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Title 24 Ventilation

Title 24 Ventilation

Title 24 Ventilation

(OP)
Can anyone explain why the 2007 California Energy Code requires a mechanical system "capable" of providing an outdoor air rate no less than the larger of a) that prescribed by table 121-A or b) 15 cfm/person, yet the 2007 California Mechanical Code refers to ASHRAE 62.1?

The way it reads it sounds almost as though the system is required to supply air per ASHRAE 62.1, though it must be "capable" of supplying 15 cfm/person (more OA).  

The size of a system capable of 15 cfm/person on a design day compared to 5 cfm/person + 0.06 cfm/sqft can be significantly different.  In my case I am finding a tonnage around 20-25% more by using 15 cfm/person.  Any comments?  Thanks.          

RE: Title 24 Ventilation

Providing you are not attempting to maximize your liability, errors &  ommission, go with the most strict in the case of a code discrepency. I've never had or heard of a rejected calculation using the larger between table 121 vs. 15cfm/occ method. Perhaps you may find the 2008 nonresidential compliance manual/chapter 4 to be helpful.  And yes, upfront and lifecycle costs of providing/treating ventilation air is a serious consideration, will DCV and it's drawbacks ease your concerns.
Your local building official will have the final say so on the matter, pose your question to him/her and let all of us know the outcome.

RE: Title 24 Ventilation

Make sure you are thorough in your 62.1 calculations.  Many times you end up with higher than 15 CFM/person on the initial calculation, especially with multi-zone units, such as VAV systems.  I find myself adjusting zones in an iterative fashion until I get a decent zone ventilation effectiveness.   

RE: Title 24 Ventilation

What tys90 states is very important.  Many people new to 62.1 calcs, and unfortunately those who had been doing it for some time around here, were not understanding the procedure as a whole.  

In my experience, utilizing 62.1, I think the lowest result I've had on a VAV system is 13.5 cfm/person.  This does not mean that is what I expect your answer to be, but just make sure you understand the procedure.  

The main mistake I've seen is ignoring the minimum zone cfm conditions.  Think about a space when there is no skin load, little or no lighting load, but is fully occupied.  That is the condition that must be analyzed if the critical zone (think meeting room or assembly space when a presentation is being given.)  I believe the 62.1 User Manual gives a good description of this.

RE: Title 24 Ventilation

For California T-24 compares zones.  The zones are compared by Occupancy and by Area.  The Occupancy is easy, number of people * 15 CFM.  Area Calcs are area * X CFM/ft^2 (usually 0.15, but need to look up).  The higher of the Occupancy and Area calcs is the number you use for that zone.  Then all your zones outside air CFM is added up for you total OA.  Some zones may have higher by occupancy (usually auditoriums or lounges) and some by area (usually hallways, storages, offices).  They use both methods because just saying 15 CFM per person would not work for different conditions.  For auditoriums and lounges using T-24, I get high OA CFM, usually 40-50% of supply air which means very large units.  Got to use DCV then.        

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