40. ‘heat generator’ means the part of a heating system that generates useful heat for uses identified in Annex I, using one or more of the following processes:
(a) the combustion of fuels in, for example, a boiler
(b) the Joule effect, taking place in the heating elements of an electric resistance heating system
(c) capturing heat from ambient air, ventilation exhaust air, or a water or ground heat source using a heat pump