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Two-stage thermostat

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Published
July 10, 2009
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Two-stage thermostat

How does a two-stage thermostat operate?

Mike Thies replies:

A staged thermostat is all about the deviation between the desired set point and actual room temperature.  

Say you allow a one-degree Fahrenheit deviation both above and below your desired room temp set point.  

You set AND LEAVE the thermostat at 68-degrees F with the heat turning on at 67-degrees F and off at 69-degrees F. As long as the room temp stays in this range, only the first stage will operate. Let the room temp fall below 67-degrees F and the second stage will kick in because the first stage alone is inadequate. BOTH stages will be running.  

Now say you raise the thermostat by more than two degrees F. Both stages will call with the second stage dropping out once room temp is 1F below set point.  

Such is the simplest, but far from the only, method of staging.  

Things can get FAR more complicated when you're talking about staging supplemental baseboard in a space ordinarily heated by a radiant floor. While not always true, the baseboard may well require a higher supply temperature from the boiler, even to provide the second-stage boost. Unless the system is designed to provide two independent supply temperatures, the first (radiant-floor) stage may be "locked out" whenever the second (baseboard) stage is calling. If it's not "locked out", and independent supply temperatures are not provided, then the second stage may never get hot enough because the first is dragging it down.