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issue: August 2005 APPLIANCE Magazine

Technology Report
High-Temperature Thermostat


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A patented thermostat is said to provide a simple, more reliable switching mechanism for high-temperature applications, displacing sensor/relay combinations and, thus, saving cost.

Although originally developed for the hearth industry, Condar’s new thermostat can also be used in cooking appliances and HVAC equipment. According to the company, the device allows for the use of simple, rugged bi-metal snap-disc thermostats in applications that exceed the temperature limitations of former bimetal thermostats.

Patented by Condar Company of Columbus, NC, U.S., the new thermostat combines two technologies—a standard, off-the-shelf snap disc and Condar’s high-temperature meter probe, creating a new product with versatile properties. According to Greg McCarren, inventor of the device, an ordinary snap-disc thermostat is a reliable, inexpensive way to make or break an electrical connection, but it has a temperature limit of only about 550°F. This new method, however, enables electrical connections in response to much higher temperatures that are said to greatly exceed the limitations of conventional bi-metal snap-disc switches.

The thermostat was originally developed to solve a real-world problem Condar was experiencing when developing a fireplace heat recovery ventilator. The company required a simple thermostat that could turn a draft inducer on and off at low flue temperatures inside a chimney. “We wanted to close a circuit at 115ºF and reopen at 95ºF,” Mr. McCarren explains. “During normal operation, a snap-disc thermostat in contact with the outside of the chimney would stay below its rated limit of 550ºF, but during the required UL 127 test, it would reach temperatures of 1,500ºF and higher, destroying the bi-metal element in the thermostat.”

Mr. McCarren adds that while the thermostat always failed in the open position, there was no guarantee that it would do so every time, leaving a potentially live circuit following an extreme occurrence. “Even if we could guarantee that it would fail safely (by adding a fusible link, for example), the appliance would still require a new thermostat to resume operation,” he notes. “The probe thermostat was an inexpensive way to guarantee that the thermostat survived the extreme occurrence without failing.”

In the end, the new thermostat ended up being a superior answer to Condar’s dilemma because it could be inserted into the gas stream. According to Mr. McCarren, this wasn’t possible with the original thermostat due to the higher temperatures and long-term stress on the metal. “Insertion into the gas stream allowed a faster, more direct measurement of the target temperature,” he says. “The probe itself is chrome-plated and has a proven track record in this environment.”

In fact, Mr. McCarren says the conductive probe can withstand sustained temperatures up to 2,500ºF and that the thermostat can be designed to react to either surface or ambient temperatures. “There can be substantial heat loss along the probe, so that the actual temperature at the snap-disc thermostat is within its design range when the measurement region is hundreds of degrees hotter than the thermostat’s tolerance,” he notes. “We can calibrate the difference by manipulating several factors, including length of probe, conductivity of the probe, and bi-metal sensitivity within the thermostat.”

 

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