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Wood burning stoves as secondary heat sources

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(@derek-m)
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@majordennisbloodnok

I carried out some research on the properties of wood when I was working on a project at Slough Heat and Power, when they had just converted from burning coal to wood chip.

If my memory serves me well, 1 tonne of dry wood contains approximately 20Gj of energy, hard woods being more dense occupy a smaller volume but contain the same amount of energy. The actual structure of wood contains a lot of open space, which readily absorbs moisture, to the point where it can become water-logged. Removing this absorbed moisture can be achieved by heating the wood, or just leaving it to dry out in the open air, after removal I think the moisture content is in the region of 10% to 20%. Once the absorbed moisture is removed, the remaining moisture is contained within the cells of the wood and is more difficult to remove.

Putting wood with a higher moisture content into a wood burner will require some energy to be needed to dry the wood before it actually burns. The higher the water content, the more drying energy that will be required.

 

This post was modified 2 years ago by Derek M

   
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Majordennisbloodnok
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Posted by: @prjohn

@majordennisbloodnok Unfortunately I can't find the source of that information. My understanding is that wood contains more than just "wood" oil and resin being just two elements within the wood, both of these have a calorific value. Oil is lost when kiln-dried thereby losing calorific value. Granted these components will create more smoke/gasses and this is where secondary burn comes into its own by recirculating these gasses for a second burn thereby creating more heat and higher efficiencies. Burning gasses is a feature of modern stoves to create high efficiency and to eliminate/reduce co2 emissions.

 

That makes sense, @prjohn, although I'd prefer to see some empirical evidence to back up the apparent common sense before I were to adopt that stance myself.

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Majordennisbloodnok
(@majordennisbloodnok)
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Posted by: @derek-m

@majordennisbloodnok

I carried out some research on the properties of wood when I was working on a project at Slough Heat and Power, when they had just converted from burning coal to wood chip.

If my memory serves me well, 1 tonne of dry wood contains approximately 20Gj of energy, hard woods being more dense occupy a smaller volume but contain the same amount of energy. The actual structure of wood contains a lot of open space, which readily absorbs moisture, to the point where it can become water-logged. Removing this absorbed moisture can be achieved by heating the wood, or just leaving it to dry out in the open air, after removal I think the moisture content is in the region of 10% to 20%. Once the absorbed moisture is removed, the remaining moisture is contained within the cells of the wood and is more difficult to remove.

Putting wood with a higher moisture content into a wood burner will require some energy to be needed to dry the wood before it actually burns. The higher the water content, the more drying energy that will be required.

 

@Derek-m, that is precisely my understanding give or take the potential new caveat @prjohn has mentioned about the theoretical loss of more volatile constituents that might otherwise burn and add to the calorific value.

The addendum to what you've laid out, of course, is that kiln drying is one of those ways to remove some of the remaining moisture contained within the cells of the wood, hence the preference for burning logs treated like that. Nonetheless, from the point the logs are removed from the kiln, my understanding is that atmospheric moisture will start to be reabsorbed until the wood reaches an equilibrium again with the surrounding humidity. Given a load of logs will last me six months or so, that rather suggests the logs I'm burning later on might just as well have been air dried anyway, so that's what I buy from the get-go. I might be wrong and find the logs take longer than that to get back to air-dried levels of moisture, but I went to fairly great lengths when I built the log store to ensure good air flow and protection from the elements.

105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and inverter
Raised beds for home-grown veg and chickens for eggs

"Semper in excretia; suus solum profundum variat"


   
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(@derek-m)
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@majordennisbloodnok

If my memory again serves me well, to drive out the tars and oils from wood requires heating to several hundred degrees. When done in an oxygen free atmosphere the end result is torified wood, which does not re-absorb moisture. 


   
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Majordennisbloodnok
(@majordennisbloodnok)
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@derek-m, forget memory, you're already past my original knowledge.

I followed up what you mentioned and you're quite right. It seems the process drives off volatiles and reduces the calorific content by about 10% whilst producing a hydrophobic and stable fuel source. Or dimensionally stable material for guitars, but that's another story....

