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MarketManager » Modeling HVAC Systems
Q: How do I model DX cooling in conjunction with VAV and DDMZ systems?


In MarketManager, both VAV systems and DDMZ are central systems that MUST have central equipment (addl. zonal equipment is optional and would be used to offset zonal loads). A VAV or DDMZ system, from MarketManager's perspective, always needs a central chiller and a central boiler. The program is conditioned to bring mixed air temp to supply temp even when new zone loads may be zero (i.e. if DX zone units have already taken care of it all).

If you really want to model zonal equipment exclusively, stay away from VAV or DDMZ. They are not appropriate. Instead use a PTAC. In these systems you can specify VAV fans in the HVAC data form (which will turn the system type into a "custom" system).

There is another option--that is to allocate a plant for each system. Then you can use a central DX unit as your cooling equipment. However, if you do this, then you have to deal with splitting up your boiler into pieces. Overall, though I don't see such an arrangement doing anything substantially different from having one single PTAC system with a DX unit attached to each of its many zones.

Finally, to answer your question: you cannot model a dual duct or multizone system with dedicated DX units, per se. You can model a VAV with a dedicated DX unit (however you would use a "custom" system).

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Otherwise feel free to contact our Tech Support staff at (805) 329-6565, or via email at helpdesk@abraxasenergy.com.

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