Abstract: To keep the heat recovery unit with water as heat medium running steadily, it is essential that the system should be operated in heat balance. A heater of a delayed coking unit using water heat medium system is studied to analyze the factors affecting heat balance, such as water flow rate, fuel consumption, air consumption and temperature of water entering flue gas heat exchanger. Results indicate that the water flow rate, fuel consumption and air consumption are the main factors affecting the heat balance of system; based on the actual heater operation data, selecting suitable value of fuel consumption and air consumption as design basis in system design is important.