My stove—a Hitzer 983 fireplace insert--has been running continuously for six weeks now. Since it is marketed to heat a 3000 ft.² living area, and I have 4000 ft.² of living space, I knew before I got it that it would only supplement my existing heating system (propane forced air), not replace it. However, due to my enthusiasm for the stove, as temperatures have gotten colder I’ve tried to “push it” to transfer as much heat as possible to my living space. Generally, this involves running stove temperatures of 400 to 500°F. My draft is good—really, too good—and I am generally burning 60 to 70 pounds of coal per day. The firebox is approximately 20" x 20" x 9" deep. I shake and load twice per day and easily get 12 hour burn times. The fireplace is in a vaulted family room, and the warmest spot in the home is the second-floor hallway above, and open to, the family room. The family room itself is comfortable in terms of temperature, but not especially warm—I assume in large part because of the amount of air rushing back to the stove in order to support combustion, and because heat output from the stove flows in an unobstructed fashion to the second floor. This setup works fairly well, and the rooms off the upstairs hallway are comfortable, but not too warm. The coldest areas of the house are on the first floor, where the insert is located, farthest removed from the family room. Still, these rooms are warm enough for reasonable comfort. Only when the outside temperature has been around 20 or below has it been necessary to enlist the aid of the propane forced air system.
With this as my experience so far, I learned something about my stove when temperatures recently went into the upper 50s, and I tried to idle my stove for a total of about 36 hours. At these times, I would load about 12 pounds every 12 hours, and would arrange it in a pile in the center of the firebox. I would shut down the ash pan vent entirely, since with my stove, some air can still get into stove through that vent even though it is entirely shut down. This small amount of air would support combustion of the pile in the center of the firebox and, although I typically reloaded, because of my work schedule, about every 12 hours, I have no doubt that that pile would remain healthy for at least 18 hours. The other thing I did at these times was to slow the convection blower fan speed by about 65%. What I learned from this experience was that, even though when I was pushing my stove and achieving temperatures of 400 to 500°F, and was indeed getting more heat output from the stove into my home, my heat output from my idling strategy was still pretty good. Even while idling, I would get temperatures of 280-300°F, measured at the stove.
This experience has me thinking about issues of efficiency. Although I do not know the precise meaning of this term as it applies to heating/ventilation, what I mean by the term efficiency is: how effectively does the stove release the heat from a given amount of fuel into the living space of the home. I suspect that when I’m pushing the stove, and when stove temperatures are as high as I’m comfortable running and the convection fan is at its maximum speed, my stove efficiency, as defined above, drops pretty substantially. After all, burning a little over 20 pounds of coal per day while idling still produces respectable stove temperatures, while burning three times that amount of coal increases stove temperature only by 33 to 66%. If it is true that my stove efficiency drops, this means that less of the heat content of the given amount of coal is being transferred to my living space, and more is being lost in some way.
If my recollection of physics is correct, there are three ways to transmit heat: radiance, conduction, and convection. A traditional potbelly stove would rely primarily on radiance, plus a small amount of passive convection to the air surrounding the stove. An insert, on the other hand, relies primarily on convection. Given my stove and my installation (the stove rests on top of several fire bricks in order to be high enough to fit into my fireplace correctly), very little heat would be conducted away from the stove. Also, because the vast majority of the exterior surface of my stove consists of jacketing for the convection fan and ductwork, there is very little surface area of firebox wall that is not jacketed, and from which radiant heat transfer could take place.
In a perfectly efficient insert scenario, all of the heat radiating from the firebox wall would be retained by the jacketing, and all of this heat would be transmitted by the forced convection blower fans out the heat vents and into the room. In addition, as much heat as possible would be retained in the stove rather than sent up the chimney.
In the less efficient scenario when I am pushing the stove to high temperatures, I suspect that the extra heat is lost in several ways. First, the convection fan and ductwork approach may be limited in its ability to retain and transmit heat via convection, so that some of the heat radiating from the firebox is transmitted to the outer wall jacket, and lost via radiance or convection to the masonry of the fireplace and chimney. Also, some of this heat may be lost by going around the insulation surrounding the stove pipe inserted up into the chimney, and lost out the top of the chimney. Second, it may be lost directly from the firebox out the stove pipe and out the chimney, especially since my draft is so high, and airflow supporting draft would be increased with increased rate of combustion. I suspect that the second possibility is leading to more heat lost than the first possibility, although I do not know for sure.
If this is true, it seems to me that there must be an ideal rate of combustion/stove temperature that permits the stove to operate at maximum efficiency. Other than general experience, is there a more precise way to identify temperatures at which the stove would be operating most efficiently? If my other considerations about heat loss in my situation are true, it would seem important to find ways to limit draft further. Is it possible or advisable to use a barometric damper when I have no easy access to the stove pipe, and the stove pipe consists only of a 5 or 6 foot section inserted into the chimney?