How well do Pellet Stoves heat , say 1200 SQ FT, when the temps hit MINUS 10 to MINUS 20 Degrees ? Will they keep the inside temp at 70 to 75 Degrees ? House is insulated and have those double insulated replacement windows , but the 85,000 BTU gas fired hot water boiler is having a hard time keeping up. Not to mention Pellet fuel HAS to be cheaper than Gas ....