The old round thermostats that were common in homes and buildings for many decades are often referred to as round mechanical thermostats. These thermostats controlled heating and cooling systems by using a bimetal coil and mercury switch to sense the temperature and adjust the heating or cooling accordingly. Here are some more details about these classic round thermostats:
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Names and Nicknames
Some common names and nicknames for the old round thermostats include:
- Round thermostat
- Mechanical thermostat
- Manual thermostat
- Mercury switch thermostat
- Bimetal coil thermostat
- Clock thermostat
- Clock-style thermostat
The “clock” nickname comes from their visual resemblance to a clock face with numbers around the dial. Since they had to be adjusted manually by turning the dial, “manual thermostat” is another apt description.
History and Background
The first mechanical thermostats were invented in the late 1800s and patented by Warren S. Johnson. His company, Johnson Controls, produced the first electric room thermostats in 1883. Throughout the early 1900s, the technology improved with more precise calibration and enclosed mercury tilt switches.
By the 1950s and 1960s, the classic round style thermostat had become commonplace in homes and commercial buildings. They provided a simple, moderately accurate way to control heating and cooling systems. These electromechanical thermostats didn’t require power to operate and functioned for decades.
How They Worked
Here are the key components and functioning of the classic round thermostats:
- Bimetal coil – This coil was made of two metals bonded together that would bend and flex with temperature changes due to the different expansion rates of the metals.
- Mercury tilt switch – A glass bulb filled with mercury would tilt and make/break contact as the bimetal coil flexed.
- Adjustable set point – A knob allowed adjusting the target temperature set point.
- Calibrated scale – Printed numbers represented target temperature settings.
- Interior wiring – Insulated wire connected the thermostat to the heating/cooling control circuitry.
As the bimetal coil sensed the ambient temperature changing, it would flex and cause the mercury switch to tilt, triggering the heating or cooling system to turn on or off. This provided a simple proportional control system.
Advantages
Here are some of the advantages of the old mechanical thermostats:
- Simple and inexpensive to manufacture.
- Easy for homeowners to understand and use.
- No batteries or power required – operated purely mechanically.
- Long lifespan, often functioning for 30-60 years.
- Fairly accurate temperature control, within +/- 1-2 degrees.
- Easy to mount on wall and wire into existing systems.
Disadvantages
Some drawbacks and disadvantages include:
- Limited temperature range and calibration.
- Prone to mechanical wear, debris buildup, and mercury leakage.
- No programmability or advanced features.
- Only basic temperature control, no humidity or multi-stage capabilities.
- No remote monitoring or adjustment.
- Mercury toxicity made disposal difficult.
Replacement by Digital Thermostats
As microelectronics advanced in the 1970s and 1980s, digital electronic thermostats capable of much more precise temperature control and programmability began to take over the market. Through the 1990s and 2000s, digital thermostats virtually replaced the old electromechanical models as they offered more accuracy, features, and reliability.
Many homes today now use smart WiFi-enabled thermostats that can be controlled and accessed remotely via smartphone. But the classic round thermostat design remains iconic and recognizable as a symbol of home heating control for over a century.
Conclusion
The old round mechanical thermostats with bimetal coils and mercury switches provided simple and reliable temperature control in homes and buildings for decades. Also called clock thermostats for their circular shape, these purely mechanical devices use clever physics principles to function automatically without batteries or power. While replaced today by advanced digital thermostats, the classic round models remain a iconic symbol of vintage temperature control engineering.