Unusual Wood-Burning Insert Installation
Energy expert continues to learn from a deep energy retrofit of his Montana Home. This time he's installing a wood-burning stove to save money, add ambiance, and provide security during emergencies - like when runaway trains knock out a town's power.
As part of a low-energy retrofit of my 123-year-old home, I installed a wood-burning insert into the enameled cast-iron fireplace on the north exterior wall of my living room. I wanted to save money, create ambience, and provide security during disasters or extreme weather. Back in 1987, a rail car filled with explosives broke loose from its train up on McDonald Pass and came rolling down into Helena. The tanker car derailed and exploded, breaking all the windows at Carroll College and badly damaging the P.E. Center. The explosion knocked out Helena’s power for about six hours. Oh, yeah—the temperature at the time was minus 30°F with a 15 mph wind, creating a wind chill of minus 70°F. Frozen pipes in my kitchen turned my ground floor into a skating rink. This minor disaster would have caused me no problem if I'd had a woodstove.
Clearing Out the Old Shallow Hearth
This fireplace used to burn coal in a steel basket placed on a hearth that was only 9 inches deep. I believed that the masons had cordoned off the narrow coal hearth inside a deeper hearth 123 years ago, when they built the fireplace. I measured from the interior masonry surface to the outside of the brick chimney exterior—a distance of around 36 inches. To test my big-hearth hypothesis, I employed a 4 lb hammer and a heavy flat bar to demolish the narrow hearth in hopes of exposing a deeper one in which to install a wood-burning insert.
I picked a spot on the vertical wall behind the hearth and drove the flat bar straight through 3 inches of baked fireclay. Behind that was a single brick wall and then assorted stone and broken brick. After removing several hundred pounds of masonry materials, I was left with a level brick hearth 25 inches wide and 27 inches deep. I cleaned up the mess and went shopping for an insert.
The Wood Stove Insert
I selected a Buck #20 wood stove insert. This model is noncatalytic and firebrick lined, with a secondary combustion air tube supplying air above the fire. The Buck #20 is mobile-home approved, meaning that it has a knockout on the bottom for supplying outdoor combustion air from the bottom. It was the smallest wood-burning insert I could find for my energy-efficient home. The insert has a fan that draws air in along the bottom of the firebox and exhausts heated air through a slot above the door.
The Buck #20 is only about 20 inches deep, so I decided to install some high- temperature insulation and a brick wall between the insert and the double-brick wall behind the hearth. Here is what I was thinking: I was insulating the exterior of the north wall with 4 inches of polystyrene foam. The chimney stuck out 2 inches from the old wood siding, giving me 2 inches of space inside the new insulated exterior wall to insulate the chimney. I chose polyurethane foam for the chimney insulation because of its high R-value per inch.
There are layers of masonry materials between the fire and the urethane foam, so I’m not worried about this foam becoming too warm. From the fire inside my new insert, these are the materials from the inside out: 1 inch of firebrick, ¼ inch of steel, a ½-inch air gap, a single-brick wall, 1 inch of fiberglass insulation board, and a double-brick wall before you get to the foam. I believe that the foam will never reach 100˚F, and its limit is approximately 160˚F.
Installing a Chimney Liner
The chimney had an unlined 9-inch × 13-inch brick flue opening. I chose 6-inch rigid stainless steel flue pipe for the chimney liner for a couple of reasons:
- I believed the rigid liner would produce more stable draft and collect less soot and creosote than the flexible liner.
- The rigid liner was better for packing sand against, in order to create something like a heat-storing masonry stove.
- The chimney travels up through the master bedroom above the living room and protrudes into that bedroom from the exterior wall.
- I planned to fill in the space around the liner with sand, which added about ¾ ton of mass. I believed that the sand would stabilize the flue temperature and store heat to be radiated into the bedroom slowly, the same way the massive exterior-insulated masonry fireplace in the living room stores and releases heat downstairs.
My work partner Joel Repnak and I strapped the chimney to the old siding with heavy steel strapping for earthquake safety before insulating the wall around it. I extended the masonry chimney on top by about 4 feet, despite my lack of bricklaying skills. Keeping those four little walls plumb, keeping the top level, and maintaining the rectangular dimensions were not a paragon of precision. My chimney extension has a noticeable S-curve, but not enough to interfere with the liner or to diminish the chimney’s dignity.
