Thermostat Controller

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In many circumstances a properly located thermostat will work well to maintain comfortable temperatures. However, the performance of a heating and cooling system may suffer when the thermostat is not in the right location, or there is significant heat gain or loss in parts of the your home. Some thermostats also lack an Auto mode, which may result in difficulties during conditions that require both heating and cooling. Thermostat Controller allows more control of your connected thermostat by creating a virtual thermostat device to manipulate the set-points of the controlled thermostat, thus taking over most of its functionality and becoming the Controller thermostat for establishing set-points of your heating and cooling system.

Features

  • Supports additional temperature sensors, allowing any connected device with temperature readings available to Hubitat Elevation®, to also participate as a remote sensor.
  • Allows control of thermostat hysteresis to set the difference between the temperature at which the thermostat switches OFF, and the temperature at which it switches ON again.
  • Auto mode is available to the Controller thermostat for conditions where both heating and cooling may be needed
  • OFF function for when manually disabling the heating and cooling system is desired.

Additional Temperature Sensors

It may be desirable to average the temperature of more temperature sensors in the conditioned space than just the one built-in to the existing thermostat. If the existing thermostat is not positioned well, this may be necessary to maintain comfortable temperatures consistently. Thermostat Controller allows any number of sensors, averages their readings. It offers the ability to offset a given sensor's readings, or place more or less weight in the average for each sensor. This average temperature becomes the controlling value for determining when to call for heating or cooling, replacing the sensor built-in to the thermostat itself. In most cases, the built-in temperature sensor of the controlled thermostat will be one of the sensors used in the average.

Control of Hysteresis

Hysteresis is the amount of temperature swing that must occur between calling for heat or cool, and the system having satisfied that demand. Using cooling as an example, and the default hysteresis value of 1.0 degrees F, with a setpoint of 75°, cooling will be called for when the temperature exceeds the setpoint plus half the hysteresis -- in this example, 75.5°. Once cooling begins, it will remain on until the average temperature has been reduced to the setpoint minus half the hysteresis value, or 74.5°. By careful setting of the hysteresis value, the efficiency of the HVAC system can be optimized while ensuring comfort in the conditioned space. With a narrow hysteresis (lower value), the HVAC system will cycle more frequently, and the temperature swings in the conditioned space will be lower. With a wider hysteresis, the HVAC will cycle less frequently, and there will be correspondingly larger swings of temperature in the conditioned space.

Auto Mode Thermostat

Once Thermostat Controller takes control of a thermostat, its operation becomes auto as to heat versus cool mode. The determination of whether to call for heating or for cooling becomes entirely automatic and dependent on the setpoints established for the controller thermostat. It is important to maintain an adequate setpoint separation between heating and cooling, typically at least 3° F. While the temperature of the conditioned space falls between these two setpoints, the HVAC system will be idle, neither heating or cooling. Once the average temperature falls below the heating setpoint (less half the hysteresis), heating will be called for. Should the temperature rise above the heating setpoint (plus half the hysteresis), the system will return to idle. If the temperature continues to rise, when it reaches the cooling setpoint (plus half the hysteresis), cooling would be called for. Etc.

If one does not want auto mode, and only wants the system to work for cooling or for heating, this can be accomplished by setting the setpoint for the unwanted mode well out of meaningful range.

How It Works

Thermostat Controller manipulates the setpoints of the controlled thermostat to control its operation. This usurps most of the functionality of the controlled thermostat. To call for cooling, for example, Thermostat Controller will set the cooling setpoint of the controlled thermostat to a level well below the current temperature, thus forcing the controlled thermostat into cooling operation. The amount by which the setpoint is set below is determined by the Control Offset, a settable number of degrees. When the demand for cooling has been met, the controlled thermostat's cooling setpoint will be set well above the current temperature, thus forcing it to idle, and again using the Control Offset. The Control Offset defaults to 2° F, and should be set high enough to cover any difference between the average temperature of all of the sensors, and the temperature reported by the controlled thermostat itself.

Cautions

Bear in mind that this app usurps the control and most safeguards of the existing thermostat. Avoid setting the Control Offset higher than needed for successful control. The Control Offset should be kept low so that should the hub fail, the real thermostat will not be in a run-away condition. Be careful in establishing a proper Hysteresis setting.

Use with Thermostat Scheduler

To add scheduling to the controller thermostat, it may be selected in Thermostat Scheduler. Do not use Thermostat Scheduler with the controlled thermostat, once it is under control of Thermostat Controller.

Thermostat Fan Mode

The created controller thermostat inherits the supported thermostat fan modes, if any, from the controlled thermostat. The thermostat fan mode may be set for either thermostat, controller or controlled, and that setting will be set for the other. Thermostat fan mode may be set by Thermostat Scheduler, acting on the controller thermostat, and such a setting would be passed on to the controlled thermostat.

Basic Instructions

When you first set up Thermostat Controller, the controlled thermostat is not yet put under the control of the controller thermostat. The first section of the app UI shows the current setting of the controlled thermostat. These can be adjusted by opening that page. On that page is a setting called "Free/Controlled", and it starts out as Free. This means that the controlled thermostat is Free from being controlled. Only when you switch that selection to Controlled, does the actual control function of the app kick in.