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                          The Physics of Heating with a Fireplace
 

             A few years ago I was staying with some friends in a mountain cabin and on a cold morning they built a huge fire in on of those open fireplaces in the center of the room with a hood over it. The trouble was that the room stayed cold. I rearranged the logs in the fire and suddenly we got much warmer....Please explain. Wouldn’t that be a neat examination question for a physics class but perhaps they don’t teach that kind of physics any more. Anyway, here’s the answer.

               Heat can pass by conduction, convection and radiation. The Sun heats the earth by radiation. Radiation is rays of heat and that is how a fire feels hot when you warm your hands by holding them to the fire. Convection is the circulation of hot air. Hot air rises. The smoke goes up the chimney because hot air rises. The hot smoke heats up the chimney. The material from which the chimney is made conducts the heat from inside to outside. Conduction is the transfer of heat in a solid. Copper conducts heat very well. Cement conducts heat poorly. The hot outside of the chimney heats the air in the room which circulates. That circulation is called a convection current.

             An open fire in a fireplace heats a room almost exclusively by radiation. Conduction and convection from a fireplace are not much unless there is a Heat-O-Lator taking air from the floor, passing it around a metal firebox and heating the room by convection. Most of the time however, radiation is the only way in which the fire in the fireplace heats a cabin.

             Any decent fire has coals glowing red in the middle. The chemical reaction, burning, is going on and the heat released has to go somewhere. The center of the fire gets hotter until the rate of heat loss by glowing equals the rate of heat production by burning. The red hot coals in the middle of a fire are really the only way in which an open fire heats a room.

             This is where the imp comes into the picture. The fireplace imp lives up the chimney and the object of the fire builder is to prevent the imp from seeing the glow of the fire by blocking his view. If a log falls apart in the middle of the fire then the fire builder must carefully rearrange the fire so that a new and cold log blocks the view of the red hot coals from the imp who lives up in the chimney.

             In the cabin where I was staying people just threw logs indiscriminately onto the fire and mostly the cold back sides of the logs were facing the room and blocking the heat in the center of the fire from reaching the room. The imp was having a field day and could see every glowing coal. The solution was to rearrange the fire so that the cold tops of logs were blocking the imp's view from up the chimney. The heat from  the glowing center of the fire poured out into the room as it was no longer blocked off by the cold backs of logs. The imp up the chimney could no longer see the hot glowing center of the fire. Well, heat has to go somewhere, so it came out of the side into the room and we all got much warmer immediately.

             This is how I made the fire in the fireplace heat the cabin. The simple rearrangement of the logs so that the imp up the chimney couldn’t see the glow any more dramatically boosted the heat coming into the cabin.

             One reason that an open fireplace is such a rotten room heater is that the air supply needed for the fire comes out of the room you are trying to heat. Once I had a landlord in England who did such a good job of caulking up all the leaks in the living room that the fire went out unless Mrs. Watson left the door open a crack. That was also why the roaring fire in the cabin was so ineffective.  A howling gale of hot air was going up the flue and being replaced by a howling gale of freezing air pouring in through the chinks in the cabin. I have never seen a fireplace built so that the combustion air came from outside, but the idea would seem to have merit. In fact, outside air coming round the Heat-O-Lator would appear to be perfect. I have just never seen it.

             The imp lives up the chimney of your living room fireplace too. The best way to build a fire to heat a room is with a back log, two side logs and one or two top logs. The glowing combustion area shines straight out into the room and the imp hardly gets to see anything.. The logs make a sort of three walled house with a two log roof. Grates are not often helpful. They tend to let the glowing bottom of the fire shine onto ashes which heat up the air supplying the fire and the heat goes up the chimney just the same. The object is to have the glow shining out into the room.

             So, there it is. That is the story of the imp that lives up the chimney. The object of the game is to stop the imp from getting your heat for which you had to work so hard cutting and splitting wood. It is a neat game and a fun way to make your fireplace work better.

            Just make sure that the imp up the chimney cannot see the glowing coals in the fire.