This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The principal systems of automatic temperature control now in use consist of three essential features: First, an air compressor, reservoir and distributing pipes; second, thermostats, which are placed in the rooms to be regulated; and third, special diaphragm or pneumatic valves at the radiators. The air compressor is usually operated by water pressure in small plants and by steam in larger ones; electricity is used in some cases. Fig. 30 shows a form of water compressor. It is similar in principle to a direct-acting steam pump, in which water under pressure takes the place of steam. A piston in the upper cylinder compresses the air, which is stored in a reservoir provided for the purpose. When the pressure in the reservoir drops below a certain point, the compressor is started automatically, and continues to operate until the pressure is brought up to its working standard.
A thermostat is simply a mechanism for opening and closing one or more small valves, and is actuated by changes in the temperature of the air in which it is placed. Fig. 31 shows a thermostat in which the valves are operated by the expansion and contraction of the metal strip E. The degree of temperature at which it acts may be adjusted by throwing the pointer at the bottom one way or the other. Fig. 32 shows the same thermostat with its ornamental casing in place. The thermostat shown in Fig. 33 operates on a somewhat different principle. It consists of a vessel separated into two chambers by a metal diaphragm. One of these chambers is partially filled with a liquid, which will boil at a temperature below that desired in the room. The vapor of the liquid produces considerable pressure at the normal temperature of the room, and a slight increase of heat crowds the diaphragm over and operates the small valves in a manner similar to that of the metal strip in the case just described.
The general form of a diaphragm valve is shown in Fig. 34. These replace the usual hand valves at the radiator. They are similar in construction to the ordinary globe or angle valve, except the stem slides up and down instead of being threaded and running in a nut. The top of the stem connects with a flat plate, which rests against a rubber diaphragm. The valve is held open by a spring, as shown, and is closed by admitting compressed air to the space above the diaphragm.
In connecting up the system, small concealed pipes are carried from the air reservoir to the thermostat, which is placed upon an inside wall of the room, and from there to the diaphragm valve at the radiator. When the temperature of the room reaches the maximum point for which the thermostat is set, its action opens a small valve and admits air pressure to the diaphragm, thus closing off the steam from the radiator. When the temperature falls, the thermostat acts in the opposite manner, and shuts off the air pressure from the diaphragm valve, and at the same time opens a small exhaust which allows the air above the diaphragm to escape. The pressure being removed the valve opens and again admits steam to the radiator. Thermostats and diaphragms are also used for operating mixing dampers in a similar manner.