The high pressure alarm protects the compressor when the pressure on the high side of the refrigerant circuit exceeds the permissible limit.
In heating mode
Low water flow, a clogged exchanger, or an excessively high supply temperature can increase the condensation pressure.
In cooling mode
Restricted airflow to the external exchanger or a fan failure can lead to high pressure.
Other causes
Incorrect refrigerant charge or a problem with the refrigerant circuit valves.
Diagnosis
Actual pressures, temperatures, flow rates, and the condition of the exchangers are checked before any reset is performed.