Static reset, SA Reset & Ventilation Air
Rather then hijack the static reset thread, I start a new one.
I would value everyone opinion on the effect of fixed volume ventilation air supplied by the VAV system when Static pressure reset or Supply air reset are used. Also what other methods can be used to save energy when supplying ventilation air thru' the VAV system without using demand ventilation.
"There is plenty of room at the top because very few people care to travel beyond the average route. And so most of us seem satisfied to remain within the confines of mediocrity." -- Benjamin Nnamdi Azikiwe, first president of Nigeria
I am in south Florida and have set up static reset, supply air reset and chilled water reset on the same building. All three strategies worked great. Even on the hottest days something would be resetting with no ill effects. You would be amazed of the drop in current draw simply from going from 1.5" to 1". The first building i tried static reset we did it without looking at or controlling by static pressure. We simply took the highest VAV damper position and used a PI loop controlling the fan speed with a 95% setpoint. Trying to get people to grasp the fact you are moving the same volume of air whether it is 1.5" or 1" is difficult to say the least!
It might be difficult to get the customer to understand the point that one can deliver the same volume of air at 1"WC as at 1.5"WC. But I doubt if anyone in this forum has trouble grasping the concept.
Originally Posted by metasystech
Your scheme CAN indeed work, and work well.
In some locations, with certain climates, with some building designs and construction, and with certain types of mechanical installations ... it can even be easy to do.
In other cases, it can be a real ball buster.
My general opinion is that if the job spec mandates it, yah gotta try regardless.
But if its an optional item, well the programming part of it is easy enough, although I never use PID loops for such a scheme as demand based duct pressure reset, so it might well be worth a try. Depending on what equipment is installed, etc. Easy enough to abandon the loop if the task of getting everything balanced and operating smoothly proves to be too time consuming.
But I've worked with systems (the use of "systems" I use here to indicate the whole combination of mechanical equipment, air distribution system, building design and construction, local climate, required SOO's, and customer/design engineer expectations for the tightness of adherence to SP) where demand based duct pressure reset was a friggin nightmare. From the point of view of the controls guy.
In such cases it depends on how much money and time you've got.
If I've got a generous budget to work with, no problem. I'll get it to work, eventually.
However, that's rarely the case. We're almost always operating on a fixed budget, that doesn't include a large markup so that you've got some extra to work with and still turn a profit on the job.
There are cases where such a scheme is a pretty easy chore. One comes to mind immediately. A fairly small project, as compared to the usual sort which we do. The building had two air handlers, each serving distinctly different areas of the building. One AHU serving part of the building was a piece of cake. Constant volume, 100% OA, heat recovery, duct reheat for each space served. Actually it's a 2 speed, a VFD was used. Full speed during occupied times, 50% speed in unoccupied.
The other AHU served office spaces, a VAV system with 31 VAV's. Each having a reheat coil. So special, unique requirements. All pretty straight forward. HW, CHW system. With the typical, around here, OAT lockouts and enables of heating/cooling, economizer mode, demand based resets of heating/cooling systems. Yadda, yadda.
In short, nothing complicated or exceptional. No special requirements for any of the areas served.
Piece of cake. Had a good balancer who did a competent job.
Setting up the programming, was pretty much a non-issue. That's usually the case. One of the easiest parts of the job.
Verification of points, establishing that what you THOUGHT was connected to X, was actually and really connected to X, and that the device connected worked correctly, was installed and configured right, both at the device end and at the controller end, etc took MUCH more time. Always does. We verify each and every single item. And the tech has to sign off individually for every single point certifying all is correct. Those sign-off sheets are kept in permanent record, BTW. It's our method of quality control. About half the customers we deal with in fact demand such an accounting. But we do it for every job.
The device and controls points verification is standard, and the time and effort included in our project pricing and planning. So its really not much of an issue. A tech finds too many problems? Our installers get talked to. Nicely, we're not out to hammer em. Just to find what they might be doing wrong, and why, and to fix that problem so it doesn't keep reoccurring. No big deal. Its a training opportunity as versus an a** chewing.
SYSTEM testing, tweaking, tuning, troubleshooting issues found, etc is what eats up the time. As in amounts of time that are almost impossible to predict beforehand.
For instance, in this case, with the building I've been talking about, I probably spent 80 man hours dealing with issues that were NOT related to faults caused by our control system or its components. Separate mechanical and regular electricians were used on this job, as neither our mechanical nor electrical divisions got the bid.
