Dear LEED, We are designing a medical office with VRF system, to fulfill iaq requirements we add independent outdoor air unit comprises of supply and exhaust fans on each zone. We employ whole building simulation by comparing our proposed model with baseline model. According to ASHRAE, the baseline for our new building would be System-8 VAV with PFP Boxes. Should we model the identical outdoor air units in the baseline which is independent from its main HVAC system? Or we are allowed to simply include identical outdoor air requirements inside the main HVAC system-8?
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Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5868 thumbs up
March 5, 2015 - 12:06 pm
The baseline should not include a dedicated outdoor air system. Model the outside air identically.
Ivan Priatman
Senior DesignerPT. Archimetric
8 thumbs up
March 6, 2015 - 2:11 am
Thank you for the quick response. In addition, we also provide several unconditioned rooms equipped only with outdoor air units in our proposed model, no VRF units included in these rooms. If in baseline case, conditioned rooms receive their fresh air from the main HVAC system, then what about these unconditioned rooms which only require fresh air intake? In this case, should we model identical outdoor air units for these rooms aside main VAV system?
Ivan Priatman
Senior DesignerPT. Archimetric
8 thumbs up
March 6, 2015 - 2:11 am
Thank you for the quick response. In addition, we also provide several unconditioned rooms equipped only with outdoor air units in our proposed model, no VRF units included in these rooms. If in baseline case, conditioned rooms receive their fresh air from the main HVAC system, then what about these unconditioned rooms which only require fresh air intake? In this case, should we model identical outdoor air units for these rooms aside main VAV system?
Anil Bhari
Min engineering inc3 thumbs up
March 6, 2015 - 1:33 pm
Do this unconditioned room required ventilation as per ASHRAE or not?
If yes than provide supply air fan for these rooms.
If no than do not provide fan for these rooms.
In both the case you have to simulate these room as unconditioned.
Regards
Anil
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5868 thumbs up
March 6, 2015 - 4:32 pm
Any fans located in unconditioned spaces are modeled identically in both models.
Ivan Priatman
Senior DesignerPT. Archimetric
8 thumbs up
March 24, 2015 - 11:21 pm
Thank you for the solid answers. Is there any LEED regulation in limiting the tolerance of unmet hours? We use EnergyPlus as our energy model and there we can mod the tolerance up to 10 degC. Is it realistic (and allowed) to just increase this throttling range to compensate a great number of unmet hours? We have this unmet hours issue since we add independent OAU (Outdoor Air Unit) in each of VRF conditioned zone. Any suggestion?
Regards,
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
March 25, 2015 - 7:39 am
1) Calculate the extra "outdoor air" load from the OAU and create this extra load to apply only during the sizing days. In the current version of e+, ODA loads from DOAS are not accounted for during sizing, leading to undersizing during autosizing. This is not the only way to work around the issue.
2) There is a new website called unmethours which might help.
3) The proposed systems should not be autosized. You should be using the actual equipment capacities.
4) The tolerance is usually not the problem and is often indicative of an underlaying problem with the model, usually to do with the controls. If you are using radiant embedded piping systems, then a tolerance of 0.5 to 1.0 C is usually both realistic to the real throtteling range and sufficient for the model. You should really use that of the design case systems as they are in the design.
Ivan Priatman
Senior DesignerPT. Archimetric
8 thumbs up
March 27, 2015 - 5:43 am
1) Would you show us how to calculate and apply extra load from OAU in e+? I have tried to increase max dry bulb and wet bulb temp proportionally on SizingPeriod:DesignDay but it ended up in substantial rise of energy consumption, especially when I applied this on baseline model. I doubt it would solve the issue since unmet hours always appeared (even it did get shorter).
2a) I have checked the website, yet there are no specific case involving OAU and VRF in the same zone. I posted question there, wish they could help.
2b) I might missed something, but I really want to know about OAU and DOAS, are they the same? I read on somewhere that DOAS is a whole OA system which consist of many OA Terminal Units serving multiple zones. E+ model this as AirLoopHVAC equipped with fans, cooling coils, mixer etc more like an AHU. Therefore, OAU I used here is just a single unit without any link or duct and not connected to any other unit. I model this in E+ as ZoneHVAC:OutdoorAirUnit, equipped only with supply and exhaust fans. Could you light me up on both terms?
3) We are still gathering detailed mechanical specifications from our HVAC team. Currently, in proposed HVAC model I can only define gross rated cooling COP at each VRF condenser and the amount of outdoor air requirements in each zone. Is this still too far from enough?
4) In E+, it is possible to introduce OA without installing independent OAU by using mixer embedded inside VRF's TU. When I implemented this, with the exact same amount of OA provisions, there was no unmet hours problem occurred. How this could be happened? Logically, it would behave similar with the OAU equipped.
Regards,
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
March 27, 2015 - 6:02 am
Hi Ivan, this is becoming software specific and detailed. I suggest you repost this on the energyplus yahoo group support forum. You will get your best help there. If I see it there, I will reply further also. Regards, Jean