See the case study attached. It was an ASHRAE article.
cry22, do you also mean no dampers at the diffusers? How is the system self-balanced?
I can see if there are two same-cfm-diffusers, say 500 and 500. The duct can be designed to keep the same static pressure to both. But what if it is 200 cfm and 800 cfm supply diffuser? Would sizing to the same static still keep the system self-balanced?
Marcoh, in the VAV systems I have designed I use the zoning to create as much diversity as possible so that the sum of peak VAV flow at each zone is substantially greater than the selected AHU can flow (AHU is selected for peak zone load airflow at a given load).
Opening all VAVs 100% prevents system balance. Because of the numerous ways that VAVs can be designed and implemented, the AABC National Standards book states that there is no "one size fits all" method for TAB. Each system needs to be understood and balancing has to be planned.
Unfortunately what I typically see is some TAB tech that has never seen the plans or the space show up at the job site after the mechanical is "done" and 2 days before the owner expects to get their CO. Then when I get the badly done TAB report I am the bad guy for not signing off and "holding up the project."
The missing information in the above discussion is that one needs to specify "pressure independent" VAV box controllers. Then, given you are using a DDC control system of some kind, you can set up the min and max volumes individually for all the boxes, and the individual boxes will be able to operate independently no matter what the other boxes are doing, and how they affect the main supply duct static pressure (which is then used as a feedback to variable frequency drives on the main fans). So no upstream duct balance dampers are needed since the system self-compensates, and the individual VAV boxes can operate at their programmed set min and max airflows.
AnotherEllis: Yeah- it seems like that on every job lately....Owner wants to move in, General Contractor/Construction Manager is screaming at ME to sign things off when THEY haven't fulfilled THEIR contractual obligations.....Mech and Elec systems these days just aren't getting finished off - a construction site, at best, is Daycare for adults, and is basically an exercise in herding cats all the way through the process, and the shiny shoes MBA types that are all popping up as "Project Managers" these days couldn't project-manage their way out of a wet paper bag.
Make sure it is a pressure independant system as GmcD noted. If you try to go without a balancing damper with a cheap VVT system you will have problems.
MVDs are not needed upstream of pressure-independent
(PI) VAV boxes. Their installation in such a fashion actually would be a detriment to the system. The VAV box controls are effectively a dynamic balancing damper, absorbing the excess pressure available in the system as pressure and zone airflow requirements change.
Having read the AJ article posted by cdxx139, my conclusion is that one would not normally insert an MVD in front of a PI-VAV box but there might be circumstances where one would, such as where you have large branch length disparities with the short branch close to the AHU. This is never ideal, but there is always a job we can think of...
Making me think though. I'm going to go back through some project drawings. Good question and input from all. Also, I admit that I have personally never used the static-regain method in my life.
You may need an automatic damper upstream, not a manual, depending on whether the supporting AHU serves more than one zone with different occupancy times.
I agree all VAV boxes should always be pressure independent which will give much better temperature control.
A VAV system is in essence self balancing but I have had issues where a VAV box (particularly the ones closest to the fan) can be noisy as they need to close too much to kill the duct pressure. Installing a VCD before the VAV box can assist in removing some of the static pressure.
ASHRAE 90.1 now requires static pressure reset. Use of MVD's resulting into set airflow at all times to all branches will actually violate 90.1
About the article:
As a lesson learned - Always add a note in your general notes saying that "in case of conflict between drawings and specifications, the highest cost item shall apply." It removes the stupid notes left out in details that are never edited, such as in this
In the case study, I see ovewhelming evidence that the A/E was right. I bet that the outcome will be in favor of the government.
The case references "high velocity" and is defined per SMACNA 1975. SMACNA 75 is absolute, we no longer talk of high pressure, low pressure, etc, we talk about 2", 4", 6" etc. Matter of fact, ductwork for VAV systems upstream of VAV boxes is considered "Medium pressure", not high pressure, thus the note referring to HP in the article is actually not applicable at all.
And finally, just because someone wrote an article about a case study involving a litigation and got "published" in ASHRAE does not mean that this is the new ASHRAE guideline -References to ASHRAE should be pulled from the 4 books, not from articles written by its members.
In my opinion, installing MVD's upstream of VAV boxes is simply stupid, it is stupidely expensive and useless on everyone, it would be buying disease with your own money.
The one exception I know of, as above, is for zone dampers, as per 90.1-2007, 6.4.3.3.4. The dampers would have to be automatic by energy code and for fire code override.
