HVAC: 15cfm/person, OR renewal 12 times per hour? Rod?

After reading about everything there is on this site about HVAC, I guess I'm still a bit dense about some aspects of it, namely the bit about changing the volume of air in a room 12 times per hour. I cannot see anything in the bible (Rod's book) regarding that, the only thing I could see is that he recommends 15cfm per person present in a room, for which the size of the room appears irrelevent. Taking my example: my CR is 6.20m L x 4.20m l x 2.60m H ie. 70m3, or 2470cf. If I apply Rod's recommendation for 5 people I need to move 75cfm. If I apply the 12 times rule I need to move 494cfm. So... what gives?

Multiply 12 times the volume of your room and that should give you the volume per hour

maybe this could help a little? look here greetings, carsten

> Javier: I know this rule, it's in my question :mrgreen: Like I said there's a huge difference between this rule of 12 times renewal/hr and Rod's rec which is 15cfm per person - 419cfm to be precise, ie. 6.6 times more!!!! And as I respect Rod's authority but also the one of people of recommend the 12 times rule I am very confused. > Carsten: I previously missed the thread you linked to so thank you for posting this link here, got one thing to say: wow. Oh, and thank you for this, too. So that's two things. This is incredibly documented but unfortunately, I'm still confused for the reason I stated above...

I thought the same thing when reading Rods book. In my case I used the 12 times rule. My HACV suppliers also recommended that rule

also. the 12 time rule is per hour. nor per minute

Ta Javier - 12 times per minute would create a hurricane inside the studio. Not good. So we still don't know for sure about this one... I'm about to order all that stuff so obviously I'm keen to know! Anyoen would know for sure?

No takers for that one? No one? This is pretty major and I would have thought that it would have triggered replies! One calculation takes into account the number of persons in a room without taking the room size into account, the other calculation does the opposite... My gut instinct tells me that Rod's one (the first one) is correct but my guts also want to be reassured and relaxed so I can put Guinness into them instead of worrying for HVAC... So thank you in advance for my guts!

Every HAVC company here in Chile had told me to use the 12 times rule

6 times is minimum, 12 times is recommended. So you might do with less if there's no other way.

Thanks for your replies guys - so should I take it that it's now a matter of public record that the '15cfm/person' rule is wrong and the volume-dependent '6 to 12 times renewal/hr' rule is right? If yes maybe someone should let Rod know so that he can amend his book for the next edition, and if not maybe Rod will come here and correct us? Or... maybe both apply and I'm just being so dense that I don't see how? Rod or any expert in HVAC please...

the rod's method is kind of strange. I can't imagine for instace a 1,000m2 building with 5 people with just that airflow

I could ask my good friend, who's a HVAC specialist, to write a little ditty about this very subject. There's many questions here on the forum about that specific topic. See if he can shine some light on the matter...

Ro that's a great idea, thanks. Thing is, I get Rod's idea, that depending on the number of people in a room the air that needs to be renewed is not the same. After all if there's no one in a room then the need to renew the air is very minimal, however big is that room. On the other hand in a tiny room if there's a lot of people then the need to renew the air might be much more than 12 times per hour. Which shows that the 12 times/hr rule is flawed or at least not one to blindly rely on - maybe that's why it's 12 times, just in case there are 12 hyperactive people in a tiny room? the Rod's 'relative' rule sounds logical to me, but as it's six times less (in my case) than the 12 times rule I just don't get how it could be so different. So an expert shedding light on this topic would be of great help!

jbassino wrote:

the rod's method is kind of strange. I can't imagine for instace a 1,000m2 building with 5 people with just that airflow

That's a pretty absurd comment....... most rooms have occupancy loads based on X s.f. per person.......... the mechanbical codes in the US set a minimum of 15 cfm per person per hour - this again based on occupancy per s.f. of persons per rooms. The problem with using the "12 times the volume" rule for a room is that you could have 5 rooms - with exactly the same footprint - meaning each room will have exactly the same occupancy load - yet all 5 rooms have different fresh air supply rates... let's make it simple - room is 10 x 10 x 8 - 800 cf of volume or 100 s.f. of floor space - For stages and patforms (most closely resembling a studio environment) the occupancy load is 15 s.f. per person - for this room then account for occupancy of 7 persons max........ by the mechanical code that would require 105 c.f.m for a total of 6300 c.f.h. - or 7.875 complete air exchanges per hour. At 12 times the volume it would require 9,600 c.f.h. OK - keep the same footprint - same number of people - and increase the ceiling height to 10 (i know all about the bad ratios - so don't even bother to mention it - this is just easy math to see the flaws with the volume method) - By the mechanical code - seeing as we did not increase the foot print - we are still 7 people - 15 cfm - 6300 c.f.h.. But the volume method has now increased our fresh air load to 12,000 c.f.h. Do those same 7 people in that room somehow need more fresh air just because the room is taller? I try whenever I can to build rooms with ceiling height running from at least 9' to 17' in height -so an average of 13' Now that same room - with those same 7 people will have 15,600 c.f.h. of fresh air to suck on - but do they really need it? I hope you can see how it just doesn't work (realistically) basing this on room volume. The current mechanical code sets a minimum fresh air at 15 cfm per person (I don't make these numbers up as I go along) if you don't ike that then set up whatever feels good for you on a basis PER PERSON - because that is who you are concerned with breathing - not the room - not the wall coverings - just the people - and then use your confortable number to determine your needs. Warmest regards, Rod

Rod, you're a good guy - thanks so much for taking the time to write this detailed post to clarify. It does make more sense to me to calculate depending on the consumption of air (number of people) than to calculate according to the size of the room, as this later method doesn't take into consideration the primary variable. One thing I don't get is

the mechanical codes in the US set a minimum of 15 cfm per person per hour - this again based on occupancy per s.f. of persons per room

One variable is the number of persons, is the occupancy per s.f. another variable?

TheFrenchVoice wrote:

One thing I don't get is

the mechanical codes in the US set a minimum of 15 cfm per person per hour - this again based on occupancy per s.f. of persons per room

One variable is the number of persons, is the occupancy per s.f. another variable?

FV, In the States the building codes proscribe the (maximum) occupancy loads for pretty much everything. Some examples: Business Use - 100 s.f. gross per person (this is measured for the footprint of the building - to outside face of walls (hence "gross") 101 s.f. = 2 persons Assembly Use, without fixed seating and meeting the following conditions: Concentrated (Chairs not fixed) - 7 s.f. net Standing Space - 5 s.f. net Unconcentrated (chairs and tables) - 15 s.f. net "Net" is measured from inside face of wall to inside face of wall within any given space. These are just a few examples of roughly 36 proscribed occupancy loads covering everything from merchantile buildings to airports terminals. The first use of these loads is to determine required egresss widths as well as the numberand sized of stairways and exits...... another use is to determine the requirement for fresh air within any given space. Although there are quite a few manners of determining actual fresh air supply, i.e. 25% of total air flow required to maintain a space as a minimum, up to 100% fresh air fed to a space when certain outside air conditions are met - this is a condtion of the model energy code - the heretofore mentioned 15 cfm which is taken from the IMC - table 403.3 entitled "Required Outdoor Ventilation Air" - which has a large number of different requirements - for example" In Education - music rooms - estimated persons per 1,000 s.f. = 50 - required Outdoor Ventilation Air = 15 cfm per person I hope this helps. Rod

And help it does Rod, thank you so much. So I'm set for 15cfm/person, I'll round it up to 100cfm total for the control room and to 50cfm for the booth, will be plenty. Thanks again (you didn't manage to get rid of that man behind the curtain yet?)