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Old 01-21-2011, 08:30 PM   #1
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How Much Savings From Burning Wood?


Right now, it is 25 outside and my thermostat is at 70. I have a high efficiency heat pump and an insert in my fireplace. I am burning wood, the glass doors are closed, and the insert fan is blowing out hot air. The heat pump is running at full speed, but the back-up resistance heat is not coming on (thank God).

I am wondering how much I save. Combustion air comes from the heated house and is replaced by cold outside air coming in through every crack in my decently sealed and insulated house. The colder it gets, the more important that I try to keep it in heat pump mode to avoid high electric bills. But the air that comes in to replace combustion air is also colder. What do you all think?

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Old 01-21-2011, 09:04 PM   #2
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How Much Savings From Burning Wood?


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. Combustion air comes from the heated house and is replaced by cold outside air coming in through every crack in my decently sealed and insulated house
you might actually be losing money. .

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Old 01-21-2011, 09:51 PM   #3
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How Much Savings From Burning Wood?


Have calculations been done? There is a whole industry of these products. Is it a huge scam or does it actually save a little money?
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Old 01-21-2011, 10:26 PM   #4
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How Much Savings From Burning Wood?


figuring if you are saving or losing money is unique to each installation.

If you used outside air for combustion and made your home relatively airtight, all you have to do is heat the air once and then make up for losses due to lack of insulation. If you increase your insulation values, it helps prevent heat loss even more.

Once you start bringing in outside air, you have to heat that to the ambient inside temp before you break even. Since the air draw is constant, you have to continually be heating new air.



I used to know a guy that had a fireplace in his basement. It used inside air for combustion. If he left the furnace off and fired up the fireplace, it actually cooled his house.
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Old 01-21-2011, 10:33 PM   #5
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How Much Savings From Burning Wood?


I'm surprised they don't make wood stoves work like high efficiency furnaces these days. Or do they? It would have an intake so it takes air from outside. The exhaust flue would also have a heat exchanger to try to get the most heat to stay indoors. Possibly a fan to blow air over it. Idealy, you should be able to go outside and touch the exhaust and not get burnt. With typical wood stoves/fireplaces a large portion of the heat is thrown outside.
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Old 01-21-2011, 10:46 PM   #6
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How Much Savings From Burning Wood?


the do make fireplaces that use outside air for combustion.

To the removing the heat from the flue; you need to be careful about removing too much heat. A cool fire contributes to creosote buildup and the heat is also necessary to cause a draft which of course is what draws in fresh air for combustion.
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Old 01-22-2011, 04:32 AM   #7
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How Much Savings From Burning Wood?


If your woods free, your probably saving money as long as the strip heaters aren't coming on.
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Old 01-22-2011, 10:14 AM   #8
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How Much Savings From Burning Wood?


I am not clear whether the OPS has a fireplace with glass doors or a wood stove fireplace insert. The two are quite different. I have a Jotul fireplace insert, which is effectively an airtight wood stove that fits into my fireplace, and the physics of it are the same as a wood stove. A fireplace with glass doors is not airtight, and has very different thermodynamic properties (read efficiency) than an airtight.

Assuming you have an airtight, it makes absolutely no difference in terms of efficiency whether you bring in outside air or use house air. This sounds counterintuitive, but if you think about it, the heat loss associated with air is the amount of heat required to raise the temperature of the air to exhaust temperature, which is effectively a loss of efficiency. Think it through, and you will see that using inside air requires you to raise the temperature of the air less, because you already spent the energy to heat the air from outside temperature to room temperature. Either way, you lose heat to the air.

It turns out that the heat required to raise the combustion air temperature to exhaust temperature is not great, so long as you restrict airflow to the minimum required to allow the wood to burn. Hence the need for an airtight stove rather than an open fireplace.

Most of the loss of efficiency of a wood stove is associated with boiling off water in the wood, and the burning of hydrogen in the wood which forms steam, which goes up the chimney as lost energy. In a condensing natural gas heater, the energy of the steam is captured by condensing the steam to water. This cannot be done in a wood stove because the condensation would drop the stack temperature below minimum required to prevent creosote buildup, which is about 500 degrees F. Bottom line is that you can get no more than about 50 percent overall efficiency out of a wood stove, and an open fireplace is typically less than 10 percent.

All that said, the question of whether you save money is a function of how efficient your wood stove is, how much you pay for wood, and how much you pay for the alternative fuel. In my house, I can burn either wood or fuel oil. Prior to installation of the fireplace insert, I went through about 1000 gallons of fuel oil per year, which both heated the house and generated hot water. About 750 gallons per year went to heat the house, which is about $2500 per year at current prices for fuel oil.

After install of the wood insert, I go through about 300 gallons of fuel oil per year for heat, which is a savings of about $1500 per year on oil. I burn about 4 cords of wood per year, which cost me a nominal $400 because I purchased 9 cords of wood as logs. Figuring in cost to saw up, split, stack and retrieve the wood, I doubt I save any money, but it does give me great pleasure to watch the fire, and consider that I am mostly out of the world oil market, at least in terms of heating my house.
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Old 01-22-2011, 01:20 PM   #9
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How Much Savings From Burning Wood?


I have an insert with glass doors that are not airtight.
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Old 01-22-2011, 03:52 PM   #10
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How Much Savings From Burning Wood?


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All that said, the question of whether you save money is a function of how efficient your wood stove is, how much you pay for wood, and how much you pay for the alternative fuel. In my house, I can burn either wood or fuel oil.
Ayuh,... sweaty, you're going to have to run your numbers as stated by Daniel to get the answer to your original question...
You're probably saving Something, but only runnin' the numbers can say for Sure...

I threw the final valves on my owb project, the monday after christmas,....
I've spent nothing but sweat equity for Heat, 'n dhwater ever since...
I built the boiler from dumpster finds, 'n salvage...
My Bud, the Tree Guy gives me All the wood I want, 'n more...
I'm livin' Large, 'n for nearly Free,... Free of Oil anyways...

Oil last year went over $4,000.00....

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