When there is latent load only (i.e., when there is no sensible load), then the above logic if (PartLoadRatio > 0.0) holds true for single speed DX coil only. For variable speed DX cooling coil, the PLR can be zero when the speed ratio is greater than zero. Thus, it needs to add a second logic to avoid turning off a variable DX cooling coil when there is latent load only.

At the bottom of controlCoolingSystemToSP is:

    this->m_CoolingPartLoadFrac = PartLoadFrac;
    this->m_CoolingSpeedRatio = SpeedRatio;
    this->m_CoolingCycRatio = CycRatio;
    this->m_DehumidificationMode = DehumidMode;

I wonder if it would be better to correct this issue here because you would want the UnitarySystem to know what the PartLoadRaio is for control and reporting of m_PartLoadFrac. There is also the calcUnitarySystemToLoad function which may need review to see if an update is needed there as well.

this->m_CoolingPartLoadFrac = std::max(PartLoadFrac, SpeedRatio);

this->m_CoolingPartLoadFrac = std::max(PartLoadFrac, SpeedRatio);

This logic may work but we will be mixing up two variables. The m_CoolingPartLoadFrac and SpeedRatio have a different meaning and resetting the m_CoolingPartLoadFrac that may have un-intended consequence elsewhere.

May be use a local variable that does not affect the m_CoolingPartLoadFrac member variable.

If I understand correctly, for variable speed DX coil, the m_CoolingPartLoadFrac value is set 0.0 whenever the SpeedRatio is > 0.0. @rraustad Do you agree?

I think it's CycRatio that gets set to 0 when SpeedRatio > 0. The VS coil is tricky. I suggest plotting Unitary System Part Load Ratio to see what that shows during latent only operation. Also, CycRatio > 0 at speed = 1 and SpeedRatio > 0 at speed > 1 so my suggestion was not quite right.

I see on one place where the PLR is set to SpeedRatio this->m_CoolingPartLoadFrac = SpeedRatio; for VS DX cooling coil. Let me examine the report variables (PLR, CyclingRatio and SpeedRatio).

That could be a mistake. this->m_CoolingPartLoadFrac is similar to CycRatio and both should be 1 at speed > 1.

The same change as the above.

Now I reset the PRL in one place using the SpeedRatio and removed the previous code changes.

The CycRatio is reset to 0.0 when the SpeedNumber is = 1.

The CycRatio should be set 1.0 when the SpeedNumber is >1. I will remove the CycRatio = 0.0; statement that added above.

This commented out line will be removed.

Added a unit test to demonstrate the fix.

The SpeedRatio is saved in a local variable which is then assigned to this->m_CoolingPartLoadFrac.

Not sure where this->m_CoolingPartLoadFrac = SpeedRatio is ever used. If the speedNum > 1 then the coil never turns off during the time step and this->m_CoolingPartLoadFrac = 1. So this line should be:

    PartLoadRatio = CycRatio;

At the bottom of this function:

    this->m_CoolingPartLoadFrac = PartLoadFrac;
    this->m_CoolingSpeedRatio = SpeedRatio;
    this->m_CoolingCycRatio = CycRatio;
    this->m_DehumidificationMode = DehumidMode;

Oh, I see it now. It's used a lot in the MuiltStage electric heater. I think that's a mistake. Here speedNum > 1 so the heating coil never turns off during the time step.

case HVAC::Coil_HeatingElectric_MultiStage: {
    if (this->m_SuppHeatingSpeedNum > 1) {
        auto f = [&state, this, DesOutTemp, CycRatio](Real64 const SpeedRatio) {
            return UnitarySys::heatingCoilVarSpeedResidual(state,
                                                           SpeedRatio,
                                                           this->m_SuppHeatCoilIndex,
                                                           DesOutTemp,
                                                           this->m_UnitarySysNum,
                                                           CycRatio,
                                                           this->m_SuppHeatingSpeedNum,
                                                           this->m_FanOpMode,
                                                           HVAC::CompressorOp::On,
                                                           true);
        };
        General::SolveRoot(state, Acc, MaxIte, SolFla, SpeedRatio, f, 0.0, 1.0);
        PartLoadFrac = SpeedRatio;


this->m_SuppHeatPartLoadFrac = PartLoadFrac;
this->m_SuppHeatingCycRatio = CycRatio;
this->m_SuppHeatingSpeedRatio = SpeedRatio;