Interior walls temperature dependant on fluid
 

I am modelling boiler with ansys fluent 14.5

When fluid gets inside it gets heated from pipes. fluid is 500 K and walls are 300 K. Walls which is not heated are cooling the fluid.

Drawing is only cut part from big boiler, so walls represent also fluid which has temperature same as fluid.

So my question is how to do those walls same temperature as fluid or make it not dependent on fluid? I just don't want those wall wouldn't interupt with my calculations.

I will try to explain my model more.

I am modelling convection in boiler which symmetry line is between 2 and 3. I attached picture where I am modelling 2nd zone. Model is 3d.

My question is what boundary conditions I should put on between 2 and 3; 1 and 2.

That small circles are heating walls