Heat Pump 12 Hight Pressure trips only in Heating

Basic diagnostics scenario, High pressure trips only in heating mode but operates normally in AC mode.

This does not cover all the possibilities but a few to get you thinking about the heat pump cycle.

1st is the indoor and outdoor temperature low in the HP operating range. I have had techs call with this issue while the indoor or outdoor temperature was too high. generally the outdoors temperature should be below 70F and the indoor temperature below 80F. If the temperatures arehigh such as some days in the spring fall or in summer, this could cause the pressure to rise too high.

2nd, airflow! In cooling mode, low airflow will still be an issue but the customer may not notice but in heating mode the indoor (now condenser) coil is much smaller. With lack of airflow across the coil, the temp rises and the pressure follows. Airflow is usually more critical in Heat Pump mode than AC. This can be dirty blower wheel, blower speed, dirty coil, dirty or restrictive filter, blocked or undersized ductwork return or supply, too many registers closed and more.

3rd Indoor bypass check valve stuck closes or missing.
Even a fixed orifice can clog in one direction but not the other. Many residential TXV's have an internal bypass check valve inside. If this becomes clogged or restricted it will not allow the refrigerant to bypass causing a pressure drop and restriction. Some older TXV and all large commercial TXV use an external check valve that can cause the same problems'. If the TXV used does not have a bypass check valve at all then it will be an issue in heating mode.

4th a restriction in the liquid line such a a clogged filter drier or a kink in the liquid line.

5th The outdoor metering device is clogged up or malfunctioning

6th The outdoor coil has a restriction inside the tubing. This is more of an issue with micro channel

7th Overcharged. since more refrigerant is used in the cooling mode than heating mode, an over charge may show up in HP mode where the extra refrigerant as nowhere to go.