Calculated CFM higher than stock equipment

[CodyK]

If the calculated CFMs of a load calculation don't match stock equipment, is it best to match the required tonnage and let the cfm's be lower? Is it justifiable to purchase a larger fan?

I recently did a manual J calculation using CoolCalc and the cooling BTUH = 35,000 but the room by room cfm add up to 1900. My guess is just to get a 3 ton unit and use the nominal 1200 cfm that come with most equipment. I mostly do commercial equipment replacement calculations and HAP usually results in cfm/ton well above stock equipment. Typically we just size the equipment based on the tonnage in the load calcs and the cfm per room is lower than calculated. Is this the correct approach?

 

[willard3]

The CFM per ton is quite variable depending upon the occupancy of the space it serves. Dry bulb and wet bulb variations will influence the CFM per ton.

 

[zar45]

Yes, it is best to match the required tonnage, especially the sensible cooling btuh.

 

[nuuvox000]

I'm not quite sure why but the CFM always comes out higher than the equipment manufacturers show with the same tonnage. We've always matched tonnages and I haven't heard of any issues using a slightly lower CFM in the few years I've been doing this.

 

[IRstuff]

Assuming you meet the ACH requirements, if any

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[CodyK]

Thanks everyone. nuuvox000 that makes me feel better; I've only been doing this for a year too so I'm trying to build confidence in my calculation and selection procedures. IRstuff, yes thanks for the reminder. My first job was in stress analysis (FEA and pressure vessel code calculations) where you do calculations and you build the product based on those exact calculations. It seems to me (in my limited experience) that HVAC is a bit more of an art than an exact science (yes you do the load calculations and select stock equipment that is close, then you balance and adjust in the field to make it work the way you want it to).