Hi
The maximum allowed fan power per AHU in the Baseline Building is defined with Table 6.5.3.1.1A, using the Design Flow Rates and the pressure drop adjustments. The design flow rate is sized based on a 20 deg. F supply-air-to-room-air temperature difference for heating and cooling.
The actual maximal flow rates, delivered by the fans, after running the simulation, are usually much lower than the summed up design flow rates of each zone (VAV, different heating/cooling loads). It could easily be only 20-40 % of the Design flow rate. Therefore, the actual fan power, is much lower than the allowed fan power.
Question: Which fan power (suply/return/exhaust) should I enter in the MEPC? I know it says "These values are calculated based on system type, any pressure adjustments listed below, the total supply volume, and the ASHRAE 90.1 fan motor efficiency associated with the fan motor power.". What is meant by the "total supply volume"? Is it correct, that my fan powers here are much lower than the allowed fan power?
Thanks for a quick confirmation, am always somewhat unsure here.
Sarah
Tyler Thumma
7GroupLEEDuser Expert
65 thumbs up
December 16, 2022 - 11:30 am
The supply volume referenced (and reported in the MEPC Air-Side HVAC tab) should be the auto-sized supply airflow according to G3.1.2.9 (20 deg. F supply-air-to-room-air temperature difference), and the fan power is the maximum fan power modeled for each fan. The sum of the maximum fan power for all supply/return/exhaust fans should equal the allowance from G3.1.2.10 (which is automatically calculated for reference by the MEPC). However, it is unusual for the maximum operational flow rates to only be 20-40% of the auto-sized supply airflow. There would be expected to be periods during the warmest months when most or all of the zones served by an AHU would be in cooling mode and therefore the supply airflow would be expected to be near 100%.