The chimney opens down into the wider rectangular fireplace about 4 feet above the hearth. I installed a rectangular ½-inch steel plate there, with a 6-inch hole through it to support the chimney liner. Two pieces of ¾-inch rebar, inserted through holes drilled in the outer brick wall of the chimney and penetrating its inner brick wall a couple of inches, support this plate. I slopped some mortar around the edges of the steel plate through the 6-inch hole from the hearth underneath to seal the gap between the steel plate and the masonry chimney. Then, from the roof, we connected the liner sections and lowered the liner, with its support collar attached, down the chimney and through the plate hole, leaving about 9 inches of the liner hanging down below the plate. (Note: I should have left 12 inches hanging down.) The plate supports the weight of the liner and the sand. The support collar keeps the liner from falling all the way through the hole to the hearth.
Instead of permitting the uninsulated liner to emerge from the top of the chimney, I attached a 1-foot piece of 6-inch insulated triple-wall pipe to it. This prevents a creosote ring from forming inside the top end of the liner.
Liner installed, I dumped some soupy mortar down around the liner to bolster the seal between the liner, the chimney, and the steel plate, and to keep sand from leaking down onto the hearth. Then I poured ¾ ton of sand into the chimney between the liner and the brick. Above the level of the master-bedroom ceiling, I poured perlite—a noncombustible insulation—around the liner instead of sand. The idea is to collect heat only through the chimney bordering the living room and bedroom above. When the combustion gases rise above that bedroom ceiling, they maintain their heat, thanks to the perlite, which helps to limit creosote deposits. The liner holds its high temperature to maintain draft, and the chimney assembly warms the house after the fire goes out.
Providing Combustion Air
For outdoor combustion air, I punched a 3-inch hole through the exterior brick wall behind the hearth. The hole penetrated through 15 inches of brick to the outdoors. Luckily, the brick was soft. Through this hole, I installed a piece of 3-inch galvanized vent pipe. Then I built a rectangle of used firebrick on the brick hearth with a square hole in the middle to enclose the outdoor combustion air pipe. The insert sits on top of this pedestal, and a hole in the bottom of the insert admits combustion air to the firebox.
Installing the Insert
Before Joel and I lifted the insert into place, I buttered the top of the pedestal with fireclay mortar to seal around the base of the stove and prevent leakage from the combustion air area. (It still leaks a little.) Wrestling the insert into splace and compressing the flexible liner to fit over the flue collar on the insert was difficult. Even more difficult was fastening the S-shaped flexible stainless vent connector to the end of the rigid liner up in the chimney with sealant and a high-quality hose clamp. I sweated, cursed, and almost wept, while forcing the vent connector over the rigid liner and tightening the clamp.
Ahhh, it felt good when the insert was finally in place and at a reasonable cost (see Table 1). I crumpled some newspapers, piled some kindling on top, struck a match, and the pile burst into flames. The first fire produced smoke from various residuals inside the stove, so Joel and I watched this first fire with all of the doors and windows open. Luckily my wife, Mary, wasn’t home to experience the chemical smoke, so domestic turmoil was avoided.
Heating the HouseI’ll replace the heating-only thermostat in my dining room near the stove with a heating-and-cooling thermostat. The cooling circuit, which closes on temperature rise, will turn the furnace blower on when the temperature downstairs reaches 65°F to move the heat around. Our comfort temperature is 63°F, so at 65°F the furnace blower will share the wood heat with the rest of the house.
I could plug some thermistors into a data logger or climb up on the roof with my combustion analyzer in order to evaluate the insert’s efficiency. However, my intellectual curiosity is unlikely to overcome my expanding inertia. Last winter, when the temperature was way below zero, the insert kept the whole 1,800 ft2 house comfortable. As close as I can estimate, the stove burned about 2 cords of scrap wood and saved somewhere around 12 therms of gas—which is 25% of my heating consumption. That’s all the data I need for now.
John Krigger is the coauthor of the Homeowner’s Handbook to Energy Efficiency and the Saturn Building Shell Field Guide. He teaches classes at Saturn Online.
For more information:
You can purchase the author’s books and other resources for homeowners and professionals at Saturn Online, http://srmi.biz.
The Homeowner’s Handbook to Energy Efficiency is also available on the Home Energy website, www.homeenergy.org.
For questions about his retrofit project, contact the author at firstname.lastname@example.org.
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