A method of producing a conduit system for use on-board an aircraft, the method comprising: identifying a plurality of source apparatuses, each source apparatus being apparatus from which a respective resource is to be transferred via the conduit system, each source apparatus being located on-board the aircraft; for each identified source apparatus, identifying one or more sink apparatuses to which that source apparatus is to be coupled via the conduit system such that a resource may be transferred via the conduit system from that source apparatus to those one or more sink apparatuses, each sink apparatus being located on-board the aircraft; using relative locations of the identified source and sink apparatuses, producing a digital model for the conduit system, producing the digital model including: for each source apparatus, specifying a conduit for coupling that source apparatus to each of the one or more sink apparatuses to which that source apparatus is to be coupled; from the specified conduits, identifying a first conduit that is to contain in use relatively high temperature contents; from the specified conduits, identifying a second conduit that is to contain in use relatively low temperature contents; and arranging the conduits of the conduit system in a nested configuration such that at least part of each conduit of the conduit system shares a common conduit wall with at least part of at least one other conduit of the conduit system such that, in use, heat transfer between the first conduit and second conduit is maximised; and using the digital model, performing, by an Additive Manufacturing apparatus, an Additive Manufacturing process to manufacture the conduit system as a single metallic object; wherein each of the source apparatuses and each of the sink apparatuses is an aircraft system on-board the aircraft. identifying a plurality of source apparatuses, each source apparatus being apparatus from which a respective resource is to be transferred via the conduit system, each source apparatus being located on-board the aircraft; for each identified source apparatus, identifying one or more sink apparatuses to which that source apparatus is to be coupled via the conduit system such that a resource may be transferred via the conduit system from that source apparatus to those one or more sink apparatuses, each sink apparatus being located on-board the aircraft; using relative locations of the identified source and sink apparatuses, producing a digital model for the conduit system, producing the digital model including: for each source apparatus, specifying a conduit for coupling that source apparatus to each of the one or more sink apparatuses to which that source apparatus is to be coupled; from the specified conduits, identifying a first conduit that is to contain in use relatively high temperature contents; from the specified conduits, identifying a second conduit that is to contain in use relatively low temperature contents; and arranging the conduits of the conduit system in a nested configuration such that at least part of each conduit of the conduit system shares a common conduit wall with at least part of at least one other conduit of the conduit system such that, in use, heat transfer between the first conduit and second conduit is maximised; and using the digital model, performing, by an Additive Manufacturing apparatus, an Additive Manufacturing process to manufacture the conduit system as a single metallic object; wherein each of the source apparatuses and each of the sink apparatuses is an aircraft system on-board the aircraft. for each source apparatus, specifying a conduit for coupling that source apparatus to each of the one or more sink apparatuses to which that source apparatus is to be coupled; from the specified conduits, identifying a first conduit that is to contain in use relatively high temperature contents; from the specified conduits, identifying a second conduit that is to contain in use relatively low temperature contents; and arranging the conduits of the conduit system in a nested configuration such that at least part of each conduit of the conduit system shares a common conduit wall with at least part of at least one other conduit of the conduit system such that, in use, heat transfer between the first conduit and second conduit is maximised; and using the digital model, performing, by an Additive Manufacturing apparatus, an Additive Manufacturing process to manufacture the conduit system as a single metallic object; wherein each of the source apparatuses and each of the sink apparatuses is an aircraft system on-board the aircraft.