Best way to model multi-speed fans for zone equipment

asked 2017-01-19 11:32:54 -0500

willyJohan's avatar

updated 2017-01-19 17:46:29 -0500

I am trying to represent in OpenStudio a VRF system with 3 speed fans serving the fan coils at the zone level. Outdoor air is provided to zones through a constant volume DOAS system. In reality the zone fans will move between the low, medium and high depending on the conditioning demand. The fans will never turn wholly off, resetting to the minimum when no conditioning is required.

as I understand it, I've got two fan options Constant and OnOff, I can then set the operating mode to either 1 or 0 to operate the fans continuously or intermittently , i can also set the supply air flow rate when no heating or cooling is necessary.

My questions are as follows:

Are the inputs that impact zone fan operation outlined above comprehensive?

Which combinations of settings would best capture the actual performance of a 3 speed fan driven by an DC brushless motor?

What is the difference between a CV fan and an OnOff fan with the operation mode set to anything other than 0?

Is there a way to specify a CV fan within OS (looks like OnOff is the only option within the interface)?

What happens when the airflow when no heating/cooling is set to something less than the supply airflow rate for the available fan types?

As I see it I've got two options:

1) Set fan operation to 1 during occupied hours and set supply airflow rate when no cooling/heating is needed to minimum outdoor (Autosizing this value just gives the same air flow as that when conditioning is required)

2) Set the fan to cycle (and 0 air when cooling/heating is not needed) with the thought that this would better capture the savings of a multispeed fan running in response to variable conditioning needs

Thanks in advance for any help.

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Also, on a related note, what is the reasoning/limitations behind EnergyPlus having only single speed fans at the zone level? I consistently have problems accurately representing system fan energy use when zone level equipment is being modeled.

willyJohan's avatar willyJohan  ( 2017-01-20 15:08:32 -0500 )edit

I believe Fan:SystemModel would be the right choice. However, it is only available in OS 3.0.1 (per SDK documentation), and I am still at 2.9.1, so have not been able to try it out. I just don’t feel comfortable upgrading to the new, segregated (for application and SDK) way of installation. Additionally, I believe I just read somewhere that EP 9.3 that comes with OS 3.0.1 may have general VRF issues.

mattkoch's avatar mattkoch  ( 2020-09-13 08:35:03 -0500 )edit