Figured I'd keep the subject brief and to the point! (unlike the body of this topic!)

I have a 2 zone system....downstairs I'm sure is sized correctly.  Upstairs has a max of 1300 cfm (varible speed blower with 1300 max) ducted in a 10x10 ductboard duct across the basement, up to the second floor, then into four 8-inch flexi-ducts to the 2 bedrooms.

All my reading suggests that 10x10 is WWWAAAAYYYYY to small for 1300 cfm.  Any thoughts, suggestions, ideas?

In the meantime, I've adjusted the downstairs zone dampers to allow partial air through even when closed, thinking that it will reduce the pressure on the blower.  The online Ductulator(sp?) suggests that the duct should be twice as large (10x24 at least).  I'm wondering is the system going to be ok like this for years, or should I really replace the existing trunk upstairs (or add a second one to split the air).

Thanks.

Just to get a couple questions answered...

10 x 10 is that the outside measurement? or the inside measurement?
You say 1300 CFM , zoned systems? Is that with lower system damper closed?


Yes, to small and 24 x 10 is a rough design guide. A Manual "D", duct design would be the correct way to go.
Every change in direction can increase restriction and reduce air flow.

Also 4) 8" supplies are questionable, as 8" is a rough design can be around 200 cfm ( again Manual "D" )

And either way you go increasing size or add more supplies upstairs, or adding a Dump Zone

Relieve the pressure and don't let your unit struggle with a bad duct design.

Definitely too small. 10x10 duct board, without making it a wind tunnel (which would eventually rip it apart...), is "rule of thumb" good up to 400 cfm. I doubt you're even getting that.

Need to know the available static pressure to figure actual cfm. And even then, it's tough. I'd say, you're way under sized for 1300 cfm, if that's what is required.

How big is the unit? 2 ton, 3 ton, etc?

What determined that 1300 cfm is what's required on the upstairs zone?

Unit is 4 ton. 10x10 is the inside measurement of the ductboard trunk. It runs about 10 feet, rounded-right-angles up to the second floor, rounded right angles again horizontally, from which 4 8" flexible runs go behind the kneewalls to the 2 bedrooms, 2 runs per bedroom, about 8 or 10 feet of flex.

I'm assuming 1300 cfm max. The unit is variable speed...starts at 700 cfm, maxes out at 1300. If only the upstairs was calling for heat or cooling, the blower could potentially ramp up to 1300 cfm fan speed. Right now, winter, I tend to keep the upstairs cooler, and since heat goes up, the upstairs doesn't come on too much by itself. I'm concerned in the summer that upstairs will call for a lot more a/c cooling and therefore a lot more duct use.

Instead of the dump zone, the downstairs zone dampers are always partially open, allowing some of the extra pressure to go into the downstairs of the house. Effective, but not ideal.

If both zones are on, I'm guessing most would stay downstairs and that would be fine, but if just the upstairs goes on, that's a lot of air pressure in a very small duct.

On the other side on the coin,
You may also want look at your Return Air duct work.

If the supply side is under-sized your return may be also.
In which, your unit may be starving for air.

If your going to add a chase to get more supply airflow upstairs, make sure you take
into consideration getting enough return air from up high, back to the unit.

geotek, so is a homeowner better off going with a lower efficiency unit, or does a lower efficiency unit loose the same percentage of efficiency as a higher efficiency unit?

It's not a big house...2100 sq feet, with a central open stairwell. There is currently a return duct right next to the supply for the upstairs. However, the distance it travels would be exactly the same as the distance the air would travel if it just went down the stairs and into the return grilles on the downstairs levels. What I'm thinking is to turn the return duct into a second supply duct, and let the open stairs be the return.

That could work

Consider adding transfers or jump ducts out of any room whose door is frequently closed into the common return area , especially bedrooms (but NOT bathrooms or kitchens)

Higher efficiency systems tend to lose more efficiency owing to suboptimal field conditions since some of their higher efficiency stems from oversized coils and higher airflows / waterflows which are proportionally harder to achieve in the field.

Are you saying the biggest bang for a persons buck is not to buy the most efficient unit available due to diminishing returns resulting from other factors?

I make no such blanket statement, just the observation that the higher the nominal efficiency, the tougher it is to attain it in the field.

Did I miss the answer to the "what determined the upstairs zone requirement as 1300 CFM" question?
Personally I try to avoid zone systems and think they are over employed by incompetant duct designers.
If you don't need the units full capacity for the second floor, have an isolating relay break the Y2 wire unless the downstairs zone is calling.
Good Luck,
Joe

I'm not sure about why 1300 cfm. I don't know if the HVAC guy did a separate load calc for upstairs, or whether the system ever gets up to 1300 cfm. I do know that the unit *can* go up to 1300 (it can go higher, of course, but it's set at a max of 1300 --- a 4 ton system should be able to do 1500-1600) and that upstairs is a different zone. If just the upstairs was calling for heat/cool, it's possible the system would start at 700 and gradually ramp up to 1300 if demands were not being met. I'm not sure what determinines on the system whether to ramp up. Is it stage 2 engaging? Is it a timer? I know it's not thermostat control unless it's simply stage 2 call causes speed ramp.

The reason we ask about cfm required is the duct you have may be adequate for the heat load which would change our advice. Certainly 1300 is greater than the requirement.
J

Ok, so one solution would be to add a relay in that if upstairs is calling for stage 2, it wouldn't engage unless downstairs was ALSO calling for stage 2. Stage 1 and Stage 3(aux) would remained wired as is and uninterrupted.

Would that basically be pulling a wire off stage 2 downstairs to drive the coil of a relay. Then the SPST relay contacts would interrupt the stage 2 upstairs wire? Seems simple enough....24 vac relay with 24 vac (or more) rated contacts.

Y1 from downstairs stat would close relay allowing y2 to engage if called upon (I would break y2 between zone board and geo).
Btw you do have a 2 stage zone control; right?
This would be better but still guessing without load data.
J