Hi Andras,

Evaluate tfun, it is of the form

{{temp -> InterpolatingFunction[...]}}

which is not a form that ContourPlot3D accepts so it returns unevaluated. One option is to replace NDSolve with NDSolveValue. I specified values for MaxRecursion and PlotPoints so the plot is generated in a reasonable time for testing.

tfun  = NDSolveValue[pde, temp, { t , 0. , 600. }, { x , y , z} \[Element] \[CapitalOmega] ]
ContourPlot3D[
  tfun[x, y, z, 40.],
  {x, y, z} \[Element] \[CapitalOmega],
  RegionFunction -> Function[{x, y, z}, x < 25. || y > 90. || z < 20. ],
  Contours -> {306., 308., 310., 312.},
  PlotPoints -> 10,
  MaxRecursion -> 0]