Ventilation with Multi-Zone AHU

[gb555]

Another HVAC Consultant claims that a CO2 sensor in the return air for a multi-zone AHU satisfies the intent of ASHRAE 62.1 for a school. My claim is that the proper ventilation air must be provided to each classroom (as reduced per ASHRAE 62.1 for multi-zones) and that meeting this with a multi-zone wastes energy by over ventilating unoccupied spaces.

 

[NCDesign]

Well a multi-zone unit is effectively a VAV system with all the vav boxes directly at the unit instead of spread throughout the space. To follow ASHRAE 62.1 you would need to incorporate a control system which could measure and modulate the outside air as well do some Z factor equations on the fly. You could then modulate the amount of outside air to the minimum amount possible and meet ASHRAE 62.1.

The only way that I am aware of to control the amount of outside air at a specific space is to bring in outside air at each individual location.

The addition of a CO2 sensor doesn't really change the equation. In a multi-zone/VAV environment the most effective place to measure the CO2 level is in the common return simply because you only have one point of control. If you can directly control the amount of outside air being introduced at each location than CO2 sensors in each location would be sensible.

To really get to the point. If you have a single point to introduce outside air into the system it doesn't matter what type of system you have you are going to over ventilate unoccupied spaces, CO2 sensor or not.

The greater concern may actually be the opposite. That is since you are now using an average CO2 over a possibly large space you may be under ventilating a heavily occupied space because some of the other spaces are not occupied.

 

[atlas06]

NCDesign
Multizones are NOT VAV systems, they are CV systems, you just mix the air from the hot/cold/by-pass decks.
I agree with you in your last statement, the only way to comply to IMC is to locate the CO2 sensor in the heavily occupied space, i.e IF you want to limit OA quantities. W/O a CO2 sensor owever, you only need to count people and use normal OA quantities for CV systems.

I disagree however that you will need X/Y/Z calcs for a multi-zone.

Now, since we are talking Multi-zone, I hope that you will not mind me go two other topics.

Topic #1:

I see some people designing one smoke detector on each supply air zone (using NFPA 90A as the governing code), as many as 10 SD's for a 10-zone being provided for one 3DMZ AHU.
My take is that only one is required at one of the zones (suggest the largest zone), the rational being that the smoke in any zone will shut-down the fan anyway. And for the sake of argument, I'd throw in an IMC required SD at the return. where do you locate your smoke detectors for Multi-zone units?


Topic #2:
How do you run your calcs using Trace 700? it always gives the heating coil size for the full CFM with very high SA temp.
I tried to get a rational from Trace folks, they tell me that the program has its limitations, it only deals with theory, not the actual building of the MZ. To what CFM do you size your heating coil in a 3DMZ? I use 50% max of fan block load with SA temp as much as 105F, or else the darn heating coil will wind up larger than the cooling coil, when the opposite should be true.

 

[atlas06]

Error in Topic #2 above: meant very low SA temp.

 

[lilliput1]

Why not use a sensor at each room and have the DDC control for the most critical zone, the space that has the max CO2.

 

[gb555]

Thank you for your replies regarding the multi-zone. They still think that one CO2 sensor in the return duct of a multi-zone used in a school satisfies the intent of the ASHRAE ventilation requirements. I feel that this method of controlling outside air violates the requirements of providing the required ventilation to each classroom. Sometimes, cost of operation seems to win regardless of facts.

 

[atlas06]

liliput has a good point, it is a common practice in schools to use a CO2 in each classroom, especially those applying for LEED certification, CO2 at each space allows a better IAQ and controllability of system. Beware of the controls guy you get though.

I have used this technique to down size a chiller (down to 500 SF/ton) for a school in the past (i.e gym, classrooms, cafeteria, auditorium, lecture halls...) do not have simultaneous use, thus a diversity factor could be used for ventilation. I had the hardest time getting the controls contractor to achieve the sequence I wanted for a demand controlled ventilation (DCV) system.

I cannot trust DCV any more because of one lousy guy.

It appears that this is good only in theory for now, unless you get a really good controls guy.

 

[NCDesign]

atlas06 I think what you meant is you can depend on DCV you just can't depend on the control contractors. It's something that we have to face everyday. Is the contractor doing quality work in implementing our designs. I would also challenge you to review your DCV sequence. Is it really clear or are you generalizing it and expecting the controls contractor to just "know it".

