Mech Code Occupancy Vs. Building Code Occupancy

[msjenkin]

I am working on mechanical design for a salon that is 1955 net sf. 2103 gross sf.
Main Salon area 1843 SF
Restroom 56 SF
Restroom 56 SF

According to NC Building Code occupant load (Business= 100gross), occupant load is 2103/100= 22 occupants.

According to NC Mechanical Code 403.3.1.1, Occupant density for salon is 25/1000sf. 25/1000= .025*1843= 46 occupants.

Does anyone know why the Mech code boosts the occupant load?

For life safety/ actual max occupancy one would go with the Building Code way to calculate occupancy.
Why does Mech code calculation result in more than double occupancy? If you use Mech Code way to calculate occupancy for HVAC loads, then that greatly increases your O.A. requirements, which in turn increases your overall HVAC requirements.

Seems to me it makes more sense to calculate the occupancy based on the ACTUAL max occupancy as given by the Building code.

Anyone else run into this issue?

Thanks
Michael

 

[Sifu]

A salon has a lot of contaminates that require additional ventilation and outdoor air so they account for it by upping the density and the people and room rates.

 

[cda]

A salon has a lot of contaminates that require additional ventilation and outdoor air so they account for it by upping the density and the people and room rates.

Interesting. Wonder why not up the cfm instead of confusing the occupant load

 

[msjenkin]

Thanks for the replies.
I understand that salons use harsh chemicals and understand the need for greater ventilation.
The mech code already boosts the O.A. per occupant (20 cfm/person for Salon VS. 5 cfm/person for office).
Increasing the O.A. and the occupant density just seems like double dipping to me. But I guess there is a method to the madness.

I browsed the Mech Code commentary but could not find an explanation.

 

[Sifu]

Not sure they could just use density as the method to increase the ventilation since the code allows that density to be altered. There have been lots of studies on salons and air quality, and it may be that a minimum amount of ventilation was arrived at and then they worked backwards to determine how to best achieve it. Did they anticipate BO's allowing a reduced occupant calculation and decide to overkill the room and people rates to account for that possibility? There are some really smart mechanical types on this forum, so stay tuned, they probably have more definitive info.

 

[klarenbeek]

My understanding is that in the IBC the occupant load can change based on exiting/reconfiguration of the space, so a small remodel can change the occupant load and the HVAC system would not even be considered in the remodel. this can be a big issue in bars/ entertainment venues. The tables in the IMC are based on ASHRAE 62.1 and provide a uniform standard for all spaces regardless of configuration.

As far as salons, don't miss the fact that there is required exhaust based strictly on square footage, not occupant load. The fresh air in ventilation basically amounts to the makeup air for the required exhaust, so if you lower the fresh air, you will be shorting makeup air for the exhaust, bringing the space into a negative.