If our own guys had did that portion of the work, I'd have spent less "wasted" time. I know those guys, have worked with em before, we communicate and work together cooperatively. Its not an "us against then" situation.
But this wasn't the case in the job I'm talking about. It was complicated by the fact that by the time I was on the site, most of the work was substantially done, most of the systems were being put on line.
Translation ... most of the foreman and lead men were off to other work, and what remained were lower level worker bees polishing off punch list items.
So I'd find that VAV such and such just wasn't making space SP, during heating season. Why? I knew darn well the reheat valve was working, had checked it myself. Okay, find a ladder and go back for a closer look. First time through is usually hurried, I'm just looking to make sure valve and damper actuators are mounted correctly and move smoothly through full stroke (in the right directions), DAT is in correct place, etc. Now I go back and look at everything, in detail. Our stuff, and that which was the responsibility of someone else.
Ah..hah ! Darn reheat piping is backwards, flow is wrong through the coil.
I contact the guys from the mechanical outfit and tell them. They shrug but say they'll call the foreman or project manager. I make notes, another item I'll have to check AGAIN and follow up on.
In this case, had to follow up on it several times. Whoever back in the office for the mechanical outfit decided I was probably mistaken, dismissed my first effort to get em to fix it. Days later I check with the low level punch list item guys. They shrug and say they have no idea if anyone did anything. So I drag ladder back to the spot and take another look. Nope, still wrong.
And so it went, couple calls produced no action. I then take the issue to the general. He comes back and says so and so back in the mechanical companies office says the sucker is installed as per drawings. Period. Wonderful, I don't care what the drawings say ... it's wrong. Fire off an email to the architects, HVAC design engineer, the customer's upper management, my office, etc. Wait a good long time. Finally get word back that I should show up for a meeting with all concerned.
Great, more wasted time. Couple hours. After which decision was made ... after a lot of debating, looking over prints, etc ... "Hey, maybe we should go take a look at the real deal?"
Yeah, wonderful idea, wish I'd thought of it long before this point. They did. Finally an engineer commented, "Oops, you're right, it is backwards. We'll have to get this fixed."
ROFLMAO ... that's my world. Happens all the time.
On this project the same sort of thing occurred with several other items. And my "wasted" time, troubleshooting and identifying other people's problems, added up and added up. Found that VFD's installed and configured by someone else, were set up wrong. In that case, I just did the reconfiguration. Nobody was looking at the time, and its was less wasted time for me to just fix it as versus going through all the hoops to get the people actually responsible to correct things. Spent a couple hours teaching a mechanical service tech, from the mechanical contractor's business, how to troubleshoot ... properly ... an electrical motor on am FCU which wasn't working right. (Thermal overload was tripping at too low of a point) Did that because I said it didn't work, and he looked at it and said it was okay, must be something WE did .... 3 times. Got tired of it. Just showed him and proved my point right in front of him in such a way he could not mistake what was happening.
Not a big deal, none of us know everything about everything. A learning experience for him. I've BTDT. But it wasted a lot of my time on that job.
I'm not bad talking that contractor. Actually, all in all, whole project wise, they did good work. Some of the issues I found were in fact a matter of them putting in something exactly as shown by the HVAC designer in his detail plans. But it was wrong. Wouldn't work, couldn't work, not adequately, no matter how well my programs and control loops were working.
Some things I missed. But the balancer found when he came through. He made folks come back and fix whatever. And I had to recommission and retest. More time wasted.
Not complaining. That was a simple project. Total number of "time wasters" was pretty low. Compared to other projects I've worked.
A different project, far larger, and more complex and demanding. Done not long ago. About a year. Egads ... I wouldn't even hazard a guess as to the total number of "wasted" hours of the types I've been indicating. Would have to check my work logs. Had to be hundreds. 37 AHU's of assorted sizes, some constant volume, some VAV. 300 and some odd VAV's. 118 exhaust fans, some constant, some VFD driven. Numerous different special space ventilation and control requirements. Occupancy sensors that tripped both lighting and ventilation. CO2 resets. Humidity control resets. 30 something different space installed power, DDC system relief dampers. Special lab requirements for some spaces. Etc and so forth.
Lord, love a duck ... was that project a cluster-f**k !
And it had just about every energy savings scheme incorporated in it you could name.
