I’ve fiddled with how we work the fireplace today. It was a snowy day and we just wanted to stay home and take it easy. So we had hours to just tend the fire so, I tried to experiment to see how we could get the most heat with the least wood. After practicing today, I’ve got so much heat coming out of the fireplace that you can’t sit within couple of feet of the opening and the wood is burning much slower (hour or more per log). A cup of soup heated up to boiling in a few of minutes by placing it about 18″ in front of this fire.
Here’s the principles that seem to help the most.
- Orange coals radiate more heat than flames.
- Lower flames burn wood much slower.
To get the most heat, keep the orange coals facing out in to the room where they can radiate heat. Fires transmit by convection and radiation. Convection is the hot air coming off the fire, most of this goes up the chimney, a good thing if you don’t want a room full of smoke. That means that most of the heat going into the room is from infrared radiation. The orange coals produce a ton of radiant heat. To get the most heat, let a bank of orange coals build up under the grate. Use the shovel to keep them together and piled as high as possible, remember you want the most surface area aimed back into the room. I’ve got about 6 to 8 inches of these hot coals banked up under the grate and they radiate like crazy. You’ll want to remove anything that blocks radiation, screens, glass, etc… whenever possible. The other thing that I found that blocks a lot of heat is the new wood added to the fire. I stopped adding it to the front where it insulated the room from the radiant heat. Put new (cold) wood in at the back and pull it to the front as the other wood burns into coals. This will keep the hottest part of the fire to the front and you can use the log in the back to as a plow to conintously bring hot burning wood to the front.
You’ll also notice that there’s very little flame on this fire. Flames are hot gasses and plasma released by the fire. They are evidence of moving gas (convection currents) and mostly just carry heat up the chimney. In a fire, the movement of air is called “draft”. One of the reasons that wood stoves are so much more efficient is they control the amount of air to the fire so the burn is slow and hot. While a fireplace will never compete with a wood stove for efficiency, you can control the draft by keeping the fire tightly concentrated over the hot coals, limiting the space between the logs so that not too much air flows between and using coals on the top of the logs to ensure a complete burn. By keeping the logs tightly spaced over the bed of coals, you ensure that hot air rising off the coals flows as slowly as possible over the wood that needs to be burned. This will make the wood burn hotter with less smoke. The close spacing of the logs acts like a baffle, slowing the air as it has to pass between the tight spaces. This backed up air is very hot and can make the wood above spontaneously combust. You can further heat this air at the time it contacts the wood by putting some larger orange coals on top of the logs in the spaces between the logs. The air moving through these spaces between logs, effectively blows on the coals providing more heat and oxygen for a hotter burn. If there are some black coals near the edge of your banked coals, try putting them on top for a more complete burn.