How to ensure minimum outdoor air at zone level in VAV system 3

So what is the problem?

As you note, you will have an approximately constant ratio of outdoor-to-total supply air.

If load is dominated by occupancy, then when occupancy is high you get fresh air, and when not occupied who cares?

Or am I missing something?

MintJulep,

the biggest trouble i see with zones where load is not occupancy related.

this is scenario i have in mind: let us say that on system level outdoor-to-supply air ratio is 1:6 - for design conditions. In large Zone A calculation gives 1:4 ratio, so i have no choice but to increase supply air in that zone, adjust supply temperature, play with some iterations until i reach approximately the same ratio in all zones.

if, however, only cooling load in zone A falls significantly sometimes during day, while remaining the same in other zones, than system level ratio will still be 1:6. Or it can fall to say 1:5, but that would not save me if vav box in zone A reduces supply air by say 40% - in that case zone A should receive (1 + 40%):6, which equals to 1:4,28 ratio to receive the same quantity of outdoor air.

apparently i cannot control that, as system ratio depends on total system supply air need, and zone supply air depends on what vav box of that zone imposes, and that is where my trouble is.

VAV systems and IAQ... As I remember the first major flaw w/ VAV systems was the lack of outside air at part loads and the subsequent sick building syndromes that followed.

I understand that many of the posts are international and not US so I maybe stepping out of bounds when I mention 'code' or 'standard'. I have ALWAYS followed ASHRAE 62.1 (starting w/ the 1989 version (in college) to the current edition of 2007 I believe). There they address the setting of the corrected outside air fraction on a VAV multi-zone (or any multi-zone) system based on the critical zone. Which was the zone that had the higest ratio of required vent air to supply air AT MINIMUM. As I understood it, if you set your outside air at this CORRECTED level you would ensure the correct amount of outside air at all zones. In short you were doing a weighted average of the ventilation needs of each zone. ASHRAE beliefs were that zones that had higher ventilation percentages than the corrected fraction where gaining the 'unused' outside air in the return air path from zones that were overventilated. Sure.

So, at least according to ASHRAE, this meets the ventilation requirements. Which I think it does. The trick and the 'code' part is the TRACKING the actual outside airflow is now required by most US adopted codes such as the IMC. Thus the aforementioned corrected fraction would be set and the system would modulate a minimum outdoor air damper to maintain this value thru out the full range of the supply fan. Tracking the outside air has been very interesting. Return fan versus supply fan air flow tracking was common until somebody realized that the error in the reading instruments was usually very large in comparision to the outside air required by ASHRAE. Another was airflow stations as you mentioned. Normally these would have to be put in a 'minimum outside air duct' as the air velocity of the full economizer sized OA duct was too low for most airflow stations at ventilation airflow rates. I have done this method several times and I may have had one or two actually work correctly... Ha. Just kidding they work if you have ROOM to put the darn things in the duct. Ruskin and Greenheck now have 'split' dampers that combine the economizer damper w/ an integral airflow station - minimum outside air damper. These are pretty slick.

I have started using a plenum pressure control method that uses 'fixed' openings to the outdide air and modulation of the return air damper to maintain this 'fixed' differential thru out the full range of the supply fan. Yeah you have to correct for temperature related density changes w/ the OSA but I have had pretty good success w/ this method. And its pretty darn cheap compared to full airflow stations. More to it than I mention here but I can give you an ISBN number of a book where I found it.

Gil Avery articles at are always extremely helpfull w/ VAV ventilation control. I had the honor of meeting this guy. He is a good source of information. If memory serves he sometimes uses a minimum ventialtion air injection fan into the MA plenum to maintain vent air. Pretty simple measure.

As Mintjulep says however, when the conference room loads up w/ people, the cooling load WILL go up and more air will go into the room (including ventilation air).

Hope this helps.

Carbon dioxide sensors in each conference room can mitigate some of the issues and fears, as long as the VAV box for each has reheat to prevent overcooling. I've seen some conference rooms in which a separate variable-speed exhaust fan would keep the CO2 below setpoint by drawing new air from adjacent corridors. It's worth considering. CO2 sensors still need calibration checking from time to time, no matter what the manufacturer claims. As CO2 rises above setpoint, VAV damper switches control to maintaining appropriate CO2 level, while reheat takes care of space temperature. If airflow to that room maxes out and cannot drive the CO2 down to its upper limit, the central outdoor air damper joins the party and modulates to increase the OSA ratio. That's a mighty simplified control sequence, but it illustrates the method pretty well. CO2 control is good for pleasing inspectors and logging results, as well.

