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Report ASHRAE 62.1 with 100% Outdoor Air

asked 2016-01-18 09:31:01 -0600

updated 2020-01-27 18:46:14 -0600

I am working on LEED IEQp1 compliance and I am following the steps showed on the previous question and also using EnergyPlus/OpenStudio in order to get the Required Outdoor Air Intake Flow (Vot).

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I have got the report with no problem but EnergyPlus report shows that most of the zones have 100% Outdoor Air Intake Flow and it´s not true.

I tried to get those values on my own and it seems to me that there is a mistake in Zone Ventilation Efficiency calculation because the value for Ev and Xs are the same in Energy Plus.

I have already checked the HVAC system configuration by clicking on the dotted line on the system configuration and setting as it follows: OS: Sizing: System: 100% Outdoor air in Cooling = No and OS: Sizing: System: 100% Outdoor Air in Heating = No.

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Nevertheless, the report keeps getting OA% = 100% for most of the zones. Is there any other field I got to check in order to get a correct Required Outdoor Air Intake Flow (Vot) value? Am I mistaken anywhere else?

The simulation file is attached here

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@Pithon I am looking into this. My initial conclusion is that there is indeed something fishy. I suggest changing from VentilationRateProcedure to ZoneSum for the single-zone constant volume systems. I haven't tested this yet, but believe it will give more reasonable results.

aparker gravatar image aparker  ( 2016-01-24 21:05:29 -0600 )edit

@Pithon I am having this same issue. Were you able to solve your issue?

Anna Osborne Brannon gravatar image Anna Osborne Brannon  ( 2017-02-07 15:12:36 -0600 )edit

@Anna Osborne, unfortunately I didn't solve completely this. In the end, I was forced to do everything right in the LEED Online platform.

Geraldo Pithon gravatar image Geraldo Pithon  ( 2017-02-07 17:27:53 -0600 )edit

I have a similar situation with a model. My opinion is that VRP is for design for a one time critical scenario. But during operation, unless you have DCV, air loops in a VAV system work based on demand from the VAV terminals. Setting VRP for a year simulation results in instances were demand from VAV terminals is low, and your VRP OA calculation is close/equal to the primary VAV terminal airflow demand, lowering the ventilation efficiency Ev below 0.5. I suggest to lock the maximum OA air to a fixed number and lock the minimum airflow fractions in VAV terminals. Min Zp is not available in EP.

Carlos Vazquez gravatar image Carlos Vazquez  ( 2017-02-27 09:38:54 -0600 )edit

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answered 2016-01-22 10:24:09 -0600

Archmage gravatar image

As far as the underlying EnergyPlus model is concerned, you would want to check the Controller:OutdoorAir object. Keep in mind that EnergyPlus provides for separate input for sizing and operation. You are showing here how the sizing input is setup, but not really how the operation input is setup. In the Controller:OutdoorAir object check the input fields related to Minimum Outdoor Air Flow.

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answered 2016-01-25 21:02:19 -0600

updated 2016-01-25 21:05:20 -0600

I picked one of the systems to look at (they all appear to be similar) It is a single-zone system with a constant volume fan

From the HTML table you posted:

0A_FC -- Packaged Rooftop Heat Pump
Thermal Zone: 0A_FC

v_bz = .00236 m3/s*person * 72.15 ppl + .00032 m3/s*m2 * 268.31 m2 = .256 m3/s
e_z = 1.0 (OS defaults e_z zone air distribution effectiveness of 1.0)
v_oz = v_bz / e_z = .256 m3/s
v_ot = v_oz = .256 m3/s (for single zone systems)
v_ou = sum (v_ot) = .256 m3/s (because it's a single zone system)

v_ps_heating = 0.256 m3/s
v_ps_cooling = 1.814 m3/s

x_s_heating = v_ou / v_ps_heating = .256 m3/s / 0.256 m3/s = 100% OA
x_s_cooling = v_ou / v_ps_cooling = .256 m3/s / 1.814 m3/s = 14.1% OA

This is a system that serves a zone with a very low heating demand. If this were a VAV system, during heating mode, minimum OA flow is higher than the required heating airflow, so you'd need 100% OA to meet the min OA requirement at that flow rate. You could increase the flow rate by lowering the heating supply air temperature. However, this is a CAV system; the fan is one speed and the system is designed for that flow rate. In this case, the cooling flow rate is the driver. Based on my understanding of the VRP calcs, the %OA should be 14.1%, not 100%. I think there might be a bug in E+ VRP calcs for single-zone CAV systems, and I suggest switching to ZoneSum for single-zone systems for now.

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@MJWitte@Archmage am I missing something dumb here?

aparker gravatar image aparker  ( 2016-01-25 21:12:09 -0600 )edit

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Asked: 2016-01-18 09:31:01 -0600

Seen: 806 times

Last updated: Jan 25 '16