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Zone design loads do not account for DOAS

asked 2018-10-24 15:08:06 -0600

Oleks gravatar image

updated 2018-10-26 11:49:33 -0600

Hi All,

I am modifying Standard 90.1, 2013 DOE Prototype Primary School Model to have four-pipe chilled beams with DOAS HVAC systems in sections with classrooms. The building is situated in Phoenix, AZ. The problem is that the Calculated Design Loads for heatingare sized to be 0s for the zones where I add chilled beams with DOAS. Given the location, lesser heating loads can be expected but not 0s. The other not modified zones still have sizable heating loads. Also, I am modeling CAV for supply air to chilled beams with 55 °F SAT and this should be reflected in the sizing if "Account for Dedicated Outdoor Air System" is selected, but it is not reflected. I have disabled chilled beams heating in response to the sizing but this results in 1653 not met hours during occupied heating. Also, all of the chilled beam inputs are hardsized. I am struggling to find the source of the problem. Here is the link to the model,

Any help would be appreciated.

Thank you.

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Are the zone thermostats set correctly? Do the availability / temp schedules have the correct heating design day profiles? Do the zone sizing objects have the correct sizing air temperatures (temperatures delivered to the space)? Are the zone HVAC objects connected to a heating plant loop, and that heating plant loop runs and has enough capacity?

mdahlhausen gravatar image mdahlhausen  ( 2018-10-25 11:59:21 -0600 )edit

@mdahlhausen, thank you, I have tried increasing Zone Heating Design Supply Air Temperature which resulted in heating loads. I understand that because I had Zone Heating Design Supply Air Temperature of 55 °F and Heating Design Air Flow Method as DesignDay there would be no airflow rate that would meet the Design Day heating loads at this supply air temperature. However, I thought that zone load sizing is done irrespective of the supply air flow, meaning that there is still heating load even if there is no supply airflow.

Oleks gravatar image Oleks  ( 2018-10-26 11:43:01 -0600 )edit

In this case, supply air for sizing means supply air from the zone units, not the central DOAS air loop.

mdahlhausen gravatar image mdahlhausen  ( 2018-10-26 12:43:27 -0600 )edit

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answered 2018-10-29 18:06:12 -0600

As you discovered, if the Sizing:Zone Zone Heating Design Supply Air Temperature is less than the thermostat heating setpoint, zero load is reported. The calculation does know the heating load for that situation, so it should be able to report non-zero heating loads, even if the resulting heating supply air flow rate is set to zero. At a minimum this should have generated warnings. I have posted a new issue, #7039 to look at this further.

So, if you use a warm heating supply air temperature, this should work as desired.

In addition, this needs to be fixed:

** Warning ** ShadowCalculation: suspect Calculation Frequency
**   ~~~   ** Value entered=[15000], Shadowing Calculations will be inaccurate.

The shading factors are being calculated only once for the entire year. This field should be blank for the default of 20 days, or something smaller if desired.

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@MJWitte thank you, this clears it up. Also, thank you for the warning explanation since I was wondering why there was only one start date for the simulation.

Oleks gravatar image Oleks  ( 2018-10-30 16:53:29 -0600 )edit

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Asked: 2018-10-24 15:08:06 -0600

Seen: 222 times

Last updated: Oct 29 '18