From personal experience that can be one of the toughest things to make sure of with controls. It is very easy to become heavily reliant on the controls guys to just make it work. The clearer we can be with our specs, sequence of operation, etc the easier it is for the dumb controls guy to implement the difficult control strategy.

Going back to your questions previously. I place the smoke detectors per the codes for the area. Here in my local jurisdiction the only requirement is for SDs in the return. If the code called out for SDs in the supply I would try and find a common point in the supply before the zone head. Otherwise I would use the largest zone as well. I haven't had to get that through an AHJ yet so I haven't really had to defend it.

I use Trace, but I haven't run a MZ unit so I can't really help with that.

gb555 I understand your frustration with the lay-out. It really isn't ideal but I have trouble arguing against using it altogether. I think lilliput1 and atlas06's DCV sequences might be a very good middle of the road. You have some control to try and better match the outside air needs for individual spaces while still minimizing cost. The way I understand your argument the only way to fully meet the ASHRAE requirements would be independently supply outside air to each individual space which would be at a huge additional cost.

 

[atlas06]

NCDesign, you got my drift alright. Regarding my sequence in my previous experience, I spent a week alone and 6 hours with a controls contractor after which we had a matrix on the drawings and all. Luck has it that the control guy that worked with me did not get the job, but the CD's had even the PPM levels at which the dampers and air flow stations would reset the OA.
And this from a reputable controls manufacturer. Just becasue you had a great company does not mean that you get a great installer.

 

[NCDesign]

atlas06 situations like that are tough. When I was part of a contracting co I always hated when I knew too much about a project. It always meant our bid would be too high.

 

[imok2]

In one auther's opinion: "Adequate amounts of dilution air can be provided without energy penalty, simply with more precise control of ventilation rates. A dynamic, continuously adjusting system will provide the greatest payback and flexibility.

New products are being installed across the country in new schools that utilize advanced designs and equipment. Some installations and locations require slightly higher initial HVAC equipment costs, but offer the benefits of providing many more years of lower operating expenses and reduced maintenance requirements, with much greater flexibility in HVAC control as conditions change in the future.

Compact and economical velocity meters are currently available for installation in smaller air handling equipment with outside air intakes, including: unit ventilators, fan coil units, water-source heat pump systems, Energy Recovery Ventilators (ERV's) and desiccant-augmented make-up air units, vertical air conditioners, and the range of smaller packaged roof-top equipment. They can be easily field-installed or ordered as an unpublished option with most air handling equipment.

Sensors are permanently calibrated from the factory for equipment-specific intake conditions, with accuracies better than 2 % of Reading for flows typically ranging between 50 and 800 fpm. Combined with central Direct Digital Control (DDC) hardware, the sensor's electronic output can operate intake dampers, serve as input for return or exhaust flow control and provide school administrators with real-time monitoring and unlimited logging of ventilation rates. These products allow measurement and control of dilution ventilation rates, where historically it was not considered possible or practical.

Improved control of dilution ventilation rates can be implemented without concern for system effects or energy impacts in most of the country, generally those areas without bitterly cold winters and without jungle-level humidity in summers. In these extreme climates, additional preconditioning equipment may be needed to avoid systematic problems and/or help limit energy usage. Regardless of the geographic environment, continuously maintaining codified ventilation rates is still required. But with professional engineering assistance, great results can be competitively accomplished in today's building designs".

 

[lilliput1]

In making the control sequence, dont forget:
1. define occupied & unoccupied or setback mode of operation
2. override DCV during economizer operation
3. set absolute limit minimm OA during occupied mode such that exhaust makeup is provided & building is pressurized at say minimum 0.05 CFM/SF
4. Use of multiple CO2 sensors & control for the zone that is most critical. This is not middle of the road solution. This is the only way you can satisfy the worst load when your AHU serves multiple zones. The OA is mixed with the RA at the AHU. There is no way unless you direct duct OA individulally to each room can you get individual DCV.

 

[lilliput1]

Oh I forgot the hardest one of all:
5. Define how you want to control the VAV return fan to track the supply fan and allow DCV!