A Brief History of Thermostats

Thermostats are the ubiquitous overlord of humanity’s day-to-day comfort. There is hardly an American citizen who does not own and operate a thermostat in their own home. And yet, few people think about this little piece of technology, beyond their annoyance at the Office Manager stubbornly locking the A/C at a crisp 64 degrees. We deal with thermostats every single day, so we are here to discuss the history of our favorite little piece of technology.

The Inventors

Once upon a time, people had a much harder time regulating their home temperature. In the early 20th century the majority of homes had manually operated furnaces. These furnaces, usually located in the basement, required frequent coal-stoking and physical adjustments of valves, draft, or dampers. Basement coal-stoking every brisk winter morning sounds like a first world nightmare. The market was ripe for a simpler means of regulating temperature, and in this spirit several men invented different types of thermostats in a short amount of time.

Andrew Ure (1778-1857). Andrew Ure was a Scottish chemist who patented the bi-metallic thermostat in 1830. Ure had worked with textile mills throughout his career and identified the product’s need for a consistent temperature. Ure’s bi-metallic thermostat would bend as a result of increased room temperature, cutting off the energy supply. While this was one of the first recorded thermostat inventions, it saw little use. It took more than forty years for inventors in American to re-imagine and popularize the thermostat.

Warren S. Johnson (1847-1911). Warren Johnson was a mustachioed professor in Wisconsin who was annoyed that his classroom was never at the temperature he wanted. Warren Johnson was also an inventor. The building in which Johnson taught his students was heated by a basement furnace requiring manual adjustments by the janitor to change temperature. Johnson would have to physically seek out the janitor every time he felt a little chilly. Perhaps to fix this mutual annoyance, Johnson invented the first electric thermostat in 1883. It included a bell that would ring as a signal for the janitor to adjust the furnace damper. Johnson Electric Service Company was created in 1885 to manufacture, install, and service Johnson’s product. This company still exists today as Johnson Controls.

Albert Butz (1845-1909). In 1885 Swiss-born immigrant Albert Butz filed the first patent for his “damper-flapper” invention, a thermostat that regulated furnace heat by opening and closing the furnace door with an automatic pulley system. He created the Butz Thermoelectric Regulator Company, which passed through several hands before eventually changing its name to the Minneapolis Heat Regulator Company. This company merged in 1927 and is today arguably the most iconic thermostat manufacturer in history. That’s right, Albert Butz helped lay the foundation for Honeywell International.

Sensor Types

Early technologies included mercury thermometers with electrodes inserted directly through the glass, so that when a certain (fixed) temperature was reached the contacts would be closed by the mercury. These were accurate to within a degree of temperature.

Common sensor technologies in use today include:

  • Bimetallic mechanical or electrical sensors

  • Expanding wax pellets

  • Electronic thermistors and semiconductor devices

  • Electrical thermocouples

These may then control the heating or cooling apparatus using:

  • Direct mechanical control

  • Electrical signals

  • Pneumatic signals

Gas expansion

Thermostats are sometimes used to regulate gas ovens. It consists of a gas-filled bulb connected to the control unit by a slender copper tube. The bulb is normally located at the top of the oven. The tube ends in a chamber sealed by a diaphragm. As the thermostat heats up, the gas expands applying pressure to the diaphragm which reduces the flow of gas to the burner.

Pneumatic thermostats

A pneumatic thermostat is a thermostat that controls a heating or cooling system via a series of air-filled control tubes. This "control air" system responds to the pressure changes (due to temperature) in the control tube to activate heating or cooling when required. The control air typically is maintained on "mains" at 15-18psi (although usually operable up to 20 psi). Pneumatic thermostats typically provide output/ branch/ post-restrictor (for single-pipe operation) pressures of 3-15psi which is piped to the end device (valve/ damper actuator/ Pneumatic-Electric switch, etc.). The pneumatic thermostat was invented by Warren Johnson in 1895 soon after he invented the electric thermostat.

