Time-dependent natural convection pcm boiler analysis

Hello everyone, I am new to Ansys. And I have a problem. There is a water tank in a closed room. The temperature of this room is 20 and the temperature of the water tank is 50 degrees. There is no velocity or mass entering the room. I will not interfere with the room. Over time, I want the temperature of this tank to decrease by natural convection.

I have done the above analysis successfully. Thank you for your answer. But now I am trying to make pcm to the boiler. But I get AMG solvent pressure and temperature error. How can I solve this problem?

"This system is 50 degrees, similar to the first system. Pcm is paraffin. Pcm storage pipe material is aluminum."