ASHRAE 90.1 Appendix G - System Fan Power Distribution

asked 2023-09-16 04:49:36 -0500

Keigo's avatar

updated 2023-09-16 10:06:52 -0500

Basic questions about how to interpret ASHRAE 90.1 Appendix G.

For the Baseline System Fan Power, the following requirement was first added to the 2010 edition of ASHRAE 90.1 and it remains in the latest 2022 edition. image description

No supplementary explanation is found in Standard 90.1 User's Manual.

Let's say, there is an office building and the air-side schematic of the Proposed case is as follows. image description

The Baseline system is System 7—VAV with reheat. If I follow the requirement above, I need to model an exhaust fan in the Baseline case. image description

My first question: In my understanding, the exaust fan is constant-volume because the exhaust air flow rate is balanced to the outdoor air flow rate which is constant-volume. Is my understanding correct?

My second question: If my first question is correct and the exhaust fan is constant-volume, the Baseline case with the supply and exhaust fans would consume more energy than the Baseline case with the supply fan only. Modelling the Baseline case with the supply fan only is supposed to be the worst-case scenario (the most conservative approach) in terms of energy saving. Then, can we ignore the requirement and model the supply fan only in the Baseline case for complicated projects? In the actual buildings, the supply, return, exhaust and relief fans are often not one-to-one correnpondence, and it would take so much time to identifiy the proportion of these fans.

As a supplement, a very rough estimate that the Baseline case with Supply fan only consumes more energy than the Baseline case with Supply fan and Exhaust fan is illustrated below. image description

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