Oh, we made it work. Finally. But it took an enormous amount of time not only dealing with our end of things, but troubleshooting and identifying other people's "Oops". Everything from design errors made by the HVAC designer, to installers who had brain farts or tried to cut corners, to issues with the building envelop leaking like a sieve. On that last point, I didn't need to pull up a local weather report to find out the outdoor wind speed. I got to the point where I could just watch the displays on the front end, see the changes in building pressure for various areas, watch the changes in VAV damper positions and air handler ramping up and down, and could give yah a pretty darn good estimate of wind speed and what direction it was coming from.
If its not obvious to you. As spaces become more negative or positive, than they were, this changes the "resistance" to the air coming out of the diffusers. More, or less, differential between the space pressure as compared to the duct pressure causes more or less air to flow into the room for a given damper position. It might be slight, but its very real. The VAVs were doing their job correctly, adjusting to compensate.
We were going from a condition, previous to the start of the "big blow" of near steady state operating conditions. Everything nicely balanced out and smoothly operating. Very, very small changes in actuator positions. To non-linear hunting varying IAW whatever the wind outside was doing. Driving the previously smoothly operating PID loops to distraction. As they attempted to once again find a "sweet point". But not only were the winds variable and unpredictable in intensity at any one moment, direction changed abruptly and irregularly. Space pressure control reliefs, and return fans that responded to space pressure also started "rocking and rolling". Reheat controls also commenced hunting where previously they'd been steady as a rock. Not only because of this constant, tho minor, hunting of the air CFM controls. But because of drafts within the walls, that constantly varied in intensity, more cold air this moment, less the next, which was causing the space temp sensors to "wander" more than normal.
VFD driven exhaust fans were also ramping up and down continuously trying to compensate for the continuous variation in pressures that were occurring.
Some of this is normal, always happening. But in normal conditions, everything designed, installed, etc well its almost not noticeable.
In the case of this building ... it was definitely noticeable. Occupant complaint calls were pouring in.
There was simply no possible way (we tried) to tune and balance out the system so that everything resumed smooth, proper operation under BOTH windy and non-windy days. Not adequately. Settings for one case, didn't work well for the other.
Of course, we had to spend time investigating, testing, and PROVING our stand on the matter to all concerned. Lots and lots of hours spent. A BUNCH of them.
In the end, the solution was two-fold. A lot of work was done to seal up the worst of the "leaks". But some, including some bad ones could not be "fixed" at a cost that anyone could afford. We were also allowed to make changes to some of the control schemes, and to vary from some of the spec'd requirements that dictated precise loop response and SP recovery times, etc. In short, we applied software patches to counter hardware problems. And in the end produced results everyone could live with.
FWIW, demand based duct pressure reset was one of the items dropped. The amount of added complication and occasional system instability problems ... under certain conditions ... was finally deemed ... in this case ... as not worth it. Given all the other energy savings schemes, each of which were deemed to have more overall impact on the final bottom line, the energy bill.
Keep in mind ... I'm only speaking about THIS installation, in this climate area.
It wasn't saving em a lot anyway. The balancer who did that work for this project was good, very good. Things were tweaked to the tee. The highest maintained duct static SP, and it works summer or winter, was 1"WC. That, along with things like occupancy detection, CO2 reset, night setbacks and shutdowns, the fact that most of the AHU's have heat recovery, etc results in a heck of a lot of energy savings.
We did test and finally implement one other measure not called for in the spec. Limiting DAT high temps values from the VAV's. In some spaces, and considering the size of the building that was a lot of areas, DAT could go pretty high when the reheats were at full bore. Keeping DAT from the VAVs from going above 92'F, we found that the VAV's in question could satisfy heating requirements and increase space comfort by reducing "short circuiting" of air heated too much which stayed high in the room and traveled directly to the return grills. As versus "falling more" during its travels, as it did with the lower temps, and better circulating at the occupant level.
Just some thoughts. No criticism nor a debate about the worth of any one energy savings scheme.
By "fixed volume ventilation air" I am guessing you mean that you have a system(s) where OA intake required is a fixed minimum value in CFM?
Originally Posted by klrogers
Just trying to clear that up since sometimes in these discussions misunderstandings occur and folks start debating "apples and oranges" situations.
I am supposing that your AHU fans have VFD's and the OA intakes have flow stations measuring actual OA CFM?
If the situation is as I'm guessing, then here are my thoughts.
Don't get me wrong about anything else I've posted as concerns Static Pressure reset. Its a valid strategy, can save energy. How much energy, and how much trouble it is to implement successfully is highly variable due to a number of factors. But it is definitely worth considering.
