Hi,
I am conducting a ventilation calculation for the an office building that consists of open office space, conference rooms, corridors, etc. The office space is served by 4-pipe fan coil (terminal) units. 100% outdoor air provided from a central outdoor air unit is ducted to the return duct of each fan coil unit.
One of my zones (i.e. group of spaces serve by a since fan coil unit) consists of a 700 SF conference room and an adjacent 100 SF corridor.
The conference room has 10 chairs, so at 0.06 CFM/SF and 5 CFM/person, we get about 92 CFM OA. The air distribution effectiveness is 0.8, so the OA requirement is 115 OA CFM.
The 100 SF corridor in this zone has very little internal load - it is an internal space and only a lighting load. The 0.06 CFM/SF required by code, along with its distribution effectiveness of 0.8, requires 7.5 CFM of OA be delivered to this space.
However, since the 7.5 CFM of OA is nearly half of the primary airflow required to cool the space (as calculated by Carrier Hourly Analysis), we're getting a System Ventilation Efficiency as low as 0.20, which is driving up the OA required to the fan coil unit to a point that it is greater than the supply air requirement to cool the zone. For a zone that requires only 400 CFM of 55F supply air for cooling, the OA calculation is requiring (115 CFM + 7.5 CFM)/0.20 = 612.5 CFM of OA. The ASHRAE 62.1 Calc is increasing the supply air to this zone such that it is 100% OA and it is overcooling the space, which requires reheat. This is detrimental to the buildings energy performance because the increase OA conditioning load and zone reheat are unnecessary.
I understand that I can manually change airflow in HAP and alter the model in other ways to get a more logical answer, but I am really looking for a LEED interpretation towards whether or not this small corridor (an other small spaces that require OA by ASHRAE 62.1) really need to be included in the OA calculation in such a situation. My firm would like to use our energy modeling software to calculate loads and OA requirements for buildings, but this specific instance is preventing us from doing so.
Please let me know if you have any knowledge as to whether these small spaces requiring OA can be excluded/altered from the IEQp1 Calculation in any way. Any help in a resolution is much appreciated. Thanks!
Ilona Johnson, PE, CEM, LEED AP
AssociateLilker EMO Energy Solutions
8 thumbs up
July 27, 2015 - 9:16 am
Todd,
LEED reviewers will definitely expect you to ventilate the corridor properly. Some options for you include:
- increase the supply air to the corridor
-if the corridor is in the building interior and does not need heat, then you could design it without reheat. This would allow you to use an air distribution effectiveness of 1.0. That should help a little bit.
-you could install a transfer fan from another room to the corridor. Just note that your calculation would have to be based on "unused outdoor air".
I hope that helps!
ilona