If any mistake in selection of size is to be made, it should be in favor of excess. Most authorities urge the choice of at least a size above that indicated by the heating area. A chimney with suitable draught is imperative. The furnace should be placed in a central location and should be set sufficiently low to permit the essential rise of the heat ducts. If the basement is low the furnace should be depressed. While the heat conveyors should not ascend directly from the furnace, they should not be carried any farther than necessary in a horizontal position. The velocity of heat is diminished in carrying it horizontally, increased vertically. Crooks and turns add to the friction and decrease heating power. Therefore the pipes should be as short and direct as possible. It is not necessary to carry the register to a window on the farther side of the room, say some authorities, as the warm air rises to the ceiling anyway, and the greater length of carry involves a loss in warmth.
Pipes for the first floor should be large. Those for the upper rooms, having a longer vertical range, may be smaller. All the pipes should be double, with an inch air space between, as a protection against fire. Asbestos paper on a single pipe is not regarded as a sufficient precaution, as it is easily torn and quickly wears out.