HOWEVER, As MintJulep and 11241 hinted, all this additional thought and controls is often for naught. Stick a recording CO2 sensor in the room and see if it ever exceeds the limits (800 ppm abover ambient). I've either installed or retro-commissioned many conference room or auditorium settings that had the fancy CO2 control. In most all of the cases, the CO2 control portion of the control loop never kicked in. It wasn't needed, as the airflow rises with load. The exceptions were rooms that had close to design occupancy in them.

The HVAC designer has to consider the maximum occupancy in the room, based on established density figures (square feet per person). In practice, most conference rooms wind up with a huge table taking up half the space and a projection screen at one end that eliminates anyone being less than five to ten feet from it. There are exceptions, though. Some no-frills places jam more people in to those rooms than you'd believe.

Let us know what you decide. It's always an interesting discussion.

11241 -- I have worked with Gil Avery on and off for the past 30 years, and continuously for the past 15 years. I agree, he's a jewel! I have never found anything in his writings that wasn't sound and defendable. Bless him.


Good on ya,

Goober Dave

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Great points DRWeig, your demand based ventilation sequence is usually what I do. Increase zone airflow first and then the OA damper next. Reheat is a must. Yes Gil is a jewel. We actually had the guy come in to our main office and have a open forum discussion with our guys. Which was outstanding.... considering we are based out of Montana. Which is a bit out of everybodies way!

I know of a couple of ways.
1. Determine the O/A flow cfm from ASHRAE 62.1 for the space(s). Size and select an O/A fan for that purpose. Have it discharge into the return air plenum of the air handler. Now you have the required O/A cfm, no matter what the VAV boxes or air handler does.

2. Have the O/A fan controlled by a CO2 sensor in the space. Use a VFD to vary the fan speed to satisfy the CO2 sensor. Of course, at low occupancy, the O/A cfm goes down, but should satisfy Code.
These methods must be approved by Code authorities.

Someone else may have other ways.

Thanks for the effort, as mentioned I firstly had in mind how to avoid DCV as I feel double controls will make an overkill.

In MCGraw Hill Wang's HVAC handbook I did find description how control system would calculate air flow at each zone together with system OA ratio, and adjust system OA ratio in order to ensure each zone receives minimum amount of OA, but could not find manufacturer who offers that.

Hi Drazen,

You won't find a manufactured controller that does what you want without custom programming. Controls like this are best handled with a building automation controller, which can be programmed to almost any sequence of operation. I've either written or dissected quite a few of these, but I am no longer in the contracting business. Contact your local sales rep for Johnson Controls, Carrier, Trane, Honeywell, Siemens, Schneider, Alerton, Automated Logic, and a dozen others that aren't popping into my head right now.

Good on ya,

Goober Dave

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Ooops, forgive me for assuming that you might be in the US.

There are many alternatives in other countries too. The largest ones listed above in my last post are pretty well marketed worldwide. In Asia you have a lot more choices, from Yamatake, Yokogawa, LG, Samsung, etc...

Good on ya,

Goober Dave

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11241 is correct about the ASHRAE multiple space calculation. It can however result in higher OA conditioning year round. DRWeig is also on target as usual (in what I’ve seen in limited logins over the recent couple of years). CO2 controls however DO work, if correctly programmed and tested. They should be cal. checked quarterly and adjusted as needed or fully calibrated anually. Hopefully this post can supplement the good ones above. I’d proceed as follows.

First, size the AHU(s) as required for cooling. Typically it would be about 1 cfm/ft2 for your type of building although it may be less with today’s lighting, heat outputs from PCs, and better insulated windows and building materials.

Second, make sure the AHU coils can handle a larger percentage of outdoor air than that of a typical office system. This might involve using an air blender after the heating coil to prevent freeze thermostat trips (some other explanation might be needed after that statement but trying to limit verbiage).

Third, the AHU minimum outdoor air volume should always exceed the normally operating building exhaust value by five or ten percent. This should be a fixed minimum outdoor air value that is NOT reduced because CO2 levels in all spaces are within tolerance. Important to note: the exhaust make-up value is often sufficient ventilating air to meet personnel requirements under most conditions… The exhaust volume (and therefore minimum outdoor air volume at 5-10% higher) can vary however if there are controls in place sensing restroom occupancy, etc.

