I have a 30kW bio-comfort and that U-model grate with a topstone. As far as I know it works, but I do have air compressor cleaning installed. As for now I have not touched the burner for over 2 tonnes of pellet, so at least with compressor cleaning it does not collect ash. Don't know why I received the burner with that grate, until now I had no idea that there are other models. I installed it about 1,5 years ago, so it can't be that old. If that grate is designed to work better on lower power levels that might be an explanation as don't need the full 30kW and my boiler does not handle over 25kW very well. I mentioned this to the dealer when ordering the burner so maybe he chose this grate for that reason?