Stick a thermometer up there and check how hot it is first. You can get a nice haze from very little heat if it's cold outside. I only say because I had to sort out some tiles near my old chimney, winter, woodstove was going and I thought I would warm me hands up. I was surprised how not hot it was.
We had a ss liner up to the cap but it was not back filled so I guess the fire is pretty efficient and the rest heated up the air space outside the liner.
Just a thought