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An older mechanical style thermostat (mercury bulb or snap acting) does not call for the second stage until the room temperature drops a few degrees below the setting. This means that the room temperature can be lower than the thermostat setting which is not an ideal temperature control condition. A digital staging thermostat offers substantial performance improvement over a mechanical thermostat by computing the optimum time to bring on the second stage and maintaining the indoor temperature closer to the setpoint. Because the thermostat microprocessor incorporates a memory, a digital thermostat can track and internally record the performance of the system (room temperature vs. setpoint). This allows the thermostat to recognize good or poor system performance and calculate when additional stages of heat (or cool) are required. Different thermostat manufacturers use different control algorithms (computations) to control staging. A mathematical understanding of the algorithm is not necessary to understand how this calculation works. To achieve staging based on the performance of the system, digital thermostats constantly monitor the performance of the first stage. The following example shows how a typical digital thermostat decides when to call for additional stages.
Heating Example: Assume that the thermostat checks the setpoint and the room temperature each minute for 30 minutes. Each time it compares the room temperature to the setpoint temperature, the result is that they are less than one degree apart. The thermostat memory records that for the last 30 performance checks, the first stage was maintaining the desired setting within one degree. If the thermostat memory size is designed to hold 30 performance checks, the memory will be full of information indicating that the system is capable of making a setpoint using only the first stage.
Now, assume that the room temperature drops slightly more than one degree below the setpoint. The next time the thermostat compares the room temperature to the setpoint, the thermostat will record in the memory that first stage in not making temperature. In the memory, there will be twenty-nine checks of adequate first stage performance and one check indicating poor performance. Based on this tally, the thermostat will not energize the second stage. One minute later, if the setpoint is still not within 1 degree of setpoint, the thermostat will again record a check of poor performance. This updates the memory with twenty-eight checks of adequate performance and two checks of poor performance. This process continues until the memory fills with data indicating that the second stage is not capable of keeping up with the demand. When the memory records thirty checks ( in 30 minutes) of the setting and the room temperature is more than 1 degree apart, the thermostat will energize the second stage. This relatively long initial delay before bringing on the second stage is because the first stage was previously keeping up with the setpoint temperature. When the first and second stages approach and achieve setpoint, the thermostat turns off the heat. The memory will continue to update, recording that the system is now within one degree of setpoint each minute and writing over the poor performance marks. If the room temperature quickly drops a degree below the setpoint, second stage will come on sooner because the memory still contains information that the first stage was not keeping up with the demand.
The net effect of the staging algorithm is when the first stage is performing well and keeping you close to the setpoint, the second stage will take longer to come on. When the first stage is not performing well, the second stage will come on more quickly.
Advantages of using a digital thermostat for staging are tighter temperature control and optimized staging based on system performance. The staging method outlined above maximizes comfort and economy by using the lowest stage of heat or cool to maintain the temperature setting. A thermostat on a Dual Fuel system uses a staging algorithm like the one described above and also turns off the compressor(s) when the furnace is energized.
It is not possible to change the algorithm to prevent the thermostat from staging but staging is always based on system performance. At installation the average time for additional stages coming on is 15 to 30 minutes. After that staging time will very with system performance.