A thermodynamic solar system suitable for generating hot water (5a, 5b) in at least one area of a building comprising: at least one heat transfer panel (1, 1a) for taking in heat from an external environment including the sun, to convert a refrigerant fluid from liquid into a vapour; a heat pump for compressing refrigerant vapour from the panel into hot refrigerant vapour, and; a condenser (10, 20, 100) for transferring heat from the hot refrigerant vapour to a water supply (5, 5c); an expansion valve through which, in use, cooled refrigerant vapour from the condenser passes before being supplied back into the panel (1, 1a); a fluid control valve having a first state in which, in use, refrigerant fluid from the panel (1, 1a) passes through the fluid control valve into the heat pump and then passes back through the fluid control valve into the condenser (10, 20, 100); and control means for controlling the fluid control valve; characterised in that the heat transfer system further comprises: one or both of a temperature sensor and/or a humidity sensor for measuring ambient temperature and/or humidity conditions of the outside environment; and wherein the control means is configured for monitoring the temperature sensor and/or humidity sensor, and wherein the control means is configured to cause the fluid control valve to move from the first state to a second state in which the flow of refrigerant is reversed to divert hot refrigerant vapour from the heat pump back into the panel (1, 1a) when the ambient conditions measured by the temperature sensor and/or humidity sensor are cold enough or include sufficient air moisture for frost or ice to form on the panel. at least one heat transfer panel (1, 1a) for taking in heat from an external environment including the sun, to convert a refrigerant fluid from liquid into a vapour; a heat pump for compressing refrigerant vapour from the panel into hot refrigerant vapour, and; a condenser (10, 20, 100) for transferring heat from the hot refrigerant vapour to a water supply (5, 5c); an expansion valve through which, in use, cooled refrigerant vapour from the condenser passes before being supplied back into the panel (1, 1a); a fluid control valve having a first state in which, in use, refrigerant fluid from the panel (1, 1a) passes through the fluid control valve into the heat pump and then passes back through the fluid control valve into the condenser (10, 20, 100); and control means for controlling the fluid control valve; characterised in that the heat transfer system further comprises: one or both of a temperature sensor and/or a humidity sensor for measuring ambient temperature and/or humidity conditions of the outside environment; and wherein the control means is configured for monitoring the temperature sensor and/or humidity sensor, and wherein the control means is configured to cause the fluid control valve to move from the first state to a second state in which the flow of refrigerant is reversed to divert hot refrigerant vapour from the heat pump back into the panel (1, 1a) when the ambient conditions measured by the temperature sensor and/or humidity sensor are cold enough or include sufficient air moisture for frost or ice to form on the panel. one or both of a temperature sensor and/or a humidity sensor for measuring ambient temperature and/or humidity conditions of the outside environment; and wherein the control means is configured for monitoring the temperature sensor and/or humidity sensor, and wherein the control means is configured to cause the fluid control valve to move from the first state to a second state in which the flow of refrigerant is reversed to divert hot refrigerant vapour from the heat pump back into the panel (1, 1a) when the ambient conditions measured by the temperature sensor and/or humidity sensor are cold enough or include sufficient air moisture for frost or ice to form on the panel.