Ideal Loads Air System default mechanical ventilation setting

Hi all,

I am wondering what is the default setup for mechanical ventilation when using the Ideal Loads Air System in HB (how is the outdoor air flow rate calculated?).

The .idf contains the following:

DesignSpecification:OutdoorAir,
** zone_1OutdoorAirCntrl, !- Name**
** Sum, !- Outdoor Air Method**
** 0.0, !- Outdoor Air Flow per Person {m3/s-person}
0.0, !- Outdoor Air Flow per Zone Floor Area {m3/s-m2}
, !- Outdoor Air Flow per Zone {m3/s}
, !- Outdoor Air Flow Air Changes per Hour {1/hr}
; !- Outdoor Air Flow Rate Fraction Schedule Name**

Which made me think there was no outdoor air flow. But I guess I am wrong !

Thanks a lot for your help,

Marc

Marc,

The ‘Sum’ value means that:

zoneVentilation = (ventilationPerArea x zoneFloorArea) + (ventilationPerPerson x occupancySchedVal x occupancyLoad)

This summing is what ASHRAE recommends. For more information, see the EnergyPlus reference:

http://bigladdersoftware.com/epx/docs/8-3/input-output-reference/gr…

-Chris

Chris,

Thanks a lot for your answer.

I’m ok with the meaning of ‘Sum’ value. So you confirm that default setup for mechanical ventilation is ‘no mechanical ventilation’ when using the Ideal Lodas Air System in HB ? (Outdoor Air Flow per Person = 0 AND Outdoor Air Flow per Zone Floor Area = 0).

I still have doubts, because when adding “Zone sensible Heating Rate” as output, I get non-null results. (I thought “Zone Sensible Heating Rate” was directly related to the Outdoor Air Flow)

Marc

The default is not 0. It is set by the zone program. Use the readMe! Output of the “Set Zone Loads” component to see your Jones’s ventilation specifications.

Sensible heating is pretty much the same as normal heating in an ideal air loads system. It’s dependent on ventilation as well as everything else.

-Chris

Hi Chris,
I have one similar question.
As defined by EnergyPlus, the ideal loads air system changes the supplied air flow rate to satisfy the heating and cooling loads.


But, at the same time, with the ‘set zone loads’ component, I set the ventilation rate per area and per occupant.

I wonder which value is used as the final air flow rate for the zone? The one that I set in ‘set zone loads’ or the one that the system calculated by itself for the heating and cooling loads?

@Tiantian,

From my reading of the (somewhat unclear) EP Engineering Reference on the topic[1], the supply air flow rate for the zone is conditionally determined based on a couple of factors.

The specified ventilation air (calculated according to the formula in the previous post) is your minimum flow rate. It’s then increased to match the maximum flow rate from the following three calculations:

  1. The air flow rate required to meet the zone sensible load at the maximum/minimum supply temperature limit.
  2. The air flow rate required to meet the dehumidification load as defined by a humidistat at your minimum supply humidity limit (if this is set).
  3. The air flow rate required to meet the humidification load as defined by a humidistat at your maximum supply humidity limit (if this is set).

So for example, let’s say your air flow rate to meet the zone sensible load at Tmax is your max flow rate, then your supply air mass flow rate (ms) is:

ms = (Qz / Cp * (Ts - Tz))
where:
ms = Zone air flow rate (kg/s)
Qz = Heat transfer in zone
Cp = Specific heat capacity of air
Ts  = Supply temperature limit (Tmax or Tmin for heating/cooling respectively)
Tz = Zone air temperature 

But if you have no heating or cooling loads, then it’ll be your ventilation air flow rate. After the flow rate is calculated, EP calculates the sensible/latent heat recovery that occurs at your mixed air box, and then, finally, your zone heating/cooling coils will supply the remaining heating/cooling energy to meet your zone loads.

S

[1] https://bigladdersoftware.com/epx/docs/9-1/engineering-reference/ideal-loads-air-system.html#ideal-loads-air-system

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Thanks for your explanation. I think it is quite clear now.:slight_smile: Sorry for my late reply.