Question

[317069]

Hello all
Two zones A&B have the same occupancy, let say 10 persons each, the load calculation told us that zone A needs 600 cfm, Zone B needs 400 cfm fore example, and both of them need 150 cfm of fresh air each for ventilation.
Now total cfm is 1000 cfm, fresh air is 300 cfm which means 30%.
So, 30% of 400 cfm is less than 150 and 30% of 600 is more than 150.
Am I right?

 

[DRWeig]

Yes.

If you want to distribute the outdoor air in a proportion that is different from the proportion of supply air, you can't just mix them and send them down the duct. A separate tempered-outdoor-air system is needed.

If not, you must increase the outside air so that the worst case room is ventilated to minimum required by code.

In your case, 38% total outdoor air will give you just over 150 CFM to zone B. Zone A will be over-ventilated (225 CFM).

Good on ya,

Goober Dave

 

[msquared48]

Dave:

If Zone A is a bathroom, over-ventilation is a good thing.

Mike McCann
MMC Engineering

 

[DrRTU]

Yes and no. If the system is CV, then zone B is your “critical” zone and it will drive your % OA. This is the dilution %. If the system is VAV and has the ability to vary minimum flow to each zone, then total OA could be as your calculation. This may require OA flow measurement, CO2 sensors, re-heat or DDC controls. I suggest you read ASHRAE 62 documents. A properly completed ventilation schedule will reveal the critical space, OA flow and damper min. position. Kruger has a few white papers on this topic and an 62 spreadsheet to run thru the math.

http://www.krueger-hvac.com/lit/whitepaper.asp

Make sure you know which code / standard your are designing to – the numbers vary. Most load programs have the ability to specify which code you are designing to and will develop the ventilation schedule. CO2 reset will help with high % OA.

 

[walkes]

Also, if zone A is a bathroom, you would likely use transfer air to make up the exhaust.

 

[317069]

Thank you for your reply, but I have points regardless to the control system for now.
1- for CV systems,
it is ok that we need 38% OA in order to have 150 cfm of OA in zone B.
I guees this is the ratio of 150/400.
Now, If I went with 0.3 as 300/1000 that mean on unit level I am ok with the code but at zone level I am not.

2- for VAV systems, here is the problem,
- zone B at 400 cfm, and zone A at 600 cfm, the OA %is 0.38%.
- what if the zone B box modulate to 250 cfm while zone A box still at 600 cfm?.
- Does that mean the zone B box has to has a fixed position at 400 cfm alwayes and zone A minimum cfm is 400 cfm or more in order to keep 0.38% of OA.?

 

[Leftyr]

I guess you are having a problem because you will be using an AHU system. Which force you to mix your fresh air and return air in the unit before supplying to your two zones. I suggest you use FCU system were you have to duct your fresh air to your different zone in a required proportioin. That is my two cent worth opinion.

 

[walkes]

Zone B may then be your critical zone that will determine the percentage of OA required for the unit. 60% assuming that the space is still occupied at that point. If the box is throttling back because there is no load then there may not be anyone in the space.

 

[tys90]

DrRTU hit on some of the points if you are under ASHRAE 62.1 for your ventilation code. It's a little more complicated than you are describing. You need to follow the method outlined to calculate the system outdoor air flow for multizone systems. It will actually end up somewhere between 30% and 38% for the constant volume system. In the variable volume case you have to account for the minimum airflow settings. It will increase your system outdoor air the lower your minimums are.

 

[DrRTU]

Run thru the numbers. First we need to assume you have run the loads correctly for the systems being applied and have correctly applied the required ventilation code. Loads will change based upon system selection.

For a CV design – Zone B is the critical space. This zone will determine the outside air minimum damper position. 150/400 = 37.5%. This will provide 150 cfm of OA.
Zone A = 600 cfm at 37.5 % = 225 cfm of OA. You will over ventilate this zone but since you have a CV system you cannot easily alter OA flow unless you apply additional controls.
Total system = 1000 cfm at 37.5% OA.

For a VAV design – Zone B will still be the critical space. Zone B maximum terminal flow = 400 cfm and minimum flow of 150 cfm (37.5% of max) from the central AHU. Zone A = maximum terminal flow of 600 cfm and a minimum flow of 150 cfm (25% of max) from central AHU. Total system = 1000 cfm at 25% OA. Next you need to make sure the VAV system will provide correct SA dilution over the AHU flow range. Controls can be applied such as DCV (CO2 sensors) or flow sensors (Trane Traq dampers) to ensure proper OA to each zone at all times.

 

[317069]

- you said "Loads will change based upon system selection." could you please explain this point alittle more.
- how can be sure that my load calculation is correct if I used the code value (300 cfm in our example) and then interduced 25% more than that(380/300),what about unit capacity in this case?
- in VAV system you said"Zone B maximum terminal flow = 400 cfm and minimum flow of 150 cfm (37.5% of max)"
if the zone B minimum is 150cfm how can be sure this 150 cfm has 150 cfm of OA,?
- if we use the same CV equation then the OA % would be 100%, and this is right if all zones are in minimum postion at the same time then the unit will provide 100% OA. but what if zone B is in minimum and zone A is not?
- also if zone A minimum is 150 cfm same question above?
- when one or both zones are in mimimum the total will be less than 1000cfm.
- when load droped down that does not mean for sure some people left, it could be north zone and west zone.

 

[tys90]

Okay, there still seems to be some confusion. I used the calculator on the Krueger website to simulate your problem. I assumed cooling mode only (Ez=1.0). Here are the results:

Constant Volume System: 325 CFM of OA required
Variable Volume System: 399 CFM of OA required

Like I said, it's not as simple as just summing your OA airflows if you are required to use ASHRAE 62.1. The ventilation effectiveness in each scenario is different because you have to use your minimum flow for Zone B.