VAV Parallel Fan-powered Boxes using HVACTemplate

asked 2020-04-02 14:00:15 -0600

Ery Djunaedy's avatar

updated 2020-04-03 07:23:32 -0600

I want to repeat the question on G3.1.3.14, this time: how to meet that clause with HVACTemplate in EnergyPlus. Especially the two requirements: (1) fans shall be sized for 50% peak primary airflow, and (2) the minimum flowrate is set to 30% of peak primary airflow.

My questions/observations are:

  1. Can these two can be met by setting the fields in HVACTemplate:Zone:VAV:FanPowered? I am looking at these four fields:

    1. Primary Supply Air Maximum Flow Rate
    2. Primary Supply Air Minimum Flow Fraction
    3. Secondary Supply Air Maximum Flow Rate
    4. Parallel Fan On Flow Fraction
  2. By default, if everything is autosized, the primary flowrate will be the same as the secondary flowrate, i.e. the secondary is 100% of primary. This does not meet the requirement.

  3. I have run everything on default/autosize (let say, this is Case 1). Then I used the autosized primary flow to calculate the secondary flow (which is: secondary = 50% of the primary), this is Case 2. However, when I run with secondary flowrate hard-coded (primary airflow remains autosized) (Case 2), then the result shows the primary flow calculated by EPlus is the same as the hard-coded secondary flowrate.
  4. If I use the the result of Case 1 to set both the primary and secondary flow rate hard-coded, where secondary is 50% primary (this is Case 3), then the result for the primary flow primary will be different from the autosized primary flow rate from Case 1. The secondary is again set to be the same as primary (back to 100% of primary, instead of 50% of primary).
  5. Is there a sizing rule for VAV parallel fan boxes that I am not aware of, especially with regards to proportion between primary and secondary? Why the sizing algorithm tries to make secondary the same as primary flow rate.
  6. What about the 30% rule? I do not even know how to set this.
  7. Which makes me wonder, can we meet the requirements with HVACTemplate:Zone:VAV:FanPowered, or do we have to work with the expanded IDF?
  8. The secondary flow rate for VAV Parallel fan boxes is only used for heating. The PNNL 90.1 PRMRM guidance also indicate so. However, LEED spreadsheet is not clear about this (i.e. if this also applies to cooling). If I am simulating a building with cooling only (no heating at all), can I safely ignore the secondary flow and set the fan flow rate to zero?
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