ASHRAE/IECC Baseline VAV Fan Curves and Min Flow Fraction

asked 2019-07-18 17:58:04 -0500

updated 2019-07-19 10:29:08 -0500

What are the appropriate fan curve coefficients and Fan Power Minimum Flow Fraction for the Fan:VariableVolume objects in an ASHRAE 90.1 PRM baseline model (System 5)?

I ask because there are some discrepancies between the ASHRAE/IECC code language and the Commercial Prototype Building Models. As noted in a previous Unmet Hours post, the prototype models use coefficients representing "Good SP Reset VSD Fan", which differ from the coefficients from 90.1 App G Method 2. Is this because the prototypes assume zone-level DDC where a SP reset would be required?

Regarding the "Fan Power Minimum Flow Fraction" field, both ASHRAE 90.1-2013 ( and IECC 2015 (C403.4.1.1) state: "Low or minimum speed shall not exceed 66% of full speed. At low or minimum speed, the fan system shall draw no more than 40% of the fan power at full fan speed." I interpret this as meaning the baseline "Fan Power Minimum Flow Fraction" should be 0.66. However, the prototype models use a fraction of 0.25. What is the appropriate Fan Power Minimum Flow Fraction for a PRM baseline?

Another unrelated discrepancy I found in the prototype models for System 5 (used in medium office). ASHRAE (G3.1.1-4 Baseline System Descriptions) lists hot water boiler heating, whereas the prototypes use gas furnace heating. Are the ASHRAE prototype models not intended to represent ASHRAE PRM baseline models?

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