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Let's assume 1kWh of electricity is used to run a fan heater. Heat pumps are more efficient, but have a range of different efficiencies, so perhaps aren't a good base comparison.

Assuming a good quality log burning fireplace, how much firewood (by weight) does it take to produce the equivalent of that 1kWh of heating? This stove claims combustion efficiency of 89.1%, but 70% is perhaps a reasonable standard to work with.

Obviously water content of the wood would make a big difference too, so let's assume either 10% or 20%, whichever is easier. I understand 20% is considered to be dry, when dealing with firewood.

  • When comparing a space heater to a heat pump, is coefficient of performance a better metric than simple efficiency? 100% of the electricity used by a space heater results in heating, while the same is not true of a heat pump. But, the heat pump produces more heat with the same amount of electricity, because it takes the "free" exterior heat for some its input energy. – LShaver Dec 21 '16 at 19:02
  • getting a usefull result could be impossible. you would need to know the temperature of the air that is replacing the air going up the chimmeny.As this air ultimatly has to come from outside, outside temperature needs to be factored in. however this air circulation is lowering the moisture in the house which also has a bearing – Peter Wildman Dec 8 '17 at 17:25
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Per Google, 1 kWh = 3412 BTU. (And a fraction, which I'll neglect.) Here's a table (one of many found with Google) of BTU/cord for various kinds of wood: https://chimneysweeponline.com/howood.htm It varies a lot by species, but let's say 20 million BTU per cord, and 3500 lbs/cord as average for hardwoods. So 1 lb of average hardwood gives you 5714 BTU. Multiply by the efficiency of your stove (70 - 89.1%) gives 4000 - 5091 BTU. So it takes roughly 0.67 - 0.85 lbs (304 to 386 grams) of firewood to equate to 1 kWh of electric heat.

  • You've ignored the fact that burning wood for heat requires managing exhaust. You won't get the heating figures you calculate in real life because some amount of heat is lost up the chimney (whereas there is no exhaust at the point of heating for resistive electrical heating). – Jean-Paul Calderone Dec 9 '17 at 0:25
  • Chimney losses are included in stove efficiency. In fact, they're a major part of that efficiency. – jamesqf Dec 9 '17 at 3:12
  • This may be what some folks claim but it's not really true. There's no way to rate a stove for chimney losses because every home and chimney is unique. The efficiency rating has something to do with the lab in which the stove is certified - optimal conditions that don't reflect anyone's actual home. – Jean-Paul Calderone Dec 9 '17 at 13:37
  • @Jean-Paul Calderone: Which, I assume, is part of the reason why wood stove efficiencies are given as a range. – jamesqf Dec 10 '17 at 4:50
  • My understanding is that they are given as a range because that is the range required by EPA for certification. If you get your stove certified, then you are in that range by definition. Then you can skip the separate testing phase (with its extra expense) for determining your product's actual efficiency. – Jean-Paul Calderone Dec 13 '17 at 13:51
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I would present a different analysis to jamesqf, because his does not take into consideration the latent heat of evaporation, or at least does not mention it.

If we're assuming a regular hardwood used typically for firewood (here in the UK at least), which has a net calorific value of 4.06 kwh/kg at a moisture level of 20%, if this is burnt in a stove with an efficiency level of 70% then it is safe to calculate that it would take a 0.352kg piece of firewood to produce 1kWh of electricity.

If the moisture content of the firewood was 10% then it would take a 0.307kg piece of firewood to produce 1kWh of electricity.

In calculating these figures I have used a net calorific value for dry firewood of 5,251. However, and this is a big HOWEVER, this calculation assumes that the 1kWh produced can be perfectly turned into electricity by a thermoelectric generator. This is a naive assumption, hence I would like to present a probable calculation about how much firewood we would need to produce 1kWh of electricity, because that's really what the question is asking. The problem however is that currently I know too little about this process, let me do some research and I may be able to provide a better answer in the not too distant future.

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The link below gives you net calorific values and energy densities for different woods with different moisture content.

http://www.woodenergy.ie/woodasafuel/listandvaluesofwoodfuelparameters-part1/

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    This is only one component of the answer and the OP is still left with not knowing how to calculate. And the link is incorrect and only for Finnish wood. And link-only answers are unwanted on SE sites because of possible link rot. – Jan Doggen Dec 13 '17 at 14:04
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Let’s look at this anther way.

  • Your house will have some insulation
  • There will be heat input from the people and electrical equipment in the house.
  • There will be heat lost from the fireplace including when the stove is not in use.

Therefore having a fireplace will increase the number of hours a year you need to heat compared to electrical heating.

So the questions as asked does not lead to a useful result, in some homes the wood burned will have a negative effect on the heat needed, assuming that the fireplace would not otherwise be there.

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    Might be useful for those considering installing a fireplace, but this definitely doesn't answer the question! – Highly Irregular Jul 13 '15 at 1:20
  • Note also that any practical wood heating would not use a fireplace, but an efficient wood stove, or fireplace insert (for those of us who bought houses with decorative fireplaces already installed). From personal experience, I can definitely state that a wood-burning fireplace insert does not cause significant heat loss, as the only times my furnace comes on is when I'm away for a day, or on cold nights when I don't want to get up to chuck in another log or two at 4 am. – jamesqf Jul 14 '15 at 21:20
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    @jamesqf, JUST having cold air coming down the chimney due to convection currents, will make any wood burning stove/inset cold. This has been shown to be an issue in VERY well insulated homes in the UK. – Ian Ringrose Jul 15 '15 at 16:35
  • @Ian Ringrose: I think I'll put my practical experience first. Now it may be true that there is a bit of overall heat loss through the chimney that would make the house a bit colder when the stove is not burning, but if you heat with wood, there are very few such times. Chimney losses would be part of overall stove efficiency numbers. – jamesqf Jul 15 '15 at 18:39

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