Static Pressure Reset

[ccnman]

Folks,
Controls guy here. Can you give me some tips on pro’s and con’s (if any) on static pressure reset in a supply vav duct ?

Most requests i see are to gradually reset duct static down if one or multiple boxes are less than 90% open and then maintain the static you achieve when the box ( or multiple boxes ) are fully open .

wouldnt many more boxes upstream of the subject boxes want to open due to the lower static in order to achive the same amount of cooling ?

I know its a highly desired practice ( static reset ) but is it more applicable to older non balanced systems ?

Thank you in advance

[WAYNE3298]

I think static reset on a VAV system has more potential problems than benefits. Static pressure does not have a high cost penalty even if a little too high. The system should be balanced and the SP set at that time. If a problem occurs in the system that requires increasing the SP I would want to know what the problem is and not simply over power the problem.
To do SP reset properly it takes a lot of sampling by the controls and the more you do the greater the chance for error. The sampling is great for alerting you there is a problem and for troubleshooting but not for total control.

[Cagey57]

ccnman,
Orion242 had comments on this in the past but I can't find them right now.
As I recall it came about as part of the IEEC or possibly an ASHRAE standard, not sure right this moment.

I have done Duct Static Reset 3 different ways, 2 worked well, 1 passible. So far the only one that stood the test of time was the one that did not allow operating personnel to Fu.k with it at all.

Based on all of the applications of the strategy I have done and verified with Cx every one demonstrates what I would term "Oscillation" of the setpoint and the VAV box dampers. Some at a higher rate than others.
My personal opinion is that it (Duct Static Reset) is a waste of time and effort "Unless" it is done correctly and operating personnel cannot mess with it.

[Artrose]

Opinion..... foolish practice sometimes promoted by young engineers. They have little field experience, and don't understand that the more complicated a system is engineered, the less likely it will operate properly after the warranty period expires.

This is another example of stepping over dollar bills in an attempt to squeeze out a nickel. Most of the folks (service, maintenance) who will be there after you've left, won't have a clue how the system is supposed to work, nor will they have the ability to make it work. (if the system can even be made to work properly)

I've never heard anybody say this is a highly desired practice, unless maybe in a special application. If used in a regular old run of the mill variable volume system, unless you're very lucky, over time it has great potential to become another dissatisfied customer.

[WAYNE3298]

I'm surprised you didn't get more responses on this. The most important thing for static pressure control is the location of the DP sensor. When a balancer sets the pressure without auto reset it is set to deliver design airflow to the box that is the hardest to get air to. That means that auto reset is useless except if you are controlling the DP to just meet the need of that box at less than full load. That also means the only time you save energy is at part load of that VAV.
The net energy savings is too small to pay back the cost if based on a life cycle cost analyses.

Your opinion Artrose is well founded in logic and common sense.

[stanbyyourword]

I agree with above but not for simple static reset This whole concept of "trim& respond" sounds great on paper , but with all the intensie commissioning and programming required to make it work properly , including increased maintenance , how much are you really saving? plus thats right next company wont have a clue

[WAYNE3298]

True word because cost determined by simple payback which is how most energy saving ideas are sold ignores a lot of the things you mentioned. Life cycle cost analysis does take all that into account and paints a different picture on all energy saving ideas and shoots down most of them.

[stanbyyourword]

I like the idea of static reset on OSA but more in favour of space temps ie all spaces satisfied, such as partly occupied floor etc. i had this discussion with orion also , and he was of opinion reset on OSA will cause the system vav boxes to fight the lower static.

[Cagey57]

ccnman,
I gave you my experience before.
My honest opinion of "Duct Static Reset" is that it is simply a waste of time. CE's specify it because it is in some standard or guide.
I have yet to find documented studies that show Actual energy savings. In theory it works, in practice I think it just complicates operation of a VAV system.

From a balance stand point (as I believe Wayne pointed out) the "Balance" has to satisfy the last terminal on the system. A VAV system does just that without any reset to being with.

Where the "Rubber meets The Road" is that operating personnel will, eventually, reduce the control system to their level of understanding when issues arise.

[WAYNE3298]

I like OSA setpoint established to minimum (by the balancer) using building static pressure and the position controlled from there by CO2 levels. The relief damper should maintain building pressure as the need for OA increases but should be closed with minimum OA.
I have seen a lot of other methods tried and most of them end up a disaster from the beginning even before I finished the balance work.