Dear eQuest users,
I am trying to model a lab building in compliance with ASHRAE 90.1 to
compare it with an as built model that represents the real case as
described in Appendix G. The actual building (the as built model) has a few
zones which have exhaust air flow rate higher than supply air flow rate as
per the building schedules. I unfortunately do not have any information
regarding the operation schedule of the exhaust fans. When I model the
exhaust air flow rates of these zones to be always lower than the zone
supply air flow rate, then the actual building is not represented closely
enough. When I increase the zone supply air flow rate such that it is
always equal to the actual exhaust air flow rate, then the zone air flow
rate is assumed to be higher than it actually is increasing the overall
energy consumption of the building. eQuest does not allow for the exhaust
air flow rate of a zone to be higher than the supply air flow rate which
would result in high unmet hours in the zone. In order to make a reasonable
comparison between the baseline and the as built models as per Appendix G,
how do you think I should represent the exhaust fans in these two models?
and why? Would you also explain the settings I should use in eQuest to
model the case you are suggesting? I would appreciate it very much.
Thank you very much,
Best Regards,
S.Andolsun, PhD
Another option can be balancing the high amount of air extracted from the
zone with infiltration instead of from the air handler, but I believe this
would increase the number of unmet hours in the system. As per Appendix G,
the total unmet hours of the baseline and as-built models need to be 300 or
less. Thus, this option did not seem like a good approach to me as well.
Thank you very much,
Best Regards,
S.Andolsun, PhD
---------- Forwarded message ----------
Hi Simge,
You have two options where the exhaust air flow rate is higher than supply
air flow rate:
1. if the exhaust fan schedule is equal to or longer than the supply fan
schedule, then re-balance the supply air in your model to account for
transfer air from other zones, or
2. if the exhaust fan schedule is shorter than the supply fan schedule, you
can de-rate the exhaust fan flow and fan power (eg 1000 cfm for 1 hour = 250
cfm for 4 hours).
Shaun
Shaun Martin LEED AP
Simge,
One idea is to take the exhaust air flow from two separate spaces-take as much as you can out of the space that actually has the exhaust, and take the rest of the exhaust air from a space that the air is probably being transferred from. Enter the fan energy for each exhaust fan so that the total matches what you need. Splitting (or lumping) equipment like this works fine as long as the part-load performance is proportional. If the part-load performance curve is non-linear, then there is some error from splitting/lumping.
eQUEST does not handle air transfers from one room to another as well as some other programs. If the transfer air is at room temperature, then you might not need to model transferring the air. For example, in motels air is often supplied to the corridors and is transferred under the doors at room temperature and exhausted out of the restrooms. You could simplify the model by taking the restroom exhaust from the corridors instead of the restrooms. This approximation breaks down if the transfer air is not at room temperature and the temperature difference is needed to handle a load in the room. Keep in mind that we are trying to model the energy use of the building, not model every detail of the building.
Sincerely,
Keith Swartz, PE, BEMP, LEED AP