Thermostat variable fan-off delay

A Fault Detection Diagnostic (FDD) method for a Heating, Ventilation, Air Conditioning (HVAC), the method comprising:

• determining, adjusting, and providing a variable fan-off delay P2 at the end of a heating on cycle or a cooling on cycle to improve energy efficiency;
• where the variable fan-off delay P2 is based on at least one HVAC parameter selected from the group consisting of:
• an off cycle time P11,
• a heating cycle duration P3 including at least one heating cycle selected from the group consisting of:
• a thermostat call for heating, a heating on cycle, and a heating off cycle,
• a cooling cycle duration P4 including at least one cooling cycle selected from the group consisting of:
• a thermostat call for cooling, a cooling on cycle, and a cooling off cycle, and
• a Conditioned Space Temperature (CST) threshold selected from the group consisting of:
• the CST reaches a heating fan-off delay differential offset, the CST reaches a cooling fan-off delay differential offset, the CST crosses the upper heating differential a second time, the CST crosses the lower cooling differential a second time, and the CST reaches an inflection point where the rate of change of the CST with respect to time equals zero plus or minus a confidence interval tolerance.

A Fault Detection Diagnostic (FDD) method for a Heating, Ventilation, Air Conditioning (HVAC) system controlled by a thermostat with a fan control having an AUTO setting and an ON fan-only setting, the method comprising:

• monitoring active or inactive signals from the thermostat to determine if the fan control has been accidentally set to the ON fan-only setting which can cause a continuous fan-only operation;
• detecting the ON fan-only setting based on an active fan signal and an inactive heating signal or an active fan signal and an inactive cooling signal from the thermostat or the presence of the ON fan-only setting without a thermostat call for heating or a thermostat call for cooling;
• if the continuous fan-only operation proceeds until a Threshold Fan-only Time (TFT), then performing at least one action selected from the group consisting of:
• de-energizing the fan relay to override the ON fan-only setting and turning off the HVAC fan, and
• monitoring the HVAC system parameters during the off cycle;
• and
• if the fan control turns off the fan prior to reaching the TFT, then performing at least one action selected from the group consisting of:
• de-energizing the fan relay and turning off the fan, and
• monitoring the HVAC system parameters during the off cycle;
• if the heating signal or the cooling signal are detected or the thermostat call for heating or the thermostat call for cooling are detected during what was previously the continuous fan operation and prior to reaching the TFT, then performing at least one action selected from the group consisting of:
• energizing the fan relay to continue operating the fan, and
• monitoring the HVAC system parameters,
• waiting for the completion of either the heating cycle duration P3 or cooling cycle duration P4 while energizing the fan relay to continue operating the fan,
• upon completion of either the heating cycle duration P3 or the cooling cycle duration P4, performing at least one action selected from the group consisting of:
• calculating the variable fan-off time delay P2,
• energizing or continuing energizing the fan relay to continue operating the fan for the variable fan-off delay P2,
• waiting for the completion of the variable fan-off time delay P2, and
• performing at least one action selected from the group consisting of:
• de-energizing the fan relay and turning off the fan at the end of the variable fan-off delay P2, and de-energizing the fan relay and turning off the fan at the end of the TFT.