Economizer and ASHRAE 90.1

[317069]

A commercial space (small store for example) with 8 tones cooling load, if we serve it with 8 tone rooftop unit then we need an economizer as per ASHRAE 90.1,
do we still need economizer if we serve it with two 4 tones rooftop units, each one has its own discharge duct system?
the question is about economizer itself, not the ventilation, if we don't need economizer, a 25% or 50% fresh air inlet will be used

 

[urgross]

You'd have to mention which weather zone you are in and AHU efficiency for avoidance of economizer.

 

[317069]

I already mentioned that with 8 tones unit the economizer is required by ASHRAE 90.1

 

[cdxx139]

I interpret 90 as an energy efficiency per piece of equipment. If a 4 ton unit does not require economizer, then you dont need to provide it by code. Its by equipment, not by building.

knowledge is power

 

[urgross]

It's by climate zone, capacity, occupancy, and efficiency. By 6.5.1, no economizer is needed for zones 1a or 1b. For other zones, limit is at 54,000 Btu/hr. By exception given at 6.5.1(I), if efficiency meets or exceeds that given by Table 6.3.2 for the appropriate zone, no economizer is needed. By 6.5.1, exceptions are also noted for occupancy and expected hours of operation.

So what climate zone is it for, what is equipment efficiency, what is occupancy, and is operation expected to exceed 20 hr/wk?