EnergyPlus Infiltration Heat Load Calculation

asked 2023-01-10 09:50:43 -0500

nsmith's avatar

updated 2023-01-10 12:16:22 -0500


I am using EnergyPlus v9.5 for load calculations. The zone we are looking at has an exterior wall area of 221SF and a window area of 75 SF. The design day outdoor air temperature is -1.4F and the room heating setpoint is 75F (deltaT=76.4F). This project is at elevation near Denver.

We are using the ZoneInfiltration:DesignFlowRate object (see image).

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The infiltration schedule is a constant value of 1.0.

The resulting hourly variable Zone Infiltration Current Density Volume Flow Rate m3/s = 0.0140250881894948 (29.7174789090896 cfm). The Zone Infiltration Standard Density Volume Flow Rate m3/s = 0.0138260174506981 (29.2956719 cfm). The resulting sensible heat loss is 2530.8 btu/h.

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2530.8 btu/h = x * 29.2956719 cfm * (75-(-1.4)) results in x being 1.13 (which is at least close to 1.08).

What I am not understanding is how the infiltration flow rate is calculated. A wall area of 221 SF * 0.11cfm/sf = 24.31 cfm, not 29.3 or 29.7. It is almost as if the calculation is double counting the window and wall area or is using the floor area (270sf) instead of the wall area

I have tried messing with the sizing parameters changing them from 1.25 to 1 and nothing changed. I also thought about adjusting for wind, but it was my understanding that that only mattered for the ACH calculation.

Am I missing something? Could this be a bug?

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None of your images are coming through. Can you fix that?

shorowit's avatar shorowit  ( 2023-01-10 12:04:52 -0500 )edit

Sorry about that. I didn't see that you could attach images instead of typing out the file path.

nsmith's avatar nsmith  ( 2023-01-10 12:17:05 -0500 )edit

Returning to my own issue, I think we may have sort of figured it out. All of the math appears to be right. When I look at the DesignDay outdoor air temperature at the surface of the zone, the zone temperature, the OA specific heat and density at the altitude and temperature conditions, the area of the wall and design infiltration rate, all of it lines up. What throws me off is the ESO reporting that shows the infiltration flow rate. This number must not be in reference to the OA conditions, but must be something like the flow rate after the air has reached to the internal zone temperature.

nsmith's avatar nsmith  ( 2023-09-25 09:52:31 -0500 )edit