In 2009, Harry Sim was awarded a patent for a pneumatic-to-digital interface that allows pneumatically controlled buildings to be integrated with building automation systems to provide similar benefits as DDC.

Electrical and Analog Electronic Thermostats

Bimetallic Switching Thermostats

Water and steam based central heating systems have traditionally had overall control by wall-mounted bi-metallic strip thermostats. These sense the air temperature using the differential expansion of two metals to actuate an on/off switch. Typically the central system would be switched on when the temperature drops below the set point on the thermostat, and switched off when it rises above, with a few degrees of hysteresis to prevent excessive switching.

Bi-metallic sensing is now being superseded by electronic sensors. A principal use of the bi-metallic thermostat today is in individual electric convection heaters, where control is on/off, based on the local air temperature and the set point desired by the user. These are also used on air-conditioners, where local control is required.

Expanding Wax Pellets

For the purposes of this article, discussion is limited to household (e.g., shower and other water temperature regulation). A thermostatic mixing valve uses a wax pellet to control the mixing of hot and cold water. A common application is to permit operation of an electric water heater at a temperature hot enough to kill Legionella bacteria (above 60 °C (140 °F)), while the output of the valve produces water that is cool enough to not immediately scald (49 °C (120 °F)).

Simple Two-Wire Thermostats

The illustration above is a diagram of the common two wire heat-only household thermostat, used to regulate a gas-fired heater via an electric gas valve. Similar mechanisms may also be used to control oil furnaces, boilers, boiler zone valves, electric attic fans, electric furnaces, electric baseboard heaters, and household appliances such as refrigerators, coffee pots and hair dryers. The power through the thermostat is provided by the heating device and may range from millivolts to 240 volts in common North American construction, and is used to control the heating system either directly (electric baseboard heaters and some electric furnaces) or indirectly (all gas, oil and forced hot water systems). Due to the variety of possible voltages and currents available at the thermostat, caution must be taken when selecting a replacement device.

Millivolt Thermostats

In the use of the millivolt thermostat, left, all of the power for the control system is provided by a thermopile which is a combination of many stacked thermocouples, heated by the pilot light. The thermopile produces sufficient electrical power to drive a low-power gas valve, which under control of one or more thermostat switches, in turn controls the input of fuel to the burner.

This type of device is generally considered obsolete as pilot lights can waste a surprising amount of gas (in the same way a dripping faucet can waste a large amount of water over an extended period), and are also no longer used on stoves, but are still to be found in many gas water heaters and gas fireplaces. Their poor efficiency is acceptable in water heaters, since most of the energy "wasted" on the pilot still represents a direct heat gain for the water tank. The Millivolt system also makes it unnecessary for a special electrical circuit to be run to the water heater or furnace; these systems are often completely self-sufficient and can run without any external electrical power supply. For tankless "on demand" water heaters, pilot ignition is preferable because it is faster than hot-surface ignition and more reliable than spark ignition.

Some programmable thermostats - those that offer simple "millivolt" or "two-wire" modes - will control these systems.

24 Volt Thermostats

The majority of modern heating/cooling/heat pump thermostats operate on low voltage (typically 24 volts AC) control circuits. The source of the 24 volt AC power is a control transformer installed as part of the heating/cooling equipment. The advantage of the low voltage control system is the ability to operate multiple electro-mechanical switching devices such as relays, contactors, and sequencers using inherently safe voltage and current levels. Built into the thermostat is a provision for enhanced temperature control using anticipation. A heat anticipator generates a small amount of additional heat to the sensing element while the heating appliance is operating. This opens the heating contacts slightly early to prevent the space temperature from greatly overshooting the thermostat setting. A mechanical heat anticipator is generally adjustable and should be set to the current flowing in the heating control circuit when the system is operating. A cooling anticipator generates a small amount of additional heat to the sensing element while the cooling appliance is not operating. This causes the contacts to energize the cooling equipment slightly early, preventing the space temperature from climbing excessively. Cooling anticipators are generally non-adjustable.

