In the first place, all hot air pipes which are not intended to be exposed must be put in position. If a hot air system has been selected the position of the pipes and registers will have been marked on the plans, but it frequently happens that in the execution, changes will suggest themselves or will be required by unforeseen circumstances, so that the superintendent should make a personal study of the piping at the building. Where it is possible to arrange it, the pipes should be run through closets, exposed to view, but this cannot always be done, and it often happens that the chimney breasts and sometimes the partitions must contain hot air pipes. The best and safest way to do this is to make the pipes double with at least a half-inch air space between the outer and inner pipes. This is expensive and is not generally done, but instead the woodwork which comes close to the pipe is lined with bright tin, and heavy wire lathing is used in front of the pipes instead of wood laths. Where the pipes are carried through the partition or woodwork a sleeve of bright tin half an inch larger than the pipes all around should be provided as a safeguard against over-heating of the pipes.