Demand control ventilation or Efficiency control ventilation

[orator]

An air handler is serving 10 classrooms each served by dedicated VAV box. As the classrooms are high occupancy areas, Ashrae 90.1 (2010) section 6.4.3.9 can be applied. The basic requirement to apply Demand control ventilation as per Ashrae 90.1 is that the area of the space should be greater than 500 sq.ft and design occupancy for ventilation shall be greater than 40 people per 1000 sq.ft area.
I have two questions:
1) Should I consider each classroom as a separate space to check the requirement of DCV or should I consider all 10 classrooms served by single AHU to be as one space. I understand that DCV is a zone level control but am not 100 percent sure. Any Advise on this??
2) What would be a better ventilation control option for classrooms? DCV as per Ashrae 90.1 section 6.4.3.9 & Ashrea 62.1 section 6.2.7(1) OR Control based on Ventilation efficiency based on Ashrae 62.1 section 6.2.7 (2).
As the occupancy in each classroom is less than 40 people per 1000 sq.ft and also the Air handler is provided with heat recovery following Ashrae 90.1 section 6.5.6.1, I think DCV is not applicable and Ventilation control based on Ashrae 62.1 section 6.2.7 (2) would be a better option. Am I correct? Waiting for Valuable suggestions.

 

[berkshire]

Do you have classrooms that are empty at same time as you have full classrooms ?
If you do you would require demand control ventilation.
B.E.

You are judged not by what you know, but by what you can do.

 

[SAK9]

Whether you require DCV or not a better system would be a dedicated ventilation AHU to serve all the rooms and individual FCUs for each class rooms.When the rooms are not occupied,you turn the FCU off and cut off ventilation air with a motorised damper.My experience with working on Uni projects is that lecture rooms are either occupied 80-100% of seating capacity or none at all.I will also have the FCU cycle on/off to maintain a higher unoccupied set point say 26C.

 

[orator]

Thanks for the response. Berkshire, the classrooms are all occupied at same time. SAK9, Your option is good but the client is more in favor of AHU and even in your case, ventilation has to be controlled in some way. Can you please put some light on my case.

 

[GMcD]

It's unclear what the terminal HVAC system is in each classroom. Are you saying that the central AHU is supplying a full variable air volume system using an all-air supply of mixed fresh air and return air from a main AHU? If so- you have a controls challenge to be able to control the blend of fresh air and well as complying with ASHRAE 90.1 AND 62.1. for each room.

Typically in my neighborhood, most of the School Districts and Post-Secondary Institutions are favouring a DOAS 100% tempered outdoor air supply central AHU with "air valves" (VAV boxes) controlled on a room by room demand basis, and the individual rooms heating and cooling is done by some form of terminal system like a four-pipe fan-coil, radiant heating and cooling ceiling panels, chilled beams, etc. The air valves would be controlled from a room CO2 sensor when occupied to modulate the DOAS fresh air supply, and the air valve would be closed when the room is unoccupied.

 

[orator]

Yes GmcD you are right. A central AHU is supplying a full VAV system using an all air supply of mixed fresh air and return air. Yes the controls are challenging thats the reason I want to understand it more clearly.
Thanks everyone for sharing your experiences but I am really looking forward for the confirmation that my interpretation about the Ashrae requirements are correct. Waiting for more suggestions/recommendations.

 

[dbill74]

I think we need a little more information about the scenario.
Are these classrooms in an elementary school, high school, university, or something else?
For something like an elementary school you will need full ventilation for most of the day.
If for a university where classrooms are occupied off-and-on throughout the day, that becomes more complicated.
1) Each classroom/VAV box/FCU would be its own zone.
2) It depends.
3) It depends.

I've done classrooms is elementary schools with Bard unit hung on the exterior wall and I've done central VAV AHU. Is one better than the other, not really, it depends on the situation. With schools, providing a system the maintenance personnel are familiar with and can work on is a factor that must be considered in your design. These people are working on tight budgets and often are responsible for multiple campuses, keeping things as simple as possible and using familiar systems helps them out. (Also, more likely to get called on to design the next school.)

There is an old saying: There is more than one way to skin a cat. Similarly there is more than one way to design an HVAC system, you often need to look beyond what is in codes and standards when deciding what to do.

 

[orator]

Thanks Dbill74. That was great. Yes these classrooms are for elementary and high schools.

