Which of the following is included in the definition of plumbing according to building regulations?

The definition of plumbing according to building regulations includes plumbing appurtenant to a structure because it encompasses all components directly related to the functioning of plumbing systems within or associated with that structure. This includes pipes, fixtures, and any related equipment that serve the plumbing needs of a building. Plumbing appurtenances are essential for the conveyance of water and waste, both of which are crucial elements in building design and maintenance.

The other choices detail systems that do not fall under the plumbing category defined by building regulations. For instance, electrical systems and HVAC systems are specialized fields governed by their own sets of regulations and standards, distinct from plumbing. Additionally, plumbing not located within any structure signifies an absence of direct relation to the intended plumbing systems being regulated, making it irrelevant in defining what constitutes plumbing under these building regulations.