In general, the greatest payback in energy savings is if your distribution system (ducting) is leaky, if you are allowed to establish a fairly broad spread between heating SP and cooling SP for individual spaces, if you haven't got a bunch of "special requirements" spaces in the same area served by the AHU in question, and you have the time/money available to identify and eliminate "rogue" VAV's which would cause the system to stay at higher pressures than need be.
I see that you are in Virgina, so I'm assuming that on an annual basis your building isn't in a heating mode more often than a cooling mode. Here, we're in heating mode a lot, and in said mode VAV's are operating in a much reduced max CFM allowed rate, usually 50% or 60% of the max cooling mode rates. So any added energy savings realized by a Static Pressure reset scheme is much less than as might be the case in an area where a building spent more time in cooling mode.
In general, based upon my own experience (don't count that for more than its worth ... its not worth much), and based upon studies I've read the various temperature reset schemes offer the best bang for the buck in energy savings (biggest % of total HVAC energy costs) and should be considered and implemented first. Granting that the heating and/or cooling plant can handle significantly reduced load operating conditions without mucking up something.
Next up would be being allowed to spread the differential between heating and cooling SP's. Depends on the occupants. On some projects I can do that. For others it is not allowed.
Yeah, I know, in some of the threads on this site some folks claim that ASHRAE REQUIRES this or that. But the fact is that ASHRAE does not in fact have regulatory power. The State where you live does. The County or City does. ASHRAE itself does not. They're not elected legislative officials. Can't make law. Have no enforcement powers. Now, in some places local legislative bodies have in fact dictated one follow ASHRAE suggestions, in total or in part. Sometimes with exceptions allowed where such and such other criteria are met.
But I can't address that for where you live and work.
Anyway, after the other steps, I'd consider a Duct Pressure reset scheme as one of the next on my list, if I'm allowed to put things in order of priority.
However, since you seem to have a fixed minimum OA intake requirement, I'm not sure that you'll realize much savings from a Duct Pressure reset scheme. Some certainly.
But it'd be more likely to be significant if you could also employ some demand based ventilation scheme. By "demand based ventilation" I'm referring to any scheme that allows you to monitor occupancy and then drop back the total required minimum OA intake, as well as cutting back on the CFM being delivered to that space.
Excess ventilation requirements can be an energy savings killer.
Where I live, fixed minimum OA schemes (at the air handler, to met "fresh air" requirements) are being dropped like crazy. Except in situations with special requirements.
Where they've been kept (for ordinary spaces such as classrooms, offices, etc) because doing the retro-work to install the needed components to make a demand based ventilation scheme work is considered cost prohibitive, typically heat recovery devices of one sort or another are being installed.
But mostly, anyone who can, is ditching fixed minimum OA CFM requirements and going to something more flexible.
For instance, one customer. They already had occupancy sensors for lighting in most all their spaces where such could be reasonably used. But said occupancy sensors were not linked to the HVAC system DDC equipment.
Making that connection wasn't any big chore or expense. DDC controlled VAV boxes for each space were close to, if not actually in said spaces. The DDC controllers had available, unused inputs. Simple, short wiring runs. Piece of cake. The rest was just a matter of programming. And not complicated programming. If the room remained unoccupied for a user adjustable period, put it into setback mode. This wasn't just go to normal minimum air flow, that was overridden and required air flow went to 10%. The front end also monitored this, and from table data previously set up, reduced SP for min OA air by an appropriate value.
Another customer set up a variable "master schedule". Since it was known that certain "groups" of rooms were occupied 24 hrs a day, other groups were occupied 12 hrs a day, and yet other groups were occupied by 9 to 5 office workers. Each VAV was assigned to an appropriate "group" schedule. While still others, having highly variable occupancy actually had occupancy sensors installed (a low number of the whole count). Thus as each group went into scheduled unoccupied, air flow was dropped AND a certain amount deducted from the min OA CFM.
Then there are those who opted for RA CO2 sensors, and an OA reset based upon CO2. Not as effective as individual space CO2 sensors, especially if there is a high variation in occupancy in the spaces served by that particular AHU. But cheaper to install. And better than just bringing in a constant OA minimum that might be in excess of current need. And wasting heating or cooling energy to condition that air.
Quite a number of our customers are now opting for individual CO2 sensors for each significantly sized space where occupancy can be highly variable. Especially in new construction. And are using a demand based ventilation scheme.
That's some of my thoughts.
Gotta get off to some real work and earn a living.