Described herein is a system and a method for controlling operation of a heat pump. The system comprises an electronic expansion valve (EXV) configured to fluidically connect an indoor unit of the heat pump with an outdoor unit of the heat pump, and a controller operatively connected to the EXV, where the controller is configured to issue a first control signal, during a first defrost mode, to at least partially close the EXV to increase discharge pressure in an outdoor coil of the outdoor unit to facilitate defrosting of the outdoor coil.

A heating system having a plurality of outdoor air source heat pumps. An indoor heat transfer system receives heat from the plurality of outdoor air source heat pumps, and uses the heat for a heating application. The heating system is configured to control the plurality of outdoor air source heat pumps so as to reduce an occurrence of the plurality of outdoor air source heat pumps simultaneously being in a defrost cycle. Water to water heat pump(s) are provided in a cascade configuration with the indoor units to produce higher temperature water for the heating application.