Some models of heat pump require a Hydrobox to be installed inside the property, which needs to be accounted for for pressure loss and space purposes. The basic Hydrobox has a single inlet and outlet (it sits on the heat pump "flow", with the return pipe going directly back to the outdoor unit without connecting to the heat pump" and contains the pump for the system (which is omitted from the outdoor unit. This could be simply represented by a volumizer or valve with a circulating pump attached