eQuesters
I'm having some difficulty with properly modeling exhaust fans in for LEED and I'm hoping someone can provide me with some guidance.
Here's my problem: I can enter the exhaust volumes and associated KW/cfm for each zone where necessary, but some of the spaces have design exhaust volumes greater than design supply air volumes (negative pressure rooms). If I select "infiltration" or "balanced infiltration" as the source of exhaust makeup for these zones, then the unmet hours for that zone become very high. If I select "air handler" as the source of makeup air, then eQuest increases the design supply air volume to match the exhaust air volume for that zone. This is problematic because the SV-A reports now have values that do not match my mechanical schedules. Additionally, the energy consumption associated with those systems increases as a result of the increased supply air volume. All in all, it's just not an accurate representation of the design. Most of these zones are expected to make up the air through zonal air transfer (something I realize eQuest is unable to do).
At this point, I'm considering adding the exhaust energy as a direct load to my meter along with appropriate usage schedules, and associating it to "ventilation fans". I believe the net effect on energy consumption and peak demand should be the same as if the exhaust fans were explicitly modeled. However, I'm curious as to any dynamic system or thermal effects that I may be circumventing by not explicitly modeling the exhaust fans for each zone. My gut feeling is that there shouldn't be much of an effect, but I'm just not sure. I'm also concerned that by modeling exhaust energy as a direct load, rather than explicitly, the SV-A reports won't show any exhaust volumes and thereby call attention as to the validity of the model.
At this point, I would like the guidance of some of the members on this forum that have more experience with these issues. Is this even an acceptable way to account for exhaust fan energy for LEED? I'm open to any suggestions from anyone who has run into these issues and/or may have experience in how to deal with it.
Thanks in advance
Seun Odukomaiya, EIT, LEED AP
I've run into the same problem too. One thing I've tried is to decrease the
zone level EA cfm until the unmet hours are reasonable, and then increase
the kw/cfm the commensurate amount. That way, at least there's some zone
level exhaust airflow and the SV-A report shows EA fan power equal to what
is on the exhaust fan schedule. A narrative to the GBCI reviewer explaining
the simulation limitations would hopefully suffice.
Anthony Hardman, PE
LEED AP BD&C
Seun,
I've used a few different approaches to address the situation of a zonal exhaust fan that has a flow rate close to, or larger than, the supply air flow to that zone:
- Model the zone as being part of the zone that it draws transfer air from. (An example would be a single zone representing a corridor and a janitor closet.)
- Increase the supply air flow to the zone with the exhaust fan, and decrease it by the same amount in the zone that it draws transfer air from.
- Create a different thermostat schedule(s) to allow the temperature to float more, and use the same schedule in both Baseline and Proposed. (If appropriate, for example, use a 60?F heating setpoint.)
- Make the zone with exhaust fan an unconditioned zone. This might be appropriate for a trash/recycling or soiled linen room where you are not trying to condition it within a certain temperature range.
Regards,
Bill
I've run into this at least once, and an approach that has worked for me is a riff on Bill's first suggestion. Start with: where is the air coming from?
If the negative space is a bathroom that in reality is drawing air from an adjacent open office space under the same system, why not specify the exhaust (wholly or in part) in the open office space? Perhaps another large minimum-airflow space under the same system would work as well? Airflows and cooling requirements should balance out at the system level provided the exhaust does not exceed the local zone airflow rate.
The balance of airflows may not work for every case, but this can potentially be the simplest approach. Consider carefully whether the quantity and scheduling of the exhaust will cause it to exceed the hourly supply air rate for the zone(s) you're considering.
`Nick
NICK CATON, P.E.
I second Nick's approach: put the exhaust flows where you think the air will come from.
In my experience, that is the best way to handle air transfer through exhaust in eQuest.
And I believe that the wizard is also doing it for you if you specify zonal exhaust before leaving it ...
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Demba NDIAYE
What about if the exhaust fans use 1 hp or less? Does everyone follow the
directions in 6.5.3.1.1?
Beelzebub
Oopsie! That will teach me to read my page numbers more carefully. Bill has
kindly informed me I slipped into the prescriptive section. I say darn it!
Carol