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Originally Posted by bretthunting
JAK
i kind of agree, i heat soley with wood,i have an indoor parlor stove,cut wood myself etc., my delima is that we work in town and are not home all day and are not comfortable leaving a fire burning in the house while no one is home.i am wanting something that can sit outside and is safe to leave burning while we are gone so that on those really cold days we do not come home to a 40 degree house. it is probably not a really effiecent way but hopefully it will be safer.
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That might work very well because you could use the indoor stove once or twice a day when you are home during the shoulder seasons. On the really cold days you could use the outdoor stove. If it could burn efficiently for an 8 hour stretch it could be started in the morning and restoked when you return, and again when you go to bed.
We have 9000F degree-days here and the coldest cold snaps would be close to 100F degree-days. Most people here that heat exclusively with wood so so efficiently and use 4-5 cords on average, some less, some more. If you used 4.5 cords for 9000F degree-days you would need 1/20 of a cord for the coldest days which is about 100 pounds of wood. You could do 20 pounds of that on the inside stove, for 8 hours from when you get home to when you go to bed. 2.5 pounds/hour is averaging 12,500 BTU/hour which is a typical parlour stove good for burning 4-20 pounds a day. The other 80 pounds you would want to do in 3 firings, so a firebox capacity of close to 30 pounds. That firebox would need to be large enough to hold 30 pounds and well insulated enough to burn 30 pounds very hot. That is still only 20,000 BTU/hr average, but probably 40,000 BTU/hr peak. It is not that much bigger or different than an inside wood stove on the very inside, but it is surrounded by lots of baffled runs to extract heat by convection rather than radiation.
There would be baffles between the firebox and enough distance for combustion to be completed before any heat is removed. Those runs would wrap around the combustion chamber to keep it hot. Then there would be the sections for the air to air heat exchanger with enough surface to transfer heat and cool down to at least 150F before hitting the flue. These runs would need to run in the opposite direction as the cold air return and hot air exit from the house, and the hot runs would be closer to the firebox, and the warm runs closer to the exterior. Outside of all this would be lots of insulation, perhaps R60. The bottom of the stove should be on the ground. In fact, the entire stove should be buried.
Whatever temperature the flue gas is as it leaves the heat exchanger will be a few degrees warmer than the temperature entering the house. Perhaps 20F degrees in order to get heat transfer with a limited flow rate. If the air is entering the house at 130F while delivering close to the peak at 30,000 BTU/HR the volume of hot air into and out of the 70F house is about 30,000 ft3/hour.
(30,000 BTU/hr / 0.018 BTU/ft3degF) /(130-70)degF = 27,777 ft3/hr
That would require a fan at 463 cfm. That's a lot of forced air. At peak 40,000 BTU/hr you would need 140F and 530 cfm. You really don't want to go much higher than 140F and 500cfm. In fact, that is a rediculous amount of hot air blastic out of your furnace and into your living room. Even if you did average a steady 20,000 BTU 24 hours a day, that would still be 370cfm at 120F or 300cfm at 130F.
300 cfm though a 6" air vent requires a lot of power even over a short distance. That power is heating up the air inside all those baffles in the wood stove. In the end you would burn less wood and more electricity, but that is not the point of the excercise. With a square 6"x6"vent that is 5 feet per second. Because of all the baffles for heat exchange your would need perhaps a 200w motor more or less, because of the baffles in the heatexchager of the stove. Its a tradeoff between motor size and the flue temperature and thus stove efficiency. 200w to deliver 20,000 BTU or 6000w of heat ain't bad. Anyhow, its still way too much heat to blast at the feet of somebody in their Lazy Boy recliner. Blasting it up to the ceiling and letting it mix and diffuse through the house on the way to the cold air returns would be better.
Not a bad system for intermittent use on coldest days. We still haven't figured out how much heat is going to the outside during operation in -30F weather that you will be using it. If the air to the house is 130F that is a temp difference of 160F from the outer layers of air exchangers sourounding the firebox and other baffles. If the firebox is 12"x12"x24" and its surrounding baffles and firebrick insulation are another 6" for 24"x24"x30" and the air-air exchanger is another 12" for 48"x48"x48" we will want another 12" of insulation outside of that at R60 for R5/inch. So we have a 5'x5'x5' BOX with 150 ft3. The loss to the outside would be 160x150/60= 400BTU/hr
400/20,000 = 2% So in this extreme case would lose and additional 2% in efficiency by being outside. With only R20 insulation you would lose 6%.
You would have 60 cubic feet of insulation for R60, which cost $12/cuft and thus $720, also because you are inside. R20 would be more economical if used for only the coldest 25% of degree-days. The volume and surface area go down a little faster also, but lets say the cost of being outside would be $250 plus 5% efficiency. If you did bury it in earth that would not really work because you are not operating continuously. During the winter thaws all that earth would eventually cool, and all that mass would have to be heated up again. Some of that heat would be extracted by running the fans after the fire is out, but not all of it.
So if the stove is well enough designed to be 75% efficient for 8 hour operation, you will lose 5% for being outside, plus perhaps another 5% for having to maintain higher flue temperatures to get the heat tranfer with limited surface area. So we might easily be down to 65%. It would be preferable to have a thermal mass type masonry heater underneath the house. Even the stove shown would work better under the house. The flue could give off its extra 5%, you wouldn't need the outside wrapper of insulation, you wouldn't need to lose the 5% for losing heat to the outdoors, and you wouldn't neccessarily have to run a 200w fan. You could just run at 60,000 BTU/hr for 8 hours in you basement each night and let the heat transfer slowly to the rest of the house. To store 640,000 BTU for those remaining 16 hours you would need about 2,500,000 lbs of earth, or about 1200 tons, which is about 1200 cubic yards. You could achieve this by excavating a 6'x6'x12' furnace room in the centre under your house, leaving all the earth around it and then insulating around the perimeter of you home down 6' to 12'. Rather extreme perhaps, but you could run it every night and not just on cold nights and just circulate the heat up a little faster with a fan on the coldest nights. In this case just a regular wood stove would do, and could be 80% efficent at burning the same ammount of wood each night, perhaps 8 hours of burntime every evening. You could still burn 2 cord upstairs, and then another 3 cords in the basement over 100 days, and so perhaps 60 pounds each night in 3 firings 6pm,9pm,12midnight. Basement woodstove work very well if the basement walls are insulated.
