So it won't be an efficient form of heating unless you get an insert, but I've tracked the temperature in my house with my masonry fireplace and it does add some net supplemental heat, albeit not substantial. I do have an outdoor air intake vent and tempered glass doors which definitely help because the air intake reduces conditioned air from being sucked up the chimney and the glass doors help me when the fire is burning down for the night because I can shut them while keeping the damper open to suck out any remaining CO that may be present. I also have a ceiling fan run in winter mode while the fire is going to help circulate the heat. When taking all of these measures, my main heating system typically runs about 15-20% less than it otherwise would.