I have an oil heater in my workshop with a low roof height and had the same problem, although we always had the window open on even the coldest days if the heater was running. I put a vent in the ceiling and noticed the difference immediately. Mind you I'm always tired anyway after spending all day trying to earn a few bucks from customers who tell me that their car is so important, an absolute lifeline that they can not be without it as they discuss the new kitchen that just cost them 15K. Colin