A thermostat is an automatic switch used to control a heating, ventilation, air conditioning and refrigeration (HVACR) system. The word thermostat has Greek origins: thermos means “hot,” and statos means “standing.” The first systems to use thermostats controlled heating systems and kept space at a “hot standing” temperature as desired. Cooling control came later with the advent of air conditioning.
Most thermostats operate several parts of an HVACR system, including the blower as well as the cooling and heating components. The blower is usually a fan with a duct system that evenly distributes the air to given spaces. The cooling or heating arrangement may control airflow or water flow.
A thermostat is a device that regulates the temperature inside a designated area, such as a building or a refrigerated box. The purpose of the theromstat is to maintain a desired setpoint temperature in the space. The setpoint is the temperature at which the thermostat is set.
The human comfort range for most people is 72° F to 80° F and 40% to 60% relative humidity. Sometimes the indoor air in the winter months gets too dry. A dry, low humidity indoor environment will create a static electricity shock when touching a doorknob or someone else. A humidistat is used to control the moisture or humidity level in an area and can be used in conjunction with a humidifier or dehumidifier. A humidifier adds moisture to an area, whereas a dehumidifier removes moisture from an area.
The oldest thermostatically controlled heating units in the late 1800s used a mercury switch. Today’s modern digital thermostats use a solid-state thermistor to sense the air or water temperature. The semiconductor thermistor element changes resistance with the slightest change of temperature.
In a digital, solid-state thermostat, the resistance is converted into a signal that will operate the various functions that the thermostat may have available. A thermistor’s solid-state sensor is so sensitive that it can maintain a space temperature within ±0.5° F of its setpoint. This sensitivity allows the thermostat with a thermistor sensor to take thermal measurements to within ranges of ±0.02% to ±0.05%, an accuracy that makes them very effective thermometers.
Types of thermostats
The common conception of a thermostat is the control on the wall that operates a heating or cooling system. Figures 1 and 2 show standard thermostats found in homes or business applications. The mechanical thermostat uses moving parts to achieve temperature control. The digital thermostat has few, if any, moving parts. The digital thermostat uses solid-state technology to gain temperature control.
This article focuses on the digital or solid-state thermostat because its use is so widespread. Digital thermostats are more reliable and have fewer moving parts than mechanical ones, and they keep the temperature within ±1° F of its setpoint. Their display is easy to read and interpret. We do, however, still need to understand the basic operation of the old-style mechanical thermostat because some are still in use.
A mechanical thermostat is secured to a subbase that is mounted to the wall. With the cover off the thermostat, as shown in Figure 3, we can see the control wires attached to the subbase. Each subbase is designed for a specific thermostat.
Next, there is the line voltage thermostat, as shown in Figures 4 and 5. In Figure 5, the sensing bulb is placed in the return air stream. When the temperature of the air or water warms the bulb, it causes the pressure diagram or bellows to expand, pushing the movable contacts closed to operate the cooling or refrigeration circuit.
Line voltage thermostats have a pressure-charged sensing bulb that should be in the return air stream. You might find this type of thermostat in a walk-in cooler, a domestic refrigerator or on a window unit just behind the air filter.
The first thermostats used in heating and refrigeration applications were line voltage thermostats. Even today, line voltage thermostats are used in domestic and commercial refrigeration systems. They are also found in window air conditioners. In many designs, the line voltage thermostat must be able to handle the current draw of the compressor since it controls the compressor and a blower motor directly. They are selected to handle the temperature range and the contact’s RLA and LRA.
The temperature range is:
- High temperature for air conditioning and/or heating applications
- Medium temperature for refrigeration (≈40° F) applications
- Low temperature for freezing applications.
Many other types of temperature controls are used, and not all thermal switches are called thermostats. For example, the internal or external overload in a compressor is a thermostat that opens the power circuit when too much current is drawn, or the motor winding becomes too hot. There are discharge line thermostats that open the control circuit to the compressor if the discharge line becomes excessive.
Parts of a thermostat
The thermostat is a thermal switch that controls heating, cooling or refrigeration. A common comfort heating or cooling thermostat is shown in Figure 5 and is composed of two parts:
- Thermostat: thermal switch and switch options such as cooling, heating, and fan on/off operation
- Subbase: connection for the thermostat wires and wall-mounting holes. The thermostat fits over the subbase.
Not all thermostats have a subbase.
The thermostat can be selected to operate on low voltage or high line voltage. A low-voltage thermostat can handle around 30 V. High voltages can damage the device. Line voltage thermostats can handle low or high voltages.
Most thermostats used in residential or light commercial air conditioning applications use a 24-V control design. Some residential and commercial systems use low-voltage DC systems. The term "comfort control" is slowly replacing the word thermostat. This term is common when the system uses a DC voltage or for systems that use communication signals between the thermostat and the rest of the system. Finally, almost every refrigeration system and window unit use line voltage thermostats.
Figures 6, 7 and 8 show three views of an air conditioning line voltage thermostat. Shown are the front (Figure 6), the thermostat with the cover removed (Figure 7), and the back (Figure 8). These types of thermostats do not have sensing bulbs like the line voltage thermostats shown in the previous figures. To be effective, they must be near the return air stream to control air temperature.
Notice in Figure 7 that the line voltage thermostat is less complex than a low-voltage thermostat. Figure 8 illustrates the factory 14-AWG wire gauge, which is much larger than the 18-AWG wire found in low-voltage temperature control devices.
Figure 9 appears to be a line voltage thermostat, but it is a low-voltage device. The amperage rating on the thermostat will determine if it is a line voltage or low voltage thermostat. This thermostat is often used in commercial applications, but can also work in a residential installation.
The installation of a thermostat’s control will affect its operation. The installation should be in a location that is free from outdoor temperature influences like the sun or drafts near an outside door. A thermostat mounted on a wall that is directly connected to the outside will be affected by outdoor and radiant temperatures on the wall. The thermostat should be isolated from internal heat sources such as lights, electronics, or people. The best location is on an inside wall that is near the return air grille.
Here are good thermostat installation practices:
- Follow the manufacturer’s recommendations.
- Mount at least 52 to 60 in above the floor.
- It must be level, even if it is a modern electronic thermostat.
- Conditioned air must not blow on it.
- Don’t install on an outside wall or behind a door or in corners.
- Don’t install where the sun or radiant heat from chimneys or pipes can affect it.
- Do not mount in a kitchen.
- Mount near the return air stream. A central hallway location is the right choice.
- Seal the wall opening around the thermostat wire. The air coming around this opening may affect the thermostat’s operation. Even an inside wall can have outside air influences.
- Usually, 18-AWG multi-conductor wires are used when hooking up a thermostat. If a run is over 100 feet, a larger gauge wire should be used.