Variable Differential Variable Delay Thermostat

A method for controlling a Heating Ventilating Air Conditioning (HVAC) system with a thermostat, the method comprising:

• continuing to operate an HVAC fan after turning off the heating or cooling system for a variable fan-off delay time P2;
• storing Conditioned Space Temperature (CST) measurements during the current variable fan-off delay period to obtain stored CST measurements;
• measuring a current CST during the current variable fan-off delay period;
• comparing the current CST measurement to the stored CST measurements;
• continuing the HVAC fan operation based on a comparison of the current CST measurement to the stored CST measurements;
• and
• discontinuing the variable fan-off delay based on the comparison of the current CST to the stored CST measurements.

A method for controlling a Heating Ventilating Air Conditioning (HVAC) system with a thermostat, the method comprising:

• turning off a heating or a cooling system;
• continuing to operate an HVAC fan for a variable fan-off delay time P2;
• determining a rate of change of a Conditioned Space Temperature (CST) with respect to time (dT/dt) by monitoring and storing at least two thermostat temperature measurements of a CST and two coincident times and dividing the difference in temperature by the difference in time between the two CST measurements,
• comparing the dT/dt to zero plus or minus a confidence interval tolerance to detect an Inflection Point (IP);
• and
• if the dT/dt indicates the CST has reached the IP one or more times, then turning off the HVAC fan.

A method for controlling a Heating Ventilating Air Conditioning (HVAC) system with a thermostat, the method comprising:

• adjusting the heating differential or cooling differential or differential offsets based on at least one time period selected from the group consisting of:
• a thermostat call for heating defining a heating cycle duration P3,
• a thermostat call for cooling defining a cooling cycle duration P4,
• an off-cycle time P11, and
• a fan-off delay time P2.

A method for controlling a Heating Ventilating Air Conditioning (HVAC) system with a thermostat, the method comprising:

• turning off a heating source or a cooling source;
• continuing to operate an HVAC fan for a variable fan-off delay time P2;
• measuring a Conditioned Space Temperature (CST) by the thermostat;
• and
• turning off the HVAC fan when the CST crosses a cooling fan-off delay differential or differential offset one or more times, or a heating fan-off delay differential or differential offset one or more times.

A method for performing a Heating, Ventilating, Air Conditioning (HVAC) fan-only Fault Detection Diagnostic (FDD) procedure with a thermostat fan control having an AUTO setting and an ON setting and correcting a fan-only operation fault by de-energizing a thermostat HVAC fan signal to override the ON setting and turn off an HVAC fan when an HVAC fan control is turned to the ON setting, the method comprising:

• determining if the HVAC fan control has been set to the ON setting which results in continuous fan operation based on detecting no simultaneous call for cooling and no simultaneous call for heating;
• if the fan-only operation exceeds a Threshold Fan-only Time (TFT), then de-energizing the HVAC fan signal to override the HVAC fan control ON setting and turn off the HVAC fan;
• if the HVAC fan control is turned off prior to reaching the TFT, then de-energizing the HVAC fan signal;
• if the thermostat call for heating or the thermostat call for cooling are detected during what was previously the fan-only operation and prior to reaching the TFT, then performing at least one action selected from the group consisting of:
• continuing to energize the thermostat HVAC fan signal, monitoring HVAC system parameters, waiting for the completion of either the thermostat call for heating or the thermostat call for cooling while continuing to energize the HVAC fan signal;
• and
• upon completion of either the thermostat call for heating or the thermostat call for cooling performing at least one action selected from the group consisting of:
• de-energizing the HVAC fan signal, continuing to energize the thermostat HVAC fan signal for a variable fan-off delay P2 based on at least one HVAC parameter, waiting for the completion of the fan-off delay P2, continuing to energize the HVAC fan signal until the TFT has been reached, and de-energizing the thermostat HVAC fan signal to turn off the HVAC fan at the end of the fan-off time delay P2.