Simulation isn't going to do this very well, because it can't see the air. You may also have not bonded the heater to the walls, which would drive your temperature to infinity over time. You don't say what your outside temperature is, but that's important. Also, I don't recall whether the R value for walls includes the convection on the outside or on the inside, or whether it's just the resistance through the walls, but these are both important.
This is not a good problem for Simulation. You could probably do this as an exercise in Flow Simulation, but Simulation FEA will not do anything with the internal convection which is pretty critical to your answer. Normally room temps are done with hand calcs by solving for the temperature drop through the wall that produces 1000 W (you should also include values for occupants, latent heat, any electronics etc.)
Temperature drop = P/(h A)
P = total power in the room (W)
H = Average heat transfer coefficient (W/m2 K)
A = Total wall area (m2)
Add this number to the outside temp and get the inside temp and know that it's hotter near the heater than far from it. The errors are so large no matter what you do, that sophisticated analysis is not justified.
Thaks a lot for answer!
So If I understand you correctly, I could simulate temperature raises in my model if I use Flow Simulation? I watched tutorial on youtube for visualizing relative humidity ( 7 Relative Humidity 1 Relative Humidity Introduction - YouTube ). I thought that I could setup similar analysis for temperature, but then I think that I need at least two oppening to specify inlet/outlet flow.
I'm not a flow simulation expert. But I believe you can analyze a closed volume with no inflow or outflow. But this is monstrous overkill for a problem that can be solved in excel in a minute or two--unless the goal is to practice using Flow Simulation.