In cases where the unit will not run at all and a defective thermostat is suspected, simply by-pass the thermostat to see if the compressor will resume running.

This method has a potential shock hazard, so proceed cautiously and observe safety rules.

1. Disconnect the unit from its power source.
2. By-pass the two thermostat terminals with a well insulated piece of wire with insulated “alligator” clips at each end. (The wire should be at least the same gauge as the original wire on the thermostat.) Make sure the clips touch no other part of the unit!
3. Connect the power supply. If the unit starts running, the thermostat is defective.
4. Disconnect the unit again to shut it off. Remove the by-pass wire and replace the defective thermostat.

In commercial refrigeration, there are primarily two types of thermostats in use:

1. The bulb type (see fig. 68). In this type, a bulb attached to the thermostat is fastened to the evaporator coil. The bulb is filled with refrigerant. As the bulb senses temperature changes, the refrigerant in it expands or contracts and transmits these changes through the line to the thermostat, which in turn causes the bellows to flex and open or close the contacts in the mechanism. The thermostat may be located anywhere as long as the bulb touches the evaporator coil and the line is not exposed to extremes of temperature to prevent transmission of erroneous readings to the thermostat. Be sure that the sensing bulb bracket is positioned in a way that the bulb makes full contact with the evaporator coil, and the nuts and bolts are securely fastened.

When coiling or uncoiling the line on bulb-type thermostats, care must be exercised to prevent kinking or breaking it to preclude refrigerant from escaping or becoming trapped in the line. This will render the thermostat useless.

2. The air-coil type (see fig. 69). This type of thermostat is installed somewhere within the refrigerated area, preferably not near a door that is opened frequently, or in the path of the air currents in the unit, to prevent false reactions.

The refrigerant in the coil expands and contracts with temperature changes and transmits signals to the thermostat to control the operation of the unit. When an air-coil thermostat is purchased, determine its cut-in and cut-out range from the instructions (or the dealer) to see if it is suitable for the particular application intended.

As it will be shown later, there is a relation between saturated refrigerant, vapor pressure in the sealed system, and the temperature produced. The operation of the temperature-regulating-pressure controls is based on this principle. Most commercial units use this type of temperature control. Some units employ both thermostat(s) and pressure control(s). Manually operated thermostats are still widely used in commercial kitchen refrigeration where those owners prefer to adjust temperatures periodically to meet their needs. Pressure controls require adjustment by qualified technicians and cannot be manually adjusted by the customer.

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