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View Full Version : Rule of thumb for CFM?

sonc
08-15-2004, 05:32 PM
1.1 cfm per sq/ft..Am I far off for a typical office space?

NormChris
08-15-2004, 07:07 PM
Is that how you design duct systems? By the CFM/Sq foot method?

Would it matter to you if the office space had no windows, single pane windows, double pane windows, blinds, tinted solar film or the windows faced south or perhaps north?

That is just talking windows. There are many other factors that affect the heat load. I could post a rather long list here.

What does the heat load have to do with the CFM? EVERYTHING! The load calculation for the Block Load determines the equipment size and total CFM. Then the load for each room determines how many CFM each room needs to handle that rooms load.

If you perform the load calculation for each room and break it down to CFM/Sq foot it will vary all over the place for each room.

Norm

sonc
08-16-2004, 01:30 AM
Would someone, other than Norm, tell me if 1.1 cfm/sq' is an acceptable RULE OF THUMB BALLPARK FIGURE to begin adjusting airflow to one freakin 10'x10' office with one 40 year old pencil-pusher with B/O and a drinking problem and no windows....thanks in advance

Just kiddin, Normie ;)

hvac3901
08-16-2004, 03:14 PM
using rule of thumb is based upon your experience and judgement. 1cfm is typical for most aplications on interior zones but is the gentleman overweight? is there a high density of computers? etc. etc. i would say 1cfm per sq. ft. is typical of most interior zones but not all.

for all you guys that bash the rules of thumb, of all the load calcs you've done computers rooms exempt. how many of them deviated drastically from the thumb rules? no one uses rules of thumb for system sizing, if they are a reasonable and prudent person. however the rules of thumb do work in many cases for small balancing and system checks, imo. if the rules of thumb were so far off base then there would be no rule of thumb. it would be nothing more than a wild a** guess.

but back to the if you have to ask....then you probably should'nt be using the rule of thumb.

Carnak
08-16-2004, 03:34 PM
Norm's right Sonc.

The starting point is the CFM you get from the load calc. It is based on the sensible heat gain of the zone, and how cold the air out of the diffusers are.

1.1 CFM per square foot IMHO would be a lot of air for an interior office space.

plain spoken
08-16-2004, 04:20 PM
Never mind.

[Edited by plain spoken on 08-16-2004 at 04:25 PM]

wolfdog
08-16-2004, 04:58 PM
It depends on a lot of variables. One is climate.

1 cfm/ft² is pretty close for commercial office space in my area. It would probably not be sufficient for a church or restaurant.

sonc
08-16-2004, 08:05 PM
"but is the gentleman overweight?"

Looks like a panda bear without the fur.

Guys listen..I don't balance every building I get a service call in, and if I did I wouldn't be in service for very long, and as far as heat load, ya, I did that in trade school. The comp i work for has a division that takes care of that.. I'm just runnin' and gunnin'.. I didn't want to get into a "longest service wrench" contest (but I would win.. oh ya baby)

[Edited by sonc on 08-16-2004 at 08:11 PM]

NormChris
08-16-2004, 08:23 PM
Originally posted by sonc

"but is the gentleman overweight?"

Looks like a panda bear without the fur.

Guys listen..I don't balance every building I get a service call in, and if I did I wouldn't be in service for very long, and as far as heat load, ya, I did that in trade school. The comp i work for has a division that takes care of that.. I'm just runnin' and gunnin'.. I didn't want to get into a "longest service wrench" contest (but I would win.. oh ya baby)

[Edited by sonc on 08-16-2004 at 08:11 PM]

Actually Sonic, I have wrenches of various lengths so I have the right size for the job. In this case the correct wrench is a calculator.

It is always best to use the right tool for the job.

Norm

hvac3901
08-16-2004, 09:58 PM
see there you go Sonc..
the gentleman is overweight, and has a drinking problem as you stated. i don't know about you guys but i get pretty warm when i've had a few. you can throw the rules of thumb out here. this guy sounds like his body could transfer much more btu's than the rest of us and still fell a bit on the warm side.

anybody know where the ashrae tables are for the overweight folks at sedentary office work? is'nt it 250 sensible and 250 latent for normal folks. what about table for overweight office drinkers? even a calculation wouyld'nt work here, thats my take anyway. cause even the ashrae tables are based on those cursed typicals (rules of thumb). just giving you guys a ribbing here, not trying to start a mess.

[Edited by hvac3901 on 08-16-2004 at 10:04 PM]

tab-tech
08-17-2004, 06:48 AM
.............. I don't use my thumb for a rule.
For you guys that do "comfort balancing" as part of a service call or a design build concept where an independant balancer is not required, the rule of thumb doesn't matter.. (who holds you accountable?)
As for me and my crew........ we'll get a P.E. to nail down the air changes per hour required. That's his job and if his pencil is dull he'll have to answer to it.