I have something similar but find that the main inefficiency...

I have something similar but find that the main inefficiency results from the installation which draws room air in and then up the flue all to be replaced by draughts from gaps under doors or fan openings in the ceiling so the house is heated poorly .
Have intention in the summer of making an intake through the brick wall at the rear of the heater to feed outside air into the box
directly thus leaving the heated air in the house.