Question
Question
(OP)
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?
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?





RE: Question
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
RE: Question
If Zone A is a bathroom, over-ventilation is a good thing.
Mike McCann
MMC Engineering
RE: Question
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.
RE: Question
RE: Question
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.?
RE: Question
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RE: Question
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.
RE: Question
- 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.
RE: Question
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.