Automatic Control: Thermostats

Press, Volume CVI, Issue 31349, 20 April 1967, Page 21

Automatic Control: Thermostats

Today’s electric heating men are fortunate in having a great variety of thermostats available to meet the simplest as well as the most exacting requirements. These thermostats may be divided into two basic types: Line voltage and low voltage. , In general, line-voltage thermostats are most applicable to installations requiring simple control at low cost and with moderate degrees of accuracy. Low voltage controls are most applicable to installations requiring highly-accurate control, switching of heavy loads where a relay is required, or special automatic features and system refinements. Both low-volt-age and tine-voltage types are available in models of varying degrees of cost and quality to fit the application. The line-voltage thermostat is basically a linevoltage switch operated by a temperature - sensitive element The temperature sensing is done by either a bi-metal element or a liquidfilled bellows. It provides direct switching of current to the heater.

Some line-voltage thermostats'are mounted on, or partly inside, the enclosure of the heater they control. They are usually factory installed.

The other major type of line-voltage thermostat is designed for mounting on the wall at a location some distance from the heater. These are the wall-mounted thermostats which appear in many forms and models for a variety of applications. Wall-mounted thermostats offer the advantage of being up where they are easy to see and adjust They are also away from the heater and in the room area where they get a more typical sampling of room temperature. To carry the necessary line-voltage loads, the thermostat switch must be relatively large, and so it takes considerable power to move it This means the sensing element must be quite heavy in construction. It tends to respond slowly to temperature changes. This is the reason some line-

voltage thermostat cycle with less than one on and off period an hour. As a result room temperatures may swing far beyond the normal comfort range. Long off periods allow the air to settle in layers with the valuable warm air wasted at the ceiling while cold air settles to the floor. “Cold floors” is a serious complaint of many home owners even though they may otherwise have modern systems. Faster-cycling thermostats will do a great deal toward eliminating this complaint. Another problem with any line-voltage thermostat is “control point drift.”

Heat is generated by the current passing through the switch and raises the temperature within the thermostat case. The inside of the case becomes warmer than the room. As a result, the thermostat Shuts off the heater before it should. Even a top-quality, linevoltage thermostat (with the switch largely insulated from the sensing element) may shift its control point as much as six degrees from partial to full load. This is one of the reasons why most line-voltage thermostats do not have degree-readings on the setting scale. It is also the reason why they do not have thermometers on them.

In home applications, especially, the line voltage thermostat to be used should be selected very carefully according to the quality and performance features. In many cases it may be desirable to use low voltage controls instead to obtain the greater accuracy and the added special features needed tb help the electric heating system perform as it should. Essentially, a low voltage control system is a combination of a light-duty thermostat and a heavy-duty switching relay. The low voltage for control circuit is usually obtained from a regular 230volt source stepped down to 24-volt by a small trans-

former. Low voltage thermostats offer a number of important advantages that are not as available, and often impossible to get, with other types of control. Because the thermostat transmits only the small current required to operate the relay, it can be light in construction and extremely sensitive. There is no problem of false heat from the switch as the operating current is very small. If desired, for special models, additional switching and control functions can readily be incorporated into tha thermostat mechanism. Sensitivity can be within a fraction of a degree. A built-in heater provides a

cycling rate to give at least six cycles an hour for uniform heating.

The thermostat cycling pattern is designed to give short, frequent on cycles with longer off periods when the demand for heat is slight. It shifts the length of these cycles to give longer on periods and shorter off periods as the need for heat increases.

Because low voltage thermostats are accurate, they usually have a temperature setting scale on the face. The home-owner can select the exact temperature he wants. In addition, a thermometer is usually provided to check the room temperature.

One of the biggest, overall advantages of low voltage control is the variety of control features it makes possible. The relay can be used with a complete choice of low-voltage thermostats. This makes it easy to give the home-owner the exact operating features he wants. The basic room thermostat is the kind recommended for everyday home applications where accuracy of control is desired. Home-owners want easy-to-read thermometers and setting dials as well as attractive design to blend with their room decoration. Manual timer thermostats are made for couples who are away from home much of the day or for those who have irregular sleeping schedules. A spring-wound timer on the thermostat lets the home-owner lower the temperature to save fudl while he is away or asleep. At the end of the time period, it automatically returns to the day-time comfort setting so the home will again be pleasantly warm. The electric clock thermostat is designed to provide completely carefree control, and requires no attention from the home-owner. On a pre-selected schedule, it automatically lowers the temperature each night and raises it each morning automatically. However, should the home-owner wish to stay up later than usual, he need only shift the night setting to the day-time level. Centralised control panels are primarily designed to give finger-tip control of day and night room temperature settings without the; need to walk from room to room. From one conveni-

ent location in his livingroom, hall or den, the homeowner can raise or lower the temperature in any room, or he may turn off the heat. This makes it easy to save'operating costs in an unoccupied part of the home—also to lower the temperatures for sleeping comfort as well as economy at night.

The wide choice of automatic control equipment made possible by low-volt-age control gives the electric industry a full range of features to offer in selling electric heating. Each installation can be matched not only to the needs of the home but also the personal wishes of the owner.

Like all things electrical* electric heating will becoine better as the industry makes it better, automatically.