My building has a fan coil unit to serve four-storey living quarters and the ventilation is met by a DOAS. 90.1 Appendix G calls for the outdoor air to match the proposed case. The part where I am not clear is that, while modeling in Trace700, there is an optional ventilation fan dedicated to handle the ventilation. How do I account for this fan energy in the MEPC and in the Baseline Energy model?
My outdoor air in proposed and baseline case is 18,170 cfm.
Supply air is 40,791 cfm and Return is 46,543 cfm from Trace700 System Checksums.
It does not make sense to ignore this ventilation fan energy as it is a significant portion of energy. The MEPC 'Allowable fan power' is based on the Supply CFM which you enter in the Air-side HVAC. As I understand it, the System Fan Power in the MEPC (Supply Fan Power + Return or relief fan power+Exhaust fan power) should match the Allowable fan power, correct?
It also seems unrealistic that MEPC reports an Allowable fan power of 12.2 kW for my system. What am I missing here?
Tyler Thumma
7GroupLEEDuser Expert
60 thumbs up
November 6, 2020 - 8:57 am
First, keep in mind the exceptions to 90.1-2016 Section G3.1.2.5 (especially exception 3) which often result in differences in outdoor air between the Baseline and Proposed case.
I would not recommend using the Trace optional ventilation fan for your Baseline systems. You are correct that the Baseline fan power allowance (calculated using the Baseline supply airflow only) is the total allowance for all fans in the Baseline case, and the allowance for residential systems 1-2 is 0.3 W/cfm so the MEPC is correct. However, in the Baseline case the outdoor air will be supplied by the PTAC/PTHP supply fan which must therefore operate continuously at a constant 12.2 kW. Since your Proposed design includes ventilation from the DOAS, it is likely that only the DOAS fan needs to operate continously and the fan coil fans can cycle.
Sankar R
1 thumbs up
November 6, 2020 - 10:10 am
Thanks for the information, Tyler. I did not include the optional ventilation fan on my Baseline model. The reason for my question is, the design case for my fan HP for the DOAS is about 37kW. Is this a price for having this system ? This looks disproportinately high and the penalty we're paying is too much.
Tyler Thumma
7GroupLEEDuser Expert
60 thumbs up
November 6, 2020 - 12:09 pm
Yes 37 kW seems like a lot for only 18,170 cfm (2 W/cfm).