Radiation heating

Hello,

we are trying to solve the following problem:

We have four radiant heaters on top of a rectangular cavity and two at the bottom. So, we could set up a symmetric problem two at the top and one at the bottom.
we would like to solve this problem as a radiant heating including the natural convection inside the cavity while a copper mass is included in the center (for this the heat flux going into the copper mass will be calculated).
the first question is about solving the radiation: we are applying a power boundary for the radiant heaters, but there does not seem to increase the temperature around the heaters to to start up the natural convection in the air.
when we apply a variable temperature boundary on the radiant heaters, it seems to work (but not with higher temperatures e.g. 1000 K), but the temperature of the copper mass becomes unphysical (lower than its initial temperature).
We set up the problem as surface to surface radiation problem among the walls of the cavity and the heaters. this is very similar to the `bulb` tutorial given in the comsol help files.
that`s all i can say, and any suggestion will be much appreciated.
thank you,
ferruh