The extended use of hot-water heating has led to the invention of many appliances for the improvement of the circulation and heating effects. Pulsation valves are used for retaining the water in the boiler until a definite pressure has been attained that will lift the valve long enough to dissipate the pressure. Many of these systems possess merit and some of them are great improvements over the simple plant. =Radiator Connection.=--The method of connecting the radiators to the distributing pipes depends entirely on local conditions. In a well-balanced system any of the methods shown in Figs. 38, 39 or 40 might be used with good heating effects. The method of attaching the supply pipe to the radiator is, however, an important factor in case of accumulation of air. In Fig. 41 is shown the form of connection most commonly used. The drawing is intended to represent a cast-iron radiator with the valve at _D_, and the air vent at _B_. Should air collect in the radiator it will rise to the top and displace the water. The water will continue to circulate and heat as much of the radiator as is in contact with the water, but that part not in contact will receive no heat from the water and will, therefore, fail to fulfill its function. As soon as the air vent is opened the air will escape and allow the water to entirely fill the space. [Illustration: FIG. 38. FIG. 39. FIG. 40. FIGS. 38 TO 40.--Various methods of attaching the supply and return pipes to hot-water radiators.] [Illustration: FIG. 41.--The effect of accumulation of air in a hot-water radiator with bottom connections.] [Illustration: FIG. 42.--With this method of connections, if the air collects sufficiently to force the water down to the level L, circulation will stop.] In Fig. 42 a much different condition exists, when air accumulates. In this mode of connection the water enters through the valve _V_, and escapes at the bottom of the opposite end. When air fills the radiator to the line _L_, the circulation is stopped and the radiator will grow cold. The position of the valve on these radiators is of little consequence. The valve is intended merely to interrupt the flow of the water and may occupy a place on either end of the radiator with the same result. =Hot-water Radiators.=--Radiators for hot-water heating are most commonly of cast iron and in appearance are the same as those used for steam heating. The only difference in the two forms is in the openings between the sections. Those intended for steam have an opening at the bottom joining the sections; while those for hot water have openings at both top and bottom to permit circulation of the water. [Illustration: FIG. 43.--The hot-water radiator valve.] [Illustration: FIG. 43_a_.--Details of construction of the hot-water radiator valve.]