Dan,
As an alternative to using dummy zones, I suggest trying an alternative workaround ("plenum method") I have advocated for, which may be appropriate depending on how you would design your active chilled beam system. (If your system would be constant volume, without a separate DOAS, you could just model it with the Induction Unit system type. If you want to allow for variable flow, and/or separate DOAS unit, read on.) I have had luck modeling DOAS by creating separate systems (as they are designed) and assigning them to the plenum spaces above the occupied spaces. The description of how to do this, and the merits and potential drawbacks, can be found in the "NEW (?) eQUEST DOAS workaround using plenum spaces" thread in his archive: http://lists.onebuilding.org/pipermail/equest-users-onebuilding.org/2013-November/thread.html. It would be an easy alternative for you to try modeling in your schematic model, as you have few zones and they all have plenum zones above them. Also, note that while you said your static is the same in both models, you have 1" static on supply and exhaust for the heat recovery portion of the DOAS systems, which you did not include in the VAV model. This is about 90,000 kWh as shown on the ERV reports.
Regards,
Bill
William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer
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A few questions on this approach:
Do you need to have a separate plenum zone for each "regular" zone? Currently my model is set up with one plenum zone for each floor. If it's the only way to get it to work, then I will probably have to backtrack to the wizard edit mode. Not ideal but with this model I tried to minimize how much work I did in detailed edit mode.
Did you set up the OA-FROM-SYSTEM for the zones' system or just set the OA requirement for each of the zones to 0 so only the plenums have OA? Do you need to assign the people to the plenum zones as well? I assume equipment and lighting loads would stay within the "regular" zone - correct me if this is not the case.
Thank you for the catch on the static, I thought I had picked that up but I must have missed it.
Thanks,
Dan
Dan,
Good questions regarding the plenum method for modeling DOAS.
Plenum per zone vs. common plenum per floor: I haven't tried doing it with a common plenum per floor, but I think it could work in many circumstances. A couple situations where it wouldn't work are if you wanted to model 1.) more than one DOAS per floor, or 2.) DCV using the "DCV-ZONE-SENSORS" MIN-OA-METHOD (because you'd only have one zone). In a situation where the ventilation for some zones on the floor is served by a different system, you just wouldn't model the ceilings for those spaces as air walls.
No need for OA-FROM-SYSTEM. Just assign all OA loads served by the DOAS to the plenum zones.
You don't have to assign any interior loads to the plenum zones other than OA. The DOAS return air will get sensible heat transfer across the air walls (ceilings) from the zones below. You can assign people to the plenum zones instead of the zones below, especially if you want to model DCV, but also if you want space latent loads to be handled by the DOAS system (as people are often the only latent load). Be aware that assigning people to the plenum zones instead of the lower zone(s) will affect the autosizing of the system that serves the lower zone(s).
Regards,
Bill
I've adjusted my model so that the DOAS picks up the OA/people from the plenum zones, and kept the single plenum per floor. I did notice that you get errors unless you change the ceiling construction to something with U-value input, as opposed to layers input, once you change the ceilings themselves to air walls.
I am noticing that the DOAS airflow is sensitive to the temperature difference between the DESIGN-COOL-T and DESIGN-HEAT-T in the plenum zone. The DOAS workaround describes "Pick DESIGN-COOL-T and DESIGN-HEAT-T values for the plenum zones that match the likely DOAS supply air temperature range" but if I get the temperature difference too close to the DOAS MIN-SUPPLY-T and MAX-SUPPLY-T I notice my airflow increases by quite a bit. Did you have a "standard" temperature difference between the plenum zone and the DOAS supply that you used? Also would this setup account for situations where you would not want to supply room neutral air through the DOAS?
Thanks,
Dan
Dan,
What do you mean "my airflow increased by quite a bit"? Are you letting eQUEST autosize the DOAS design airflow? Going from memory, the instructions from my workaround write-up were appropriate if you manually enter the DOAS supply flow rate, which should be easy to do once the total OA requirement is determined, and assuming a 100% OA unit. My instruction for the DESIGN-COOL-T and HEAT-T values was to ensure that system/zone flow rates and capacities for the occupied zones below would be autosized correctly, since there is a high rate of heat transfer with the plenum zones due to the air wall ceilings. Also, while the zone type for the plenum zones should be set to "conditioned", no thermostat schedules should be assigned, assuming the DOAS will not be sized to meet space sensible loads. Taking into account your issue, I would recommend the DESIGN-COOL-T and DESIGN-HEAT-T for the plenum zones be set to the range of expected return air temperatures to the DOAS, which should be closer to the range of room temperatures maintained by the systems serving the sensible loads of the lower occupied zones. Again, I don't think it will matter much once you manually enter the DOAS airflow rate (and plenum zone airflow, and OA) and remove plenum zone temperature schedules.
Regards,
Bill