joesinger12
commented
1 year ago I have done a few tests to see the impact of the using OccDens based on the selected VentSpcFunc. Instead of basing OccDens on Appendix 5.4A SpcFuncs (For VentSpcFuncs that are 0.15 cfm/ft2, or exhaust, use the SpcFunc OccDens.) Overall:
- There are noticeable changes in heating/cooling/fan/DHW energy due to the increase or decrease of # of occupants.
- Energy increases as # of occupants increase - cooling, fan, and DHW energy.
- Energy decreases as # of occupants decrease – heating.
- And vice versa.
- Can have positive or negative compliance margin impact.
- There is an even bigger impact from changing in DCV requirement due to changing occupancy density.
- In many cases the updated occupancy density results in DCV to be required where it was not required, see attached spreadsheet.
- It significantly reduces Standard Design heating energy and make it more stringent.
We believe that it is more fitting to use ventilation function to define occupancy density, but in some cases we still want to use the original occupancy and not changing the DCV requirement.
The solution we propose is:
- Define OccDens in VentilationSpaceFunctionData Table (App5.4C)
- For VentSpcFuncs that are 0.15 cfm/ft2, or exhaust = 0
- In most other cases = 2 x (Total Outdoor Airflow Rate cfm/ft2 / 15 x 1000.
- In a few cases where we want to keep the DCV requirement unchanged we manually input values.
- For simulation, use OccDens from VentilationSpaceFunctionData Table except
- Space uses VentSpcFuncs that are 0.15 cfm/ft2, or exhaust. Then use OccDens from SpaceFunctionData Table.
- Occupancy specified by users. Then use user specified value.
We've listed the cases where we recommended to manually input the OccDens In the attached spreadsheet. As reference, we also list 3 different occupancy assumptions in the spreadsheet:
- Based on Space Type – The default occupant density listed in Appendix 5.4A which is used in the current CBECC.
- Based on Ventilation Space Type – Is calculated as 2 x (Total Outdoor Airflow Rate cfm/ft2 per Table 120.1-A) / 15 x 1000
- Based on Egress Space Type – Is calculated as 1 / (Maximum Floor Area Allowances per Occupant per Table 1004.5 in CBC 2018)
- Notice that this is not the maximum but the code minimum egress occupant load factor.
- This is not the absolute minimum because lower number can be used if is approved by the building official (per exception to CBC section 1004.5)
- There can be area with fixed seating and area without fixed seating in a single space. E.g., courtroom.
Original comment by: joesinger12
As part of work on Ticket 3490, the team revisited the question of what occupant density should be used for calculating minimum ventilation and in simulating # of people in the space when design occupancy/fixed seating is not used.
For background, each SpcFunc has a default and specified list of applicable VentSpcFuncs that can be selected. The list of all VentSpcFuncs is based on Table 120.1-A of the standard (incorporated into the ruleset as App5-4C_Ventilation -T24N_2022.csv). The conditional list of available VentSpcFunc based on SpcFunc is generated in the Appendix 5.4A spreadsheet, which is then pasted into the BEMEnums.txt file for the rulesset.
Currently, occupancy density is based on the Spc:SpcFunc values in Appendix 5.4A, col 'OccDens', which is multiplied by 0.5 to arrive at the simulated occupancy and occupancy used for minimum ventilation calculations. In otherwords, the OccDens is aligned most closely with the expected CBC egress occupancy density. The occupancy density used for simulation/ventilation calculations does not change depending on the Spc:VentSpcFunc selected. This can lead to some large discrepencies between the occupancy used in simulation/ventilation calcs, and the occupancy assumed in determining the cfm/ft2 (Rt) value in Table 120.1-A.
The purpose of this ticket is to evaluate the impact of calculating occupancy used for simulation and vent calculations to be based on the value associated with the VentSpcFunc, not the value currently defined for the SpcFunc.
Reported by: joesinger12
Original Ticket: cbecc-com/tickets/3522