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In the context of optimization, redundancy in a system refers to the fact that there is more than one path for water to take to a particular node.

System redundancy must be carefully considered before initiation of optimization. The importance of redundancy in the part of the system to be sized depends on the particular system. Redundancy is important in the sizing of an entire addition to an existing system. It may be less important, or indeed unnecessary in the case where the reinforcement to an existing system is to be sized, since the existing system may already provide the necessary redundancy. Redundancy can be controlled in this optimization routine in several ways. First, it is possible in the size list of a specific pipe group to limit the search for alternatives to specific pipe diameters and not to allow the program to consider cleaning or pipe elimination (i.e. not to specify C nor 0 in the size list). The selected pipe size for the group will then be at least the minimum size specified. Redundancy is also controlled through the multiple loading constraints, by assigning a fire load to various nodes. Pipes which could be eliminated under one load case may be essential in another one in order to meet the pressure constraint. Alternatively, two pipes serving a node can be placed in the same group which would force both pipes to be included in the final solution.