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Milivolt Systems w/ Modern Thermostats
Jesse Grandbois is a tech who reads the tech tips, and he wrote a few tips that he wanted to share on some gas furnace control basics. This tip is about how to use a fancy digital stat on a millivolt system.
For those of you who don't know, a millivolt system uses a thermocouple/thermopile with a standing (constant) pilot flame to generate a tiny “millivolt” signal used to control the furnace rather than the 24v signal modern thermostats use.
On a call for heat, the relay coil will be powered, closing the SPST relay powering the millivolt system (NO terminals) for heat.