so I don't have to cut any more holes in floors and ceilings. Then upstairs, I would combine both trunks into a supply box for all the flex lines to the bedrooms.

You have serious duct design issues. Your return sizes are not even close for proper operation.I am going to assume your supplies are also not sized correctly.Noise issues are do to velocity of air ,reducing the cfm that the blower is operating at is only compounding your real problem.Not to mention there is a huge loss in efficiency and you maybe doing damage to your equipment.Call a professional to solve your problem before you spend a lot of foolish time and money.

It is old news that this project has oversized equipment w/ undersized ducts.
j

Apologies for bringing up old news. Every so often I get "inspired" to try and do something about it myself. :-) What do they say? "A little knowledge is a dangerous thing"?

You know...you look at the space where all the stuff is and you think, "Why, with a little duct board and tape, I could probably improve what's there for the price of materials and my time."

I know. I've brought it up before. I've actually contacted a couple contractors and found them unwilling to help. Seriously, none of them want to come in and examine an already-installed system. I feel like I need to tell them what I want them to do, but then I sound like I'm lecturing them. Hence my own wondering if I can change the 10x10 supply duct myself.

The downstairs operates perfectly as is, even at 1600 cfm. I think the trunk lines in the basement are in such a mess, plus the ground loops and house-supporting I-beam are right there, that no-one wants to touch it.

I say go for it. This stuff ain't rocket science. There is no problem with using a stairwell as a return path (think of it as about a 3' X 7'rectangular duct...it'll move a fair amount of air...)

Problems arise with rooms at the top of the stairs with doors that close. Ideally they have individual returns, but that can be overcome with a mixture of door undercuts, transfer grills and jump ducts.

If you feel like you know more than the contractor, chances are you probably do and need other better contractors. So many of my (apparent) competitors don't want to get anywhere near the duct system despite the fact that therein lies core system problems.

Was redoing the ductwork for the new 2nd story, and thought I'd share what I was discovering.  I think it answers some of my own questions. 

Here's the carefully constructed return duct in the joist bay that travels across the basement ceiling into the blower return.  I didn't cut the foam board.  There were a sheet metal duct (supply) that hung underneath it forming the rest of the bottom.  Notice the careful airtight construction.


And here we have the zone 2 upstairs damper.  Note how cleverly and tightly it fits inside the duct.  In fact, it fits so well, there is a big gap to the side of it.

You aren't shocked that someone who would run a 4 ton into a 10X10 zone actually has poor artisanship as well, are you? The gap next to the zone damper is simply a designed in bleed through....right? Yeah right.

That said, the fact that return was not "airtight" simply means it pulled return air from the basement.
j

Is anyone actually checking pressure balances? Ie, no matter what doors or zone dampers are open/closed, every room should be very close to neutral (ie equal to outdoor pressure). If this isn't done, then the pressure difference is causing air infiltration/exfiltration. How much - that depends, but enough pascals to exceed wind and stack effects are very easy to create.

Zoning the supply and not the return is asking for significant pressure differences. Imagine a return in a closed room with the supply turned off - it will be sucking outside air right through the walls.

I agree with pressure balancing, but ductwork that shoddy is a far greater problem than the odd wayward Pascal.

Success! During our home remodeling, we had the supply line to zone 2 enlarged, and also enlarged the return, properly sealing it and the new zoning damper. The "vacuum" in the return in very noticeable now (the previous installer had 2 joists through the return, effectively cutting the width by half in two locations, and made it narrow by a third to go around a shelf, along with the "terrific" sealing job shown in the above photos). I've adjusted the fan speed to max 1600 cfm and there are no rushing noises any more especially in Zone 2 like there was with the narrow ductwork. Thanks for all the support!

Are you leaving the fan speed on max, or did you just put it on max to test it? I'm assuming normal is the best setting, but perhaps the pros can verify that. Is it better to put the fan speed on max if the noise level is acceptable? I know my system states not to put it on "-" if you have a heat strip, but doesn't say anything about not putting in on "+".

I think it goes way higher, but I'm not sure...there are DIP switches for Heating mode, Cooling mode, and Aux mode . Each mode has 4 settings of speed, but they are not labeled in CFM. Just "Speed tap 1, 2, 3, and 4". Then there is also an "adjust" set of DIP switches, with "-", "normal", "+" and "test". I think the manual says that the "adjust" switches add or subtract 15% of the blower speed. The only way to check the blower speed is to count the CXM pulses from the LED on the board.

I read online at many websites that for heating, 350-400 cfm per ton is ideal (assuming ductwork is up to par). My original installer had dialed it down to 1200 max because of noise. I fiddled around and got it to 1800 (!) with no undue noise, but when it went on, it felt "drafty", so I tweaked it until it hit 1600. I haven't tried Speed tap 4 with the 15% + adjust yet.

In heating higher CFM per ton raises efficiency and capacity by allowing the indoor coil to operate at lower temperatures and pressures. At some point blower power, noise or uncomfortably low supply air temperatures and / or high supply air velocities cause comfort issues. It sounds like you have found a happy medium.

I had to bump CFM per ton slightly above optimum for noise on a system I commissioned last winter - geo extracts so much heat from the ground down here (Florida) that compressor amps were bumping up against rating without the extra air.

Here is a question that isn't worthy of its own post, but I need to ask. I know ducts should not be run in "unconditioned" spaces. My question is, would the space between a finished basement room and a finished main floor room be considered conditioned, or unconditioned? I'm guessing conditioned, even though the space isn't actually heated. I'm also assuming that my 2 unconditioned rooms in the basement should be air-sealed and insulated from the rest of the house, even though they are already air-sealed and insulated from the outdoors.