Fourth, when one of your spaces becomes densely occupied and CO2 levels rise, the VAV box(es) serving that space should modulate toward maximum airflow set point. If the space CO2 continues to rise while at maximum VAV airflow, the AHU should then adjust its minimum outdoor air value upward. It might start at 15% to make up for the building exhaust and rise to higher percentages depending on what you design. This was pointed out aptly by DRWeig.

So a 200 seat auditorium at 15 cfm OA per person could need as much as 3,000 cfm OA. If the maximum VAV set point for that space based on load is 6,000 cfm, the AHU should be able to achieve about 50% OA without coil performance limitations under design conditions.

I hope this added bit helps without being too redundant from prior posts… -CB

Using a DOAS might be a suitable alternative if still in design.

Thank you ChasBean!

Also to urgross's post -- I just love dedicated outdoor air systems, but so few people want to spend the extra money. Notable exception: US Corps of Engineers.

Good on ya,

Goober Dave

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Never thought I'd be on the same side of the fence as the COE; I've had quite a few unpleasant years experience with them.

I've got one small office building project at about 60% construction complete using DOAS and polled CO2 sensors. The capital outlay is higher, but the LCCA came out better over 15 years. The DOAS also makes energy recovery duct layout easier.

- if you do not use DCV, then how the VAV zone box will know that this zone( conferance room) is full or empty.?
- what about VVT system?.

Well, usually if a conf. room zone is full you have a temperature rise. Upon a temperature rise, the VAV system will modulate toward its maximum airflow. The CO2 reading to me is a backup means of ensuring there is enough ventilation. If the temp. rise from the people in that conf. room is not enough, the CO2 rise might be. Now you have two means of ensuring enough ventilation: the higher CO2 will set the room VAV to max airflow and can also raise the AHU minimum OA if the higher room airflow is not enough. Reduced minimum OA should never go below the building exhaust to keep the building positive. So CO2 DCV to me is a cost added measure to ensure enough ventilation (although that is not how it is sold).

VVT is fine, but be careful. They are generally cheaper systems that can produce humidity problems because they are not "smart" enough to adjust their controls based on outdoor dew point. An OA dew point override and reheat should be in place for humid conditions or we would have odors/mold...

Yes, Chas, I counted on temperature rise.

Thank you once more for all of answers, number of them giving me excuse not to respond to all.

I should have put more emphasis on my constraints from the beginning, and that is limited budget, which can crucially affect client decisions.

That is why I am trying to avoid DCV, as well as custom-made control system, hoping to find some manufacturer's "standard" module which will provide, if not all, majority of required functions.

Chasbean
I agree with you in general, but I have one point, you said if the zone is full the temperature will raise up then that will cause VAV to open and bring more OA.
in this case you are using space dry temperature as a sensing poit, but what if this temperature has been raised for a different reason other than occupation load in the zone, then we will bring more OA acutally we don't need it.
CO sensor is part of DCV as I know, and the main question was about finding somthing other than DCV but act as the same as the DCV

if we have two zones, A and B
both have the same total sensible load of 100000 Btu/hr each, but zone A has occuaption load of 5 persons, and zone B has occuaption load of 10 persons, the total supply cfm still the same as long as the total sensible load still 100000 btu/hr, but zone A need less OA than zone B

Keep it simple...

Ensure you seperate office areas from the likes of conference rooms or higher occupancy area so areas of equel proportions of outsdie air are on the same HVAC system. e.g. offices may require 10% outside air, conference rooms require 25% outsdie air, best have have these of seperate AHU's,. If you cannot do this the area with the higher proportion of o/a dictates the minimum outside air rate of the entire operation.

Assertain the required minimum amount of outside air for each system / area l/s/person etc... This will depend on your design standard or local authority code. You can then decide to maintain this minimum outsdie air for when ever the HVAC is ON. Even when the VAV ramps down if you can maintain this outsdie air flow rate you will comply with your design code.

Place a good quality flow measuring station on the incoming air duct to the AHU or at the damper intake (check vendor installation guideline and dont take short cuts). Set up your controls so the damper modulates to maintain the setpoint outside air flow rate, so matter what speed the fan VSD is doing the minimum o/a is maintained.