271. Air may be moved and distributed with equal facility by either of these systems, provided that the difference in pressure, above or below the atmosphere, is the same, and that the apparatus is of equal capacity. But the leakage of air which takes place through every hole and crevice in the walls, and around every loose-fitting window, etc., affects the exhaust system far more seriously than the other. When air is drawn through a heating apparatus consisting of a hot-air furnace, some of the products of combustion are certain to leak through the furnace joints and poison the air supply. With the pressure system, no leakage of gas is likely to occur. On the contrary, the fresh air is likely to force its way into the interior of the furnace.

The practical difference between the two systems may be clearly perceived by considering the exhaust system as having a single outlet, but a multitude of inlets; while the pressure system has numerous outlets, but only one inlet. The purity of the air supply must be controlled at the inlet, and it is practically much easier to supervise one large inlet than a multitude of small ones. Otherwise, the real points of difference in the two systems are unimportant.

Usually the terms exhaust and pressure are applied only to those systems which are operated by fans or steam blowers, but the same difference in principle exists between those operated by heat or gravity. Thus, the aspiration system, which depends upon the draft of a central foul-air chimney, is essentially an exhaust system, because the pressure within the rooms is slightly lower than in the external atmosphere. The ordinary natural-draft system, operated by a hot-air furnace or by indirect radiators, is in principle a pressure system, the internal pressure being a little above that of the atmosphere. The difference in either case is the measure of the force of the draft.