There is a neat trick that uses zone exhaust fans to help balance air system flows when using ZoneMixing. The volume of return air flow can be deprecated using Fan:ZoneExhaust. That is the air leaving the sending zone as transfer air can subtracted from the air returning to its air handler. Set the flow rates to be same in the ZoneMixing and Fan:ZoneExhaust objects, but don't consume power in the exhaust fan. The sending zone may not really have exhaust fans but this trick allows capturing the reduced return air flow in the air handler.
Depending on what is happening in the zone receiving the transfer air, you can control the split between a real exhaust fan and the air handler return in the Fan:ZoneExhaust using the field Balanced Exhaust Fraction Schedule. I don't think you can force more return air than enters the receiving zone via the air system supply air, but if you really have exhaust then it need not deprecate return air flow when all the exhaust should be transfer air.
In EnergyPlus, you can model the movement of transfer air from one zone to another using the ZoneMixing object.
From the EnergyPlus ZoneMixing Documentation :
"In EnergyPlus, the ZoneMixing syntax is intended to allow simplified treatment of air exchange between zones. Note that this statement only affects the energy balance of the “receiving” zone and that this statement will not produce any effect on the “source” zone. Mixing statements can be complementary and include multiple zones, but the balancing of flows between zones is left to the user’s discretion. The use of the Cross Mixing syntax (ref: Cross Mixing) can allow for automatic balancing of flows and energy flow rates between zones but is hindered by the limitation of only a single cross mixing statement per zone. The use of Refrigeration Door Mixing syntax (ref: Refrigeration Door Mixing) automatically balances the flow and energy between two zones and allows multiple mixing statements per zone. More advanced mixing calculations are possible using the EnergyPlus AirflowNetwork model for multi-zone airflow with or without HVAC system operation. Mixing is entered using the following syntax."
@aparker, this answer is helpful, but it doesn't get at the core problem which is that transfer air depends on the flow rates and operation schedule of the HVAC system. I don't think that ZoneMixing takes the HVAC system into account at all. (Sure, you could try to match your schedules and flow rate to the anticipated system operation, but that can be hard especially for variable flow.) Oh, and I generally don't want to get into AirflowNetwork because it's way too much overhead but I would still be interested in how it could be done that way (if at all).
@Peter Ellis, Brent might have a better answer on how AirflowNetwork might be used for this, but unless there is a model of pressurization then I think you're looking at interchange driven by wind and temperature. That might not be what you're looking for.
What about a case where you have an actual transfer fan that transfers air from one space to another in response to a thermostat? This is most commonly used to keep small electrical rooms from getting too hot. When the electrical room goes above a certain threshold temperature (say 85 F), the transfer fan turns on and pulls in air from an adjacent space. I could model the air transfer using equal and opposite ZoneMixing objects and specify minimum source/zone temperatures to enable the transfer only when the electrical room is above 85 F, but how can I accurately track the fan power?
If you want modelling indoor natural ventilation, you can use airflownetwork. But if you want modelling mechanical ventilation (with air treated or no) you can use plenum zone.
@Vmoreno: welcome to Unmet Hours. Would you mind detailing your answer more please? As the message above the "answer" box states, it is important to try to give a substantial answer. As of right now, this would constitute more of a comment than an answer. Thank you!
Asked: 2014-09-01 16:21:28 -0500
Seen: 2,251 times
Last updated: Oct 18 '16
