A method for utilizing heat in a plant or animal growing device includes circulating a heat transfer fluid through a circuit forming a closed fluid loop, heating, via a heat source, the heat transfer fluid in the fluid circuit to a temperature within an efficient operating range of a first heat unit, supplying heat from the heat transfer fluid to a first heat unit, the first heat unit cooling down at least part of the heat transfer fluid to a temperature within an efficient operating range of at least one additional heat unit connected in serial arrangement with the heat source and the first heat unit, supplying heat from the heat transfer fluid from the first heat unit to the additional heat unit, the additional heat unit cooling down at least part of the heat transfer fluid, and returning the cooled down part of the heat transfer fluid from the additional heat unit to the heat source in the fluid circuit.
