Heating apparatus for a glass house 3 includes a heat pump 11 through which is circulated air from the glass house 3, via ducts 12 and 13. The heat pump 11 extracts energy from the air circulated through it, and transfers the energy to liquid in a liquid reservoir 8. When it is desired subsequently to heat the glass house 3, air therefrom is no longer circulated through the heat pump 11, and the relatively hot liquid from the reservoir 8 is circulated through a pipe network 2. A boiler 1 supplies any additional heat to the liquid in the pipe network 2, but its output requirements are relatively reduced. Whilst the glass house 3 is being heated, the heat pump 11 may continue to extract heat from atmospheric air, instead of from air from the glass house 3.
