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Using Airflow Network with Ideal HVAC load system?

asked 2015-01-06 09:51:34 -0500

Xandrika gravatar image

updated 2015-11-12 16:44:24 -0500

My building has operable windows and single zone split HVAC for heating and cooling. I have set up a Airflow Network in energyplus to find out how natural ventilation can contribute to lower cooling loads. (Location: mixed humid climate and mixed dry climate, ASHRAE 4A and 4B).

However, I get strange results for the winter period because natural ventilation is happening while heating is ON.

What is the right approach to avoid this? Might it be that the IdealLoadAirSystem is not interacting correctly with the AirFlowNetwork Model. Or should I set up a schedule to avoid ventilation in winter nights?

I found this information in energyplus documentation (but not sure which HVAC system does it mean): The AirflowNetwork model is currently able to model a wide range of multizone air flow scenarios. However, only a single air distribution system with a constant volume supply fan can be included as part of the airflow modeling.

@MJWitte or @Archmage, could you, please give me some insight view on the compatibility of air flow network and HVAC objetcs. Actually, I was thinking to use ideal loads because I am not interested in HVAC details.

The objective of my simulation is to find out best passive design strategies to reduce heating and cooling loads. My location is in Nepal and I have no data about actual infiltration rates, etc. I am doing parametric analysis with jEPlus; so I run a high number of simulations.

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I haven't done natural ventilation in E+ (IESVE), but how are you governing the opening/closing of the windows? They will at least be governed by both the ambient (outside) temp and the inside space temp(s), with respect to your setpoint(s).

keb gravatar image keb  ( 2015-01-06 09:59:34 -0500 )edit

Yes. I have set up a ventilation setpoint (24C) in energyplus. That means when indoor temp goes above this setpoint the windows are opened. What happens in winter is that the heating system somehow drives ambient temperature above 24 and, thus, windows are opened. In energyplus I cannot set up a limit for outdoor temperature:-(

Xandrika gravatar image Xandrika  ( 2015-01-06 10:05:11 -0500 )edit

Aside from possible incompatibility between Airflow Network and IdealLoadAirSystems, could you not use an EnergyPlus Energy Management System routine to control window opening based on outdoor as well as indoor temperature?

Eric Ringold gravatar image Eric Ringold  ( 2015-01-06 14:33:16 -0500 )edit

@Eric Ringold. Thanks for EMS hint. I did some trial with HybridVentilation and it seems to work properly.

However, are you sure that Airflow Network and IdealLoadAirSystems are incompatible? I did not find any info on that in e+ documentation.

Xandrika gravatar image Xandrika  ( 2015-01-08 13:14:02 -0500 )edit

Oh no, I'm not at all sure about that. I meant that I couldn't speak to that part of your question. Sorry for the confusion.

Eric Ringold gravatar image Eric Ringold  ( 2015-01-08 13:32:24 -0500 )edit

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answered 2015-01-09 09:03:59 -0500

Archmage gravatar image

As for the compatibility between AirflowNetwork (AFN) and HVAC system it is true that the documentation is a bit unclear. That passage you quote about only modeling a single constant volume air system is referring to a somewhat advanced application of AFN where the pressure network includes the various components inside the air handler. That application of AFN allows modeling leaks from the HVAC and how those interact with the rest of the building.

However there is a more basic application of AFN and HVAC where a modeler isn't really interested in the behavior of the HVAC system itself, but rather the impact that a given air system operation has on the rest of the building. For this application one would want the zone's air system inlets and outlets to be included (conservation of mass) as drivers in the AFN but the air system's flow rate and operation isn't directly modeled by AFN. This is where the documentation isn't clear but it turns out that AFN is actually compatible with presence of HVAC systems. If AFN is set up properly (AirflowNetwork:Multizone:Zone ) it will pick up the air system nodes from the zone (ZoneHVAC:EquipmentConnections) and include those air flows in the pressure network model. As I understand it, this is the more common application for pressure network modelers because they are happy to take the air system's zone distribution flow rates as fixed in the design and seldom really want to model the inside of the air handler in detail.

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answered 2015-01-09 09:03:14 -0500

You can use Airflow Network with Ideal Load Systems but you have to bear in mind that Airflow Network calculation will not take into account the effect in the pressure/flow that a real system would have if working simultaneously with natural ventilation. If you have no problem with that, you can go ahead.

For the natural ventilation not being active when the heating is on you have several options:

  • The most obvious is coordinating heating and ventilation setpoints. It is quite strange that you get a higher temperature than the heating setpoint (24ºC) in a winter night (it could happen during the day, most of the times due to solar radiation). You can play with ventilation setpoint schedule but I think that the problem can be elsewhere (in the heating setpoint).
  • You can play with modulation according to inside-outside temperature difference to reduce ventilation if outside temperature is too low, which may partially solve your problem.
  • Finally you can use EMS, though it may be a bit scaring if you have not played with it earlier.
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Gracias @ecoeficiente. Actually, the building has room split ACs that are operated manually by the user and I assume that either AC will be running or the windows will be opened. I used AvailabilityManager:Hybridventilation to control NatVent when heating is On and it seems to work properly. However, I am not too confident because it is the first time that I am using AFN. Could you have a look at one of my files and results and give me your opinion. Si no te molesta?

Xandrika gravatar image Xandrika  ( 2015-01-09 13:25:19 -0500 )edit
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answered 2020-01-29 00:39:13 -0500

GARIMA.RKE gravatar image

hey i am trying to do the exact same thing. i have given cooling setpoint as 24 and ventilation setpoint as 22 with outdoor temperature range 18-35. i am using AFN and have given ideal air load system(using template). so ventilation should work if the conditions are met else ideal air load system should b working. But it seems that ideal air load system is not getting shut..so sometimes i am getting ach and cooling load together for the zone. As you have done the same thing successfully can you help me rectify this problem?

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AvailabilityManager:HybridVentilation is used to control natural ventilation vs HVAC system operation. However, this only works with an AirloopHVAC system, not zone equipment (such as ideal loads).

MJWitte gravatar image MJWitte  ( 2020-01-31 17:08:24 -0500 )edit

yes.. ryt @MJWitte thank you.. i tried the same thing with unitary air to air heat pump and AFN (without distribution). but still as the temperature goes above 24 Coolings loads are coming.. what i want is system should first check if the outdoor temp is between 18-35 and indoor temp is above 22, natural ventilation should be working and if these conditions are not satisfied only then HVAC should be running. i even tried doing it with EMS (i referred that relative humidity based temperature control) but still as the indoor temp goes above 24 cooling loads are coming.

GARIMA.RKE gravatar image GARIMA.RKE  ( 2020-02-14 01:13:17 -0500 )edit

If you use EMS to override the availability schedule in the ideal loads object, you should be able to accomplish this.

MJWitte gravatar image MJWitte  ( 2020-03-06 15:58:30 -0500 )edit

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Asked: 2015-01-06 09:51:34 -0500

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Last updated: Jan 29 '20