This class of buildings may be satisfactorily warmed by direct steam, hot water, or where ventilation is desired by the fan system. Probably direct steam is used more frequently than any other system for this purpose. Vacuum systems are well adapted to the conditions usually found in this type of building, as most modern office buildings have their own light and power plants, and the exhaust steam can be thus utilized for heating purposes. The piping may be either single or double. If the former is used it is better to carry a single main riser to the upper story and run drops to the basement, as by this means the flow of steam and water are in the same direction and much smaller pipes can be used than would be the case if risers were carried from the basement upward. Special provision must be made for the expansion of the risers or drops in tall buildings. They are usually anchored at the center and allowed to expand in both directions. The connections with the radiators must not be so rigid as to cause undue strains or lift the radiators from the floor.

It is customary in most cases to make the connections with the end farthest from the riser; this gives a length of horizontal pipe which has a certain amount of spring, and will care for any vertical movement of the riser which is likely to occur. Forced hot-water circulation is often used in connection with exhaust steam. The water is warmed by the steam in large heaters, similar to feed-water heaters, and circulated through the system by means of centrifugal pumps. This has the usual advantage of hot water over steam, inasmuch as the temperature of the radiators may be regulated to suit the conditions of outside temperature.