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OS Zone Sizing Report Question

asked 2015-12-02 19:36:49 -0500

SVVitale gravatar image

updated 2017-08-05 08:07:26 -0500

I have a question about what "User Design Load" means in the Zone Sizing Report (created using a BCL measure in Openstudio) linked here. Working to troubleshoot why my boiler for a Packaged Rooftop VAV with Reheat system is not being properly autosized, I decided to save out a copy of my model and switch back to Ideal Air Loads. Attached is a copy of the output for one of my zones.image description. Under Zone HVAC Sizing Summary, I see a heating load of 418,480 BTUH for my zone as a "User Design Load". I have no idea where I've specified any loads directly in my OS settings. Two other data points worth noting are my Peak Heating Load of 5.18 BTUH/ft2 and square footage of 3230 which when multiplied together give 16,731 BTUH which makes more sense for an ideal air heating load consisting of walls, roof, floors and etcetera.

Can anyone advise where I might have input something that would influence a user design load? I have a similar question about User Design Airflow. The values given (see attached screenshot) for both cooling and heating User Design Airflows seem very low in comparison to the Peak Heating Load and Peak Cooling Load.

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I found this post about a similar question link. Still scratching my head though.

SVVitale gravatar image SVVitale  ( 2015-12-02 21:43:48 -0500 )edit

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answered 2015-12-03 11:55:39 -0500

Looking at the components of the peak heating load listed in the rows above the bottom red circle, I see that infiltration is by far the biggest contributor to the heating load.

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Agreed. Though I still do not understand what is driving the large difference between calculated heating load of 16,731 BTUH and the user design load of 418,480 BTUH.

SVVitale gravatar image SVVitale  ( 2015-12-03 14:09:19 -0500 )edit

answered 2015-12-03 14:29:28 -0500

SVVitale gravatar image

updated 2015-12-03 14:31:00 -0500

I figured out at least part of my problem. When I changed my simplified model from one with a Packaged Rooftop VAV with Reheat to one with Ideal Air Loads, I forgot to change my zone cooling and heating design airflow method to DesignDay. I had previously set it to Flow/Zone. I still do not know why the user design heating load was so high though, nor why the user design airflow was so low by comparison.

Now my user design heating load is showing up as 18,440 BTUH (which is about 10% higher than the answer I get by multiplying the BTUH/SF by the zone floor area.). Seems reasonable.

The user design airflow is still coming out low by comparison. If my design heating supply air temperature is 105 degrees F and I'm trying to maintain 70 degrees F setpoint on my heating design day, the associated zone airflow in heating for a user design heating load of 18,440 BTUH should be around 488 cfm. The zone report now gives only 8.12 cfm as user design airflow in heating, which is too low.

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User design load = calculated load * sizing factor, so that could be the source of the difference. You can check out those values on the Simulation Settings tab at the top. The default heating sizing factor is 1.25, and the default cooling sizing factor is 1.15.

aparker gravatar image aparker  ( 2015-12-04 14:29:12 -0500 )edit

Also, check out the HVAC Sizing Summary, Zone Heating section inside the EnergyPlus results report. There might be a unit conversion issue in the Zone Reporting Measure mucking things up.

aparker gravatar image aparker  ( 2015-12-04 14:31:47 -0500 )edit

Also check out your Sizing:Zone object. This is what determines the heating supply air temp, not the Sizing:System object in the AirLoop.

aparker gravatar image aparker  ( 2015-12-04 14:32:32 -0500 )edit

I think there IS an issue of some sort with the zone reporting measure. The heating cfm from Energyplus and from OS is correct but way low in the zone reporting measure.

SVVitale gravatar image SVVitale  ( 2015-12-04 14:58:27 -0500 )edit

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Asked: 2015-12-02 19:36:49 -0500

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Last updated: Dec 03 '15