 

[orator]

Guys, I would like to have some more thoughts on this from experts. Waiting..

 

[GMcD]

Orator, hopefully you've Googled some information off the 'net like these excellent bits here:

https://www.trane.com/content/dam/T...sletters/standards-codes/admapn017en_1005.pdf

https://www.trane.com/content/dam/T...eers-newsletters/airside-design/enews24_2.pdf

http://www.google.ca/url?sa=t&rct=j...iq47zo__XMAhVN2mMKHR4WDxQQFgg0MAM&url=http://

http://www.opp.psu.edu/planning-con...GJLv9xtmov1sP5IVQ&sig2=IkRsptZaFpgZuNgZvcgOyA

I am not associated with Trane in any way, but there is a lot of material out there for review, the last link is to a Word.doc download from Penn State for VAV system control dated 2010.

 

[dbill74]

So elementary and high school classrooms. You are going to have a pretty consistent schedule and occupied times. I don't see the need for DCV with CO2 sensors and the extra controls that would be associated with it; unless you want to maximize/optimize energy savings. But this comes at an upfront cost that you may not be able to afford. The controls are also going to be 2-4 times as complicated, not just for you to write, but for maintenance personnel to maintain. It's a good bet in a few years time they end up bypassing sequences mitigating half the energy savings you intend.

So you want to use a VAV system, and mix OA with RA before entering the AHU. In this case you determine the minimum OA needed, use an air flow monitor to measure how much OA you are bringing in and have your controls modulate a damper to maintain your minimum OA requirement. You can probably have 3 OA set points, 1 for student occupied, another for occupied no student (staff only), and finally unoccupied.

With this setup, you still have the option of using a DOAS and have the DOAS supply side dump into the AHU's return to take some of the load off the AHU.

 

[EnergyProfessional]

you need to balance OA with exhaust air. Assume the school has bathrooms and lockers with exhaust requirements etc. You need to determine how much you need to exhaust vs. the ventilation requirements. then use the higher of both numbers for your design for both exhaust and OA supply. If your Oa is > EA requirement, you can reset OA down to the point that you need to exhaust. your JHA can tell if during low occupancy you can lower exhaust.

You want to keep your building neutral, so you need to balance EA and OA or you create infiltration or ex-filtration. All the OA and EA should go through an ERV.

in each classroom you can install a CO2 sensor to reset VAV minimum flow. for example increase flow to maintain 800 ppm etc. Use a global return CO2 sensor to reset OA intake (and EA at same time)

 

[dbill74]

I think you got a little backwards there EP when you said

If your Oa is > EA requirement, you can reset OA down to the point that you need to exhaust.

What you should have (based on your previous sentence & what I think you intended) was when OA requirement > EA requirement you can increase EA to what you need to maintain proper building pressurization.

When it comes to keeping your building neutral, I will disagree there. Keeping your building slightly positive will give a better performing building. Usually about 10% difference between OA CFM and EA CFM is enough. In general exfiltration is preferred over infiltration for occupant health, keeping a slight positive pressure helps maintain this and allows for the fact you will never get a building truly neutral.

 

[EnergyProfessional]

dbill74: what I meant is if your EA is 1,000 cfm and your OA is 1,500 then your design needs to be 1,500 cfm for both. But you can reduce OA and EA down to 1,000 cfm (based on CO2 values etc.)

 

[Gottago]

Hi orator,

Let me share my experience with you. I also design schools, elementary and secondary both. I only apply DCV to areas like Library, Gymnasium, Theatres. I don't apply DCV to classrooms. I prefer considering classrooms as one zone and leave it to dedicated VAVs' to take control of the individual classrooms. But when it comes to OA requirement, it varies room to room and there is no way (to the best of my knowledge) that you could satisfy this requirement. However, with the FCU arrangement, it can be achieved to some extent, by adjusting the dampers of the make-up air to each FCU. If you are really concerned and want kids to be sitting in healthy environment in terms of air quality, Displacement Ventilation is the answer. I have applied this concept with little modification in couple of elementary schools and its working fine. But remember in areas having harsh summers and humid climates, displacement ventilation will not be a good choice.

 

[Drazen]

I'm not sure to be understanding how do you plan to avoid conflict between temperature-controlled VAVs and DCV?