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 he 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.

REGISTERS

There are arguments in favor of side-wall registers.  They save floor space and obviate some dust.  On the other hand, they are not quite so effective in heating as the other sort, since the pipes for floor registers may be of larger diameter and as a rule require fewer bends.  Each register should have a separate pipe from the furnace.  Where direct heat is not desired, a register opening in the ceiling of a downstairs room will sometimes carry enough heat to the upper chamber to make it comfortable for sleeping purposes.

Since furnace efficiency is largely dependent upon air control, a strong wind sometimes makes it difficult to heat portions of the house.  To meet this emergency there is a combination hot-air and hot-water heater which supplies radiators on the upper floors, or elsewhere if desired.  The additional cost is practically all in the installation, as the same fire furnishes both forms of heat.

For an eight-room house or smaller, a first-class steel-plate furnace, securely sealed against the escape of gas and smoke, costs free on board about $150.  Each two rooms additional raises the price about $25.  Other furnaces may be had as low as $50.  Cost of tin work, brick setting, etc., depends upon locality.

HOT WATER AND STEAM HEAT

Hot water and steam heat cost more for installation, but have many advantages over the furnace.  Their chief drawbacks are the space usurped by radiators, lack of ventilation, and the possibility of an occasional breakdown.  The ingenuity of the makers, however, is partly overcoming these difficulties, mainly by the device called the indirect system.