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OpenStudio: DOAS won't match Design Specification Outdoor Air rates

asked 2016-03-29 20:11:27 -0500

mdixon gravatar image

updated 2016-03-30 18:26:24 -0500

I'm trying to model a residential tower with HRV's in each suite (approximated with one large air loop with air-to-air HX, supply/exhaust fans, and AirTerminal Single Duct Uncontrolled in each zone). Minimum fraction of O/A has been set to 1.0 via a schedule at the air loop Outdoor Air Controller. I have defined People and Design Specification Outdoor Air rates at the Space Type level. I have also tried setting flags in OS:Sizing:System so that "type of load to size on" is set to VentilationRequirement and "System Outdoor Air Method" is set to VentilationRateProcedure. All fans, intakes, terminals etc are autosized.

When I run the model, the resulting airflows are massive (10x what I have defined or more). If I hard-size all the diffusers to the airflow I know I need it works fine. Kind of defeats the purpose of schedules and hierarchies though.

I am at a loss. If anyone is able to assist, would be much appreciated.

Edit:Link to download OSM file here (thanks Julien & Jamie). I switched a few diffusers in air loop "Residential HRV Loop" back to autosize, most have been left hard-sized for comparison's sake:

Edit2: Here's an example comparison of some of the zones that are autosized airflows vs hardsized flows entered at zone diffuser. Values are taken from the standard OpenStudio reports, Outdoor Air section. As you can see, difference is pretty extreme in some cases from the defined airflow requirements.

Zone                        ft3     ach     E+ cfm  design cfm  Autosize?   Error
L1 CityHome 1 - Plus        15924   0.716   190.0   62.59       Y           204%
L1 CityHome 3 - Plus        15924   0.238   63.2    62.59       N           1%
L13 NE Unit                 7312    0.871   106.1   62.59       Y           70%
L13 North Unit              7524    0.506   63.5    62.59       N           1%
L1 Corridors                17566   1.756   514.1   585.5       Y           -12%
L1 Manager Office           2424    5.469   220.9   13.87       Y           1493%
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I would need to take a look at the model. Can you upload it somewhere else (google drive, dropbox, or even on the Unmet Hours slack channel (see here) and add a link to it?

Julien Marrec gravatar image Julien Marrec  ( 2016-03-30 03:02:15 -0500 )edit

You should be able to upload it here if you change the file extension from .osm to .png or similar.

Jamie Bull gravatar image Jamie Bull  ( 2016-03-30 05:45:02 -0500 )edit

Can you elaborate on how you got that the resulting airflows are 10x what they need to be? For example, the apartment space type ventilation is defined at 5 CFM/person * NumberOfPersons + 0.06 CFM/ft^2 * ZoneArea. Can you show one zone or something where you compare the E+ Output to the value you expect?

Julien Marrec gravatar image Julien Marrec  ( 2016-03-30 12:57:17 -0500 )edit

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answered 2016-03-30 18:57:28 -0500

Sizing can be tricky at times when setback is used during the sizing calculations. I suggest trying a constant zone temperature set point and again compare the airflow rates. Remember that at the time of setback or setup, the predicted zone load can change dramatically, which can then affect the air flow needed to meet that exaggerated load.

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Asked: 2016-03-29 20:11:27 -0500

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Last updated: Mar 30 '16