This section is from the book "A Treatise On Architecture And Building Construction Vol4: Plumbing And Gas-Fitting, Heating And Ventilation, Painting And Decorating, Estimating And Calculating Quantities", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Automatic Temperature Regulators. Regulators that are commonly used to automatically control the operations of a heating apparatus are arranged either to operate the furnace dampers and thus control the fire, or to vary the adjustment of a mixing valve, or to open and close or regulate the apertures of valves which govern the flow of the heating fluids.
There is a class of damper regulators which are operated by the pressure of the steam in the boiler, but they are useful only for regulating combustion of the fuel and so maintaining a constant pressure of steam.
There is another class of regulating apparatus which is operated by the heat of the air in the room that is to be warmed. These are called thermostats.
The internal construction of a thermostat which operates by the expansion of a volatile liquid is shown in Fig. 42. It consists of a metal shell which is divided into two chambers a and b by the flexible corrugated diaphragm shown. The chamber b is partly filled with the liquid, as shown. The pressure of the vapor above the liquid is always proportional to the temperature. Naphtha is an excellent material for this purpose, because the pressure of the vapor at temperatures of 70° or 80° amounts to several pounds per square inch, and thus furnishes sufficient force to operate ordinary dampers, etc.
When the temperature of the air around the thermostat increases, the vapor pressure rises correspondingly and forces the diaphragm towards the chamber a. The movement of the diaphragm may be utilized in any convenient manner to operate valves, etc. If the valve.or damper moves easily it may be operated by means of a diaphragm motor, the tube e, which connects the thermostat to the motor, being filled with water. The motion of the thermostat diaphragm will thus be transmitted to the motor diaphragm and to a lever, which will increase it sufficiently to operate an ordinary damper.
for the same general purpose, having various degrees of merit.
Thermostats when in use are usually covered with an ornamental grille, and are attached to the side walls of the room. Care must be taken in locating them to secure a position where the temperature will be the average temperature of the room, or nearly so. Therefore, the vicinity of hot-air inlets or foul-air outlets should be avoided.