 

https://en.wikipedia.org/wiki/Torrefaction

105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and inverter
Raised beds for home-grown veg and chickens for eggs

"Semper in excretia; suus solum profundum variat"


   
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(@charjes2)
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I wanna buy a wood-burning boiler stove along with a flue stat and an ESBE load valve. Does anybody have experience?


   
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(@george)
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We have two log burners in our house and try to source all our wood from local tree service companies who are cutting down trees and we then process it and store it until its ready to burn. I had a quick look at getting a large pallet of kiln dried logs and they used to be £280-320 and the same large pallets are now £470-500.

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(@prjohn)
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@george Most kiln-dried wood comes from Poland, so not environmentally friendly. I get my logs locally from managed woodlands and windfall. Logs are airdried to approx 15%- 20% the majority of the wood sits around 15%. Just purchased two giant builders bags which came to £200. This along with a similar order earlier is two years' worth of wood. I suspect with energy prices and my fixed-term energy deal ending in Feb, I'll be using this quantity over the colder months. I just wish I had it fitted for water heating. 


   
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(@kev-m)
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Posted by: @prjohn

@george Most kiln-dried wood comes from Poland, so not environmentally friendly. I get my logs locally from managed woodlands and windfall. Logs are airdried to approx 15%- 20% the majority of the wood sits around 15%. Just purchased two giant builders bags which came to £200. This along with a similar order earlier is two years' worth of wood. I suspect with energy prices and my fixed-term energy deal ending in Feb, I'll be using this quantity over the colder months. I just wish I had it fitted for water heating. 

I've just been stacking my recent delivery and seeing what I have. I can fit in another big bag so I'm going to order that today.  I also just ordered 1/2 tonne of coal so that will be me fully stocked. 

I had water heating on my woodburner but had to lose it when I had the ASHP fitted. Our coal stove does have a hot plate so I suppose I could heat water/cook on that if I have to 😮 


   
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(@redbuzzard)
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We installed a multi-fuel stove in our new build, although we do not intend to use "coal", only wood.

However there does seem to be a growing (no pun intended) concern about burning wood for heat. I'm a bit dismayed, having just bought a new stove.  At least professor Whitty allows that wood-burning in a rural area is not the same as everybody doing it in an urban one.  The least we can do is to ensure we burn only reasonably dry wood which is far less polluting than wet/green.

Guardian - wood burner pollution

I can't say we bought it for aesthetic reasons although I like using it  - I just hated being totally dependent for heating on something that can break down and also relies on mains power being reliable.  In a village with overhead low voltage supplies, power cuts are not unusual although generally short-lived.

Sure enough, 7 months after we moved in our heat pump failed in late November and it took 10 days to get a fix (a new outside unit).  So the stove was brought into play.

Although it's only 5kW rated it gave us one very warm 45 sq. m. room and a warm enough house (lounge doors wide open).  So I feel justified in installing it. Other than buying seasoned wood, we used a couple of fan heaters to warm the bathrooms for an hour each morning.

The stove itself is a Charnwood Skye 5 which I have just about got the hang of.  I've used it a few times since we got the heating back, on cooler evenings, and the difficulty has been in not getting too hot.  I just about have the hang of it now.  I've ordered a moisture meter so I can ensure I am always user the drier wood - it's easier to keep the air closed up a bit when it's burning easily. 

What I will say for the benefit of others in well insulated houses, apart from "don't buy too big a stove", is to look at the minimum heat it can produce.  Ecodesign stoves, among other things, are so designed that the minimum air supply will give sufficiently complete combustion to keep pollution within limits.  Unlike the little Morso that we had on a boat about 20 years ago, it can't be made to 'tick over' such that it will stay in overnight.  Apparently, operating like that is massively polluting which I wasn't aware of at the time.  The Skye 5 according to spec will produce 2-7kW.  I initially intended to buy a Nordpies, a nice stove and cheaper, but the minimum output of that stove was 3kW.  3kW will heat a largish room in a house meeting current standards of insulation, and is probably too much to use as supplementary heating unless the main system isn't running.

E&OE - I'm not quite an expert. 

2021 built 2 storey detached house, 212 sq.m. / 2300 sq.ft. heated area. EPC 87B