A district heating and/or cooling system comprising: a set of three or more conduits (2, 3, 4, 5) for carrying a thermal energy transfer fluid, each conduit carrying the fluid at a different temperature, a plurality of consumer appliances (22-1, 22-2, 23-1, 23-2, 24-1, 24-2, 24-3, 24-4, 24-5) distributed along the length of the conduits, each consumer appliances being linked on a higher temperature side to a first one of at least two conduits having thermal transfer fluid at different temperatures and linked at a lower temperature side to a second one of at least two conduits having thermal transfer fluid at different temperatures and being other than the first one, the consumer appliance being adapted for either: receipt of the fluid from the first one of the conduits and for extraction of heat energy and to discharge the fluid into the second one, or receipt of the fluid from the second one of the conduits and for absorption of heat energy for cooling and to discharge the fluid into the first one, further comprising: a plurality of heat generating plants (12, 13, 14), each pair of conduits being connected to at least one of the plants to either: raise the temperature of the fluid in the lower temperature conduit of the pair to the temperature of the fluid in the higher temperature conduit of the pair, or lower the temperature of the fluid in the higher temperature conduit of the pair to the temperature of the fluid in the lower temperature conduit of the pair, characterized in that the district heating and/or cooling system is adapted so that the flow of fluid in all of the conduits is bi-directional relying on equalising flows back and forwards within the system without a dedicated return conduit. a set of three or more conduits (2, 3, 4, 5) for carrying a thermal energy transfer fluid, each conduit carrying the fluid at a different temperature, a plurality of consumer appliances (22-1, 22-2, 23-1, 23-2, 24-1, 24-2, 24-3, 24-4, 24-5) distributed along the length of the conduits, each consumer appliances being linked on a higher temperature side to a first one of at least two conduits having thermal transfer fluid at different temperatures and linked at a lower temperature side to a second one of at least two conduits having thermal transfer fluid at different temperatures and being other than the first one, the consumer appliance being adapted for either: receipt of the fluid from the first one of the conduits and for extraction of heat energy and to discharge the fluid into the second one, or receipt of the fluid from the second one of the conduits and for absorption of heat energy for cooling and to discharge the fluid into the first one, further comprising: a plurality of heat generating plants (12, 13, 14), each pair of conduits being connected to at least one of the plants to either: raise the temperature of the fluid in the lower temperature conduit of the pair to the temperature of the fluid in the higher temperature conduit of the pair, or lower the temperature of the fluid in the higher temperature conduit of the pair to the temperature of the fluid in the lower temperature conduit of the pair, characterized in that the district heating and/or cooling system is adapted so that the flow of fluid in all of the conduits is bi-directional relying on equalising flows back and forwards within the system without a dedicated return conduit. receipt of the fluid from the first one of the conduits and for extraction of heat energy and to discharge the fluid into the second one, or receipt of the fluid from the second one of the conduits and for absorption of heat energy for cooling and to discharge the fluid into the first one, further comprising: a plurality of heat generating plants (12, 13, 14), each pair of conduits being connected to at least one of the plants to either: raise the temperature of the fluid in the lower temperature conduit of the pair to the temperature of the fluid in the higher temperature conduit of the pair, or lower the temperature of the fluid in the higher temperature conduit of the pair to the temperature of the fluid in the lower temperature conduit of the pair, characterized in that the district heating and/or cooling system is adapted so that the flow of fluid in all of the conduits is bi-directional relying on equalising flows back and forwards within the system without a dedicated return conduit. raise the temperature of the fluid in the lower temperature conduit of the pair to the temperature of the fluid in the higher temperature conduit of the pair, or lower the temperature of the fluid in the higher temperature conduit of the pair to the temperature of the fluid in the lower temperature conduit of the pair, characterized in that the district heating and/or cooling system is adapted so that the flow of fluid in all of the conduits is bi-directional relying on equalising flows back and forwards within the system without a dedicated return conduit.