What are the appropriate fan curve coefficients and Fan Power Minimum Flow Fraction for the Fan:VariableVolume objects in an ASHRAE 90.1-2013 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 (6.5.3.2.1) 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. Related to this is the VAV terminal minimum flow setpoint, which (according to G3.1.3.13) shall be 30% of zone peak airflow. Is the correct interpretation of these two rules that the Fan Power Minimum Flow Fraction should be 0.66, and the AirTerminal:SingleDuct:VAV:Reheat Constant Minimum Air Flow Fraction should be 0.3? Wouldn't the Fan Power Minimum Flow Fraction of 0.66 limit the turndown of the terminal units to 0.66 as well (assuming all zones and systems have same operating schedules)?
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?
