Abstract: According to an aspect of the present disclosure, there is disclosed a method of determining the use of hot water performed in a system including a water heater and an air handler. The method includes: sensing a temperature of influent flowing into the water heater; sensing a temperature of effluent flowing out of the air handler; calculating a temperature difference between the temperature of the influent and the temperature of the effluent; and determining whether hot water is used by calculating whether the temperature difference is greater than a predetermined value.