CFD Online Discussion Forums - Heat source on variable surface

Hello.
I want to conduct ice melting simulation. I've created volumetric heat source (w/m^3) which is limited by certain radius and height condition:

Code:

if ((prom<=r)&&(yc>=0.0009)&&(yc<=0.001))

        {                

               source = 5e12;

        }

        else

        {

                source = 0;

        }

At the beginning of the simulation everything works fine, but when the surface of the ice begins to deform temperature raise above 5k K :(
http://i.imgur.com/Olv17uw.png
In the picture you can see deformed ice surface. I think that I need to write condtition which will define that source should "follow" ice surface. Now it seems like heat source affects air - that's why temperature rise to such high values.
I got the idea to add another condtion - source should act only when the density is at specific value. But on the other hand i should know what are the coordinates of free surface - source should have same "depth" across surface.
Can anybody gave me a hint where to find solution or promt some UDF example? I don't need exact solution - just example how to track such kind of surface changes.