A thermostat is placed in the tempered-air part of the plenum room to maintain the proper temperature of the tempered air. This thermostat operates the by-pass damper under the tempering coils, and sometimes the valves on the coils. The mixing dampers at the base of the galvanized-iron ducts are controlled by their respective room thermostats. Attic-vent, fresh-air, and return-air dampers are under pneumatic switch control. A humidifier can be provided readily for this system. This system of heating is designed particularly for school houses where adequate ventilation is a necessity. =165. The Thermostat.=--One of the many examples of the expansion of metals is shown in one form of the thermostat (Fig. 148) in which two pieces of different metals and of unequal rates of expansion, as brass and iron, are securely fastened together. The thermostatic strip _T_ moving inward and outward, as affected by the room temperature, varies the amount of air which can escape through the small port _C_. When the port _C_ is completely closed (Fig. 148_a_) the full air pressure collects on the diaphragm _B_ which forces down the main valve, letting the compressed air from the main pass through the chamber _D_ into chamber _E_ as the valve is forced off its seat. The air from chamber _E_ then passes into the branch to operate the damper. When port _C_ is fully open (Fig. 148_b_) the air pressure on diaphragm _B_ is relieved, the back pressure in _E_ lifts up the diaphragm and the air from the branch escapes out through the hollow stem of the main valve, operating the damper in the opposite direction from that when _C_ is closed. Important Topics 1. Transmission of heat in fluids. 2. Convection. Drafts of a chimney. Land and sea breezes. 3. Heating and ventilation of buildings. (a) By hot air. (b) Hot-water heating. (c) Steam heating. (d) Direct and indirect heating. (e) Vacuum steam heating. (f) The plenum system. (g) The thermostat. Exercises 1. Is a room heated mainly by conduction, convection, or radiation, from (a) a stove, (b) a hot-air furnace, (c) a steam radiator? 2. Name three natural convection currents. 3. Explain the _draft_ of a chimney. _What_ is it? _Why_ does it occur? 4. Make a _cross-section_ sketch of your living room and indicate the convection currents by which the room is heated. _Explain_ the heating of the room. 5. Make a sketch showing how the water in the hot-water tank in the kitchen or laundry is heated. Explain your sketch, indicating convection currents. 6. Is it economical to keep stoves and radiators highly polished? Explain. 7. If you open the door between a warm and a cool room what will be the direction of the air currents at the top and at the bottom of the door? Explain. 8. If a hot-water heating system contains 100 cu. ft. of water how much heat will be required to raise its temperature 150°F.? 9. Why does a tall chimney give a better draft than a short one?