THERMOSTATS

The thermostat is a control that responds to changes in temperature and directly or indirectly controls the temperature. There are many different designs of thermostats. Figure 14-20 shows a few of the common thermostats used in modem heating and cooling systems. Thermostats are of three types: heating, cooling, and dual (combined heating and cooling thermostat in one). Figure 14-20(B).

Figure 14-20.-Thermostats.

The common sensing element of a thermostat is bimetal. A bimetal sensing element simply uses two different types of metal, brass and invar, which have different expansion rates. Figure 14-20(A) depicts three common profiles of bimetals used in thermostats.

Figure 14-20(A).-Bimetal profiles.

The bimetal element in 14-20(B) has a set of contacts on one end. The top contact is fixed. When the temperature changes around the bimetal, the two contacts open or close.

When the contacts close, a path is created for current to flow. The snap action in the magnetic type makes the contacts close or open quickly. This eliminates any spark and extends the life of the contacts. Figure 1420(C) shows enclosed contacts that use a

Figure 14-20(C).-Contacts.

bimetal element for movement and contacts or mercury for making contact between two electrodes.

The manufacturing engineers determine what type and design of thermostat should be installed in a particular system. Knowing and understanding the advantages and disadvantages of different types of thermostats will help you identify the type of thermostat being used in a system and enable you to troubleshoot an inoperative system efficiently.

Electrical room thermostats are in three categories: line voltage, low voltage, and millivoltage. Line-voltage thermostats are usually 115 volts. When line-voltage thermostats are installed, there is no need for lowering voltage with a transformer. However, line-voltage thermostats are dangerous for the users and the cost is higher. Normally line-voltage thermostats are located only in industrial commercial applications.

Low-voltage thermostats (24 volts) are not dangerous to the user. They are also more cost efficient than line-voltage models. The disadvantage of low-voltage thermostats is the extra requirements of wiring and additional components; they are less rugged than linevoltage thermostats.

The millivoltage thermostat operates at 750, 500, or 250 millivolts. This thermostat uses its own power source for operation and is not affected by power interruptions. The system requires only a small amount of wiring compared to other systems. However, this system is limited for use only in heating applications. The temperature control is less precise than other systems, wire length and size are critical, and the system requires a separate device to power a 24-volt control, or you must use a millivoltage control.

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