A steady state thermal analysis is performed on a fully voided channel of Indian pressurised heavy water reactor (IPHWR) by providing variable heat flux inputs of 800 W/m2, 1500 W/m2, 2500 W/m2, 3500 W/m2 and 4500 W/m2 for evaluating the numerical results of temperature distributions in pressure tube (PT) and calandria tube (CT) at concentric scenarios using numerical approach. The top to bottom temperature difference of PT and CT is found to be 0.57% and 3.31% respectively for 800 W/m2 input heat flux and this variation decreases to 0.015% and 0.05% respectively for 4500 W/m2 input heat flux. Thus, pointing out that increasing the heat flux leads to decreased top to bottom temperature difference. Also, the temperature distribution pattern is found to be similar for all the input heat fluxes and for a particular heat flux the change in circumferential temperature is found to be negligible. Boiling phenomenon starts at 2500 W/m2 and the volume fraction of moderator vapour increases as the heat flux is increased, average density of moderator inside the enclosure decreases and continuous variation in film thickness is also observed with respect to time for a particular heat flux value. The results also showed that the moderator acted as an effective heat sink for higher heat input rates.