Control method, system and device without domestic hot water inlet temperature sensor

The invention discloses a control method, system and device without a domestic hot water inlet temperature sensor, and the control method comprises the steps: starting a domestic water inlet temperature setting unit before a wall-mounted gas boiler is started, obtaining the currently collected temperature, and recording the currently collected temperature as the domestic water inlet temperature; obtaining T < setting > and T < output >; when it is detected that the water flow is larger than 2.5 L/min and T < outlet > is smaller than T < set >, a domestic hot water heating mode is started; and if the temperature difference T is larger than T1, running in a firepower mode with the maximum power, carrying out rapid preheating until the temperature difference T is smaller than or equal to T1, running a PID program mode, adjusting the firepower, and shutting down after a user turns off water. According to the control method, under the condition that no domestic hot water inlet temperature sensor exists, before the