Hi,
We are doing the energy model for a building using the default occupancy for C&S projects (as per client approach), however the systems have been designed and sized for a much higher occupancy as worst case scenario as per project brief. We got the comment from LEED reviewer pointing out a discrepancy in the equipment capacities in the model vs the designed capacities (i.e. outdoor air volume, fan volume, etc.). The reason for this discrepancy is the difference in the number of people in the building (modelled vs design documents).
ASHRAE 90.1 section G3.1.10 a. states: "Where a complete HVAC system exists, the model shall reflect the actual system type using actual component capacities and efficiencies".
Does this mean that if we are doing the proposed model with a much lower occupancy (compared to design) we still need to size the AHUs to have a capacity matching the design documents?. We have a DOAS with CAV system, so is it possible to set the maximum capacity of the AHU in the model as per design documents and keep it running at lower capacity even though in reality will not happen due to CAV? Or should we just supply the same amount of air in the model as per design documents even though this will mean a very high fresh air rate per person in the proposed model and therefore a much higher air volume against the baseline (air rate set as per standard). Hence a worse energy results.
Would the same approach apply for the district heating exchanger and chillers?
In general, if we have an HVAC system designed for a given occupancy factor, should we be using the default occupancy factor in the energy model or is this someting we should not be doing?
Thank you in advance
Manuel
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5907 thumbs up
April 15, 2021 - 5:07 pm
You should model the HVAC systems as designed in the proposed. You should model the occupancy based on what is expected and only use the defaults if you do not know what to expect. Given that you designed the systems to accomodate a higher occupancy than the default I would think you should be using that higher occupancy rate. The outside air volume must be modeled identially in the baseline as the proposed.
Manuel Martin Delgado
Buro Happold Polska Sp. z o.o5 thumbs up
April 16, 2021 - 3:32 am
Thank you Marcus for your reply.
I only wonder how the occupancy should match across the different credits. For example, architects are using default occupancy for calculating the FTE, number of bike racks, water consumption, etc. should we use a different occupancy only for the energy model?
Thank you.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5907 thumbs up
April 16, 2021 - 2:47 pm
There should be consistency throughout the submission. IMO you should only use defaults in the absence of knowing.
Manuel Martin Delgado
Buro Happold Polska Sp. z o.o5 thumbs up
April 21, 2021 - 9:33 am
The occupancy factor was only used for sizing the systems. However this factor includes a very worst case scenario when full time users and visitors occupy the building at the same time (lets say the systems are very oversized for a non likely scenario). Therefore, client and architects decided to use default FTE for LEED assessment. Technically the occupancy is unkown since no fit-out/space arrangement is being designed.
Since the energy model should use the same FTE as in other LEED credits, I wonder whether the following approach would be acceptable:
1) Hard-size the proposed model systems to match design documentation (chiller, heat source and AHUs)
2) use default occupancy (23m2/p-gross-) and design fresh air supply rate per person
3) run the systems (specially applicable for AHUs) in the proposed case at part load due to lower occupancy and fresh air design rate per person applied
The system is a CAV with variable speed control, so if a lower occupancy is agreed, the AHUs could run at lower capacity (and proably at lower efficiency as well).
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5907 thumbs up
April 21, 2021 - 2:19 pm
This all seems reasonable given your situation assuming the proposed systems adjust outside air to the level of occupancy and it is not fixed.