Electro-mechanical thermostats use resistance elements as anticipators. Most electronic thermostats use either thermistor devices or integrated logic elements for the anticipation function. In some electronic thermostats, the thermistor anticipator may be located outdoors, providing a variable anticipation depending on the outdoor temperature. Thermostat enhancements include outdoor temperature display, programmability, and system fault indication. While such 24 volt thermostats are incapable of operating a furnace when the mains power fails, most such furnaces require mains power for heated air fans (and often also hot-surface or electronic spark ignition) rendering moot the functionality of the thermostat. In other circumstances such as piloted wall and "gravity" (fanless) floor and central heaters the low voltage system described previously may be capable of remaining functional when electrical power is unavailable.

There are no standards for wiring color codes, but convention has settled on the following terminal codes and colors. In all cases, the manufacturer's instructions should be considered definitive.

Line Voltage Thermostats

Line voltage thermostats are most commonly used for electric space heaters such as a baseboard heater or a direct-wired electric furnace. If a line voltage thermostat is used, system power (in the United States, 120 or 240 volts) is directly switched by the thermostat. With switching current often exceeding 40 amperes, using a low voltage thermostat on a line voltage circuit will result at least in the failure of the thermostat and possibly a fire. Line voltage thermostats are sometimes used in other applications, such as the control of fan-coil (fan powered from line voltage blowing through a coil of tubing which is either heated or cooled by a larger system) units in large systems using centralized boilers and chillers, or to control circulation pumps in hydronic heating applications.

Some programmable thermostats are available to control line-voltage systems. Baseboard heaters will especially benefit from a programmable thermostat which is capable of continuous control (as are at least some Honeywell models), effectively controlling the heater like a lamp dimmer, and gradually increasing and decreasing heating to ensure an extremely constant room temperature (continuous control rather than relying on the averaging effects of hysteresis). Systems which include a fan (electric furnaces, wall heaters, etc.) must typically use simple on/off controls.

Digital Electronic Thermostats

Newer digital thermostats have no moving parts to measure temperature and instead rely on thermistors or other semiconductor devices such as a resistance thermometer (resistance temperature detector). Typically one or more regular batteries must be installed to operate it, although some so-called "power stealing" digital thermostats use the common 24 volt AC circuits as a power source, but will not operate on thermopile powered "millivolt" circuits used in some furnaces. Each has an LCD screen showing the current temperature, and the current setting. Most also have a clock, and time-of-day and even day-of-week settings for the temperature, used for comfort and energy conservation. Some advanced models have touch screens, or the ability to work with home automation or building automation systems.

Digital thermostats use either a relay or a semiconductor device such as triac to act as a switch to control the HVAC unit. Units with relays will operate millivolt systems, but often make an audible "click" noise when switching on or off.

More expensive models have a built-in PID controller, so that the thermostat knows ahead how the system will react to its commands. For instance, setting it up so that the temperature in the morning at 7 a.m. should be 21 °C (69.8 °F), makes sure that at that time the temperature will be 21 °C (69.8 °F), where a conventional thermostat would just start working at that time. The PID controller decides at what time the system should be activated in order to reach the desired temperature at the desired time. It also makes sure that the temperature is very stable (for instance, by reducing overshoots.

Most digital thermostats in common residential use in North America and Europe are programmable thermostats, which will typically provide a 30% energy savings if left with their default programs; adjustments to these defaults may increase or reduce energy savings.

The programmable thermostat article provides basic information on the operation, selection and installation of such a thermostat.

