Typical assumptions for Mechanical Room exhausts

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Dear All,

Specifically for energy models for LEED EAc1, what are typical assumptions
for operation of mechanical room exhaust? The mechanical rooms in a project
I'm dealing with have freeze protection heating only but as per LEED spaces
that are heating or cooling only have to be modeled as both heating and
cooling. For example, one of the mechanical rooms has 8000 CFM of exhaust
and zero Outside Air CFM. How does one model for accurate representation
yet keep the loads from ballooning disproportionately in such a situation?

Thanks,
Ramana Koti.

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I always set up a separate heating and cooling schedule for mechanical
rooms in garages (as an example). Cooling setpoint is 120 degrees for
all hours, heating setpoint is 45 degrees for all hours. Set it up as a
PSZ system, with EIR meeting minimum requirements of 90.1 even though it
will never be in cooling mode.

James Hansen, P.E., LEED AP

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Ramana,

If the heat is simply for freeze protection, the mechanical room might qualify as a "semi-heated space" instead of a "conditioned space." (See the definitions under "space" in 90.1.) Then you would not have to model the cooling system.

On the other hand you might want to keep the cooling system in to enable the air-side economizer. If you want the mechanical room ventilation system to operate based on room temperature, you could model an air system with cooling in eQUEST and enter a high limit for the economizer so that the fan will come on to "economize" when the room temperature exceeds the setpoint. Be sure to lock out the compressor during economizer mode.

________________________________
Keith Swartz, P.E., LEED AP

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The exhaust fans sound like they may be for boiler combustion air? If so,
add the exhaust fan power as an electric EIR to the boiler. Then you don't
have to worry about infiltration loads blowing up your unmet hours.

Anthony Hardman, PE, LEED AP BD&C

Anthony Hardman, PE

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