Multispeed DX coils not following specified minimum flow rates with no cooling or heating load

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
cpp
Domain
backend

Research direction

Start by running the attached in.idf.txt and inspecting the AirLoopHVAC:UnitarySystem handling for multispeed DX systems during continuous fan operation with no load. Compare the simulated no-load supply airflow with the configured minimum rate and confirm that the fan uses the minimum airflow rather than the maximum; add or update coverage for this scenario if the existing test structure identifies a suitable location.

Written by the indexing model from the issue text.

Description

Issue overview

I am modeling a multispeed DX heat pump system (representing a variable speed commercial heat pump RTU). The fan operation is set to be continuous during operating hours, and cycling during non-operating hours. I am using the "AirLoopHVAC:UnitarySystem" system type.

For timesteps when no heating or cooling load is present, but the fan operation schedule is "continuous", the fan appears to use the maximum airflow rate rather than the airflow rate specified in the "No Load Supply Air Flow Rate" field. This is resulting in the highest airflows occurring during times when there is little to no load.

This is shown in the figure below at the beginning and end of the building operating hours when there is no heating or cooling load. The unitary multispeed DX system correctly chooses speed 0, heating and cooling loads are 0, but the fan operates at the maximum flow rate around 0.65kg/s, rather than the minimum airflow rate (speed 1 airflow).

When the cooling load appears in the middle of the day, the unitary DX system speed increases stages, and the fan flow responds accordingly as expected. Additionally, during unoccupied hours, the fan and DX system appear to cycle as expected.

I have attached the IDF.

Capture
in.idf.txt

Dominant language
C++
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