The share of renewable energy share in India's energy mix of India has been increasing recently owing to the promotion and implementation of various government policies that promote the use of renewable sources of energy, depleting fossil fuel levels, increasing fuel prices, and stringent laws for emission reduction worldwide. The increase in the use of solar-evacuated-tube-collector-based domestic water heating is the result of this transition from conventional to nonconventional sources of energy. The recent urban agglomeration is challenging to install these solar water heaters in every household because the roof space is limited. Space reduction can be achieved by reducing the collector area by varying the diameter of the evacuated tubes. This study used a single tube and tank arrangement model with a fixed inclination to achieve a thermosyphon effect in a solar water heating system with three different diameter configurations of 48 mm, 54 mm, and 72 mm, with a tube measuring 1800 mm in length. Thermal assessment of the thermosyphon-based evacuated tube-solar water heater using the ANSYS CFD simulation software involves measuring the temperature at three different positions in the tank to study the thermosyphon initialization. This study signifies that the 72-mm-diameter evacuated tube is more capable of initializing the thermosyphon effect in the system. Nevertheless, the final temperature of the system is better achieved in the 48-mm-diameter evacuated tube.