Combination Heating and Cooling Regulations

Depending on what is being controlled, a forced-air air conditioning thermostat generally has an external switch for heat/off/cool, and another on/auto to turn the blower fan on constantly or only when heating and cooling are running. Four wires come to the centrally-located thermostat from the main heating/cooling unit (usually located in a closet, basement, or occasionally in the attic): One wire, usually red, supplies 24 volts AC power to the thermostat, while the other three supply control signals from the thermostat, usually white for heat, yellow for cooling, and green to turn on the blower fan. The power is supplied by a transformer, and when the thermostat makes contact between the 24 volt power and one or two of the other wires, a relay back at the heating/cooling unit activates the corresponding heat/fan/cool function of the unit(s).

A thermostat, when set to "cool", will only turn on when the ambient temperature of the surrounding room is above the set temperature. Thus, if the controlled space has a temperature normally above the desired setting when the heating/cooling system is off, it would be wise to keep the thermostat set to "cool", despite what the temperature is outside. On the other hand, if the temperature of the controlled area falls below the desired degree, then it is advisable to turn the thermostat to "heat".

Heat Pump Regulation

The heat pump is a refrigeration based appliance which reverses refrigerant flow between the indoor and outdoor coils. This is done by energizing a reversing valve (also known as a "4-way" or "change-over" valve). During cooling, the indoor coil is an evaporator removing heat from the indoor air and transferring it to the outdoor coil where it is rejected to the outdoor air. During heating, the outdoor coil becomes the evaporator and heat is removed from the outdoor air and transferred to the indoor air through the indoor coil. The reversing valve, controlled by the thermostat, causes the change-over from heat to cool. Residential heat pump thermostats generally have an "O" terminal to energize the reversing valve in cooling. Some residential and many commercial heat pump thermostats use a "B" terminal to energize the reversing valve in heating. The heating capacity of a heat pump decreases as outdoor temperatures fall. At some outdoor temperature (called the balance point) the ability of the refrigeration system to transfer heat into the building falls below the heating needs of the building. A typical heat pump is fitted with electric heating elements to supplement the refrigeration heat when the outdoor temperature is below this balance point. Operation of the supplemental heat is controlled by a second stage heating contact in the heat pump thermostat. During heating, the outdoor coil is operating at a temperature below the outdoor temperature and condensation on the coil may take place. This condensation may then freeze onto the coil, reducing its heat transfer capacity. Heat pumps therefore have a provision for occasional defrost of the outdoor coil. This is done by reversing the cycle to the cooling mode, shutting off the outdoor fan, and energizing the electric heating elements. The electric heat in defrost mode is needed to keep the system from blowing cold air inside the building. The elements are then used in the "reheat" function. Although the thermostat may indicate the system is in defrost and electric heat is activated, the defrost function is not controlled by the thermostat. Since the heat pump has electric heat elements for supplemental and reheats, the heat pump thermostat provides for use of the electric heat elements should the refrigeration system fail. This function is normally activated by an "E" terminal on the thermostat. When in emergency heat, the thermostat makes no attempt to operate the compressor or outdoor fan.

In Summary

Albert Butz's 1885 "damper flapper" patent, and his business, was snapped up and, in 1906, was bought out by a young engineer by the name of Mark Honeywell, who later unveiled the first programmable thermostat, the "Jewell". Featuring a built-in clock, it enabled users to turn down the heat at night and automatically adjust it to a pre-set temperature in the morning.

An electric clock was added in the 1930s and in 1953 Honeywell's famous round thermostat was unveiled.

"One of the world's most recognizable designs, it remains in production and adorns the walls of more households around the world than any other thermostat," says Matthew Gordon, Honeywell's marketing manager.

The next big invention was separate outputs for heating and hot water and the 1980s saw the digital revolution. Or it should have. Electronic thermostats have been surprisingly slow to catch on. John Barclay, national sales manager for Siemens, admits that their blue-to-red dials are still hugely in demand, despite the option of slimline and even wireless remote digital controls which control heating far more accurately and flexibly. "When you think that by turning your heating down by one degree, you can save 10 per cent of energy, it's remarkable," he says.


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