Space variant boundary conditions

Hello everyone,
I am modelling the combustion chamber of a biomass stove. For now, I aim to simulate just the fluid flows.
I am using the non isothermal flow interface to model this(axisymmetric). My fluid domain is continious and need to specify the heat source only in the interior of the combustion chamber.
I know this can be done using a step function. But how do you implement this step function on to a boundary condition? In my case, I am specifying a heat source and I want to implement the step function onto this. What is the syntax for the same?
Really need your help.Have been wasting too much time on this!

thanks,
Anoop