Large Single Story Building HVAC Systems

We have a 450,000 ft2 single story mental health hospital that we are energy modeling. The building size and climate zone means that we should use System 7. Our confusion is how many HVAC systems we should use with our Baseline.

From ASHRAE 90.1 G.3.1.1

For systems 5, 6, 7, and 8 each floor shall be modeled with a separate HVAC system.

In our case we only have one floor though and one HVAC serving 450,000 ft2 doesn't seem realistic either.

Exception 2 says to use System 3 or 4 when there are sufficiently different occupancy, process, or schedules. In our model though that would create a lot of System 3.

Our gut tells us we should be conservatively match the HVAC system groups in the Proposed to the Baseline. We would like to find language in ASHRAE 90.1 or LEED that some direction though.

You should follow ASHRAE 90.1 G.3.1.1, but in my understanding, this is a minimum requirement (i.e. VAV systems on two or more floors should not be combined), and the VAV system on one floor should be further separated according to other requirements.

I separate HVAC systems for zones with DCV and for zones without DCV on the same floor. DCV will not work properly if these zones are served by single HVAC system.

If some zones in the Proposed case include humidification and other zones does not include it, the VAV systems in the Baseline case should be separated into those with humidification and those without humidification as per Table G3.1 10.HVAC Systems.

Please also refer to other requirements in Appendix G.

By the way, I think you don't need to worry about whether the Baseline case is realistic or not. There are many requirements in the Baseline case that are not realistic, such as fixed pump head, no matter how large the building is.

Addendum

According to the 90.1 User's Manual, if a floor has zones with various cooling thermostat setpoints, they do not need to be separated. One system should serve them. The supply air temperature should be 11C below the lowest cooling thermostat setpoint.

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