Issue overview

Cycling issue at low speed for heating mode

From 12:30 to 13:00, the compressors are running at the lowest speed in both cases, zone temperature are the same but both system have cycling ratio = 1. This shouldn't happen as they have different compressor curves corresponding to the lowest speed.
From 13:10 to 13:40, the one with minimum speed = 1320 has larger cycling ratio (runs for longer period) than the one with minimum speed = 900.

The problem is likely related to two issues in the cycling ratio calculation.

1. The position of the cycling ratio calculation

Similar to the cycling ratio problem in the cooling side. Whether the demand is less than minimum speed capacity is checked at two places. The first one will short-circuit and prevent the Te adjustment step (orange box in the following simplified flow diagram)

The following is the diagram in the documentation

2. Discontinuity

When the demand is very low, the cycling ratio is calculated as follows. When Q_h_TU_PL <= CompEvaporatingPWRSpdMin, it will enter branch 1. When Q_h_TU_PL > CompEvaporatingPWRSpdMin, it will enter branch 2. C_cap_operation is generally close to 1. However, when Q_h_TU_PL is close to CompEvaporatingPWRSpdMin, depending on which branch it ended up in, the result could be very different. If we assume CompEvaporatingPWRSpdMin and CompEvaporatingCAPSpdMin have similar magnitude, and C_cap_operation is indeed close to one,

• when Q_h_TU_PL is slightly smaller than CompEvaporatingPWRSpdMin, then CyclingRatio is close to 1
• when Q_h_TU_PL is slightly larger than CompEvaporatingPWRSpdMin, then CyclingRatio is close to 0

        Q_c_OU = max(0.0, Q_h_TU_PL - CompEvaporatingPWRSpdMin);
        ...
            if (Q_c_OU == 0) { // branch 1
                // Q_h_TU_PL is less than or equal to CompEvaporatingPWRSpdMin
                CyclingRatio = Q_h_TU_PL / CompEvaporatingPWRSpdMin;
                this->EvaporatingTemp = OutdoorDryBulb;
            } else { // branch 2
                // Q_h_TU_PL is greater than CompEvaporatingPWRSpdMin
                CyclingRatio = Q_c_OU * C_cap_operation / CompEvaporatingCAPSpdMin;
                this->EvaporatingTemp = max(CapMinTe, RefTLow);
            }

02/20 13-1,branch 2 ----- cyclingRatio=0.5424031528859093,Q_h_TU_PL=772.3616736028802,CompEvaporatingPWRSpdMin=191.07128170025996,C_cap_operation=0.895930180163394
02/20 13-2,branch 2 ----- cyclingRatio=0.010679792435435032,Q_h_TU_PL=202.516754863906,CompEvaporatingPWRSpdMin=191.07128170025996,C_cap_operation=0.895930180163394
02/20 13-3,branch 1 ----- cyclingRatio=0.7196798169795096,Q_h_TU_PL=137.5101450440834,CompEvaporatingPWRSpdMin=191.07128170025996
02/20 13-4,branch 1 ----- cyclingRatio=0.9347582121695664,Q_h_TU_PL=178.6054496790826,CompEvaporatingPWRSpdMin=191.07128170025996

Defect files

US+SF+CZ4A+hp+crawlspace+IECC_2006_VRF_minSpd900.idf.txt
US+SF+CZ4A+hp+crawlspace+IECC_2006_VRF_minSpd1320.idf.txt
USA_NY_New.York-John.F.Kennedy.Intl.AP.744860_TMY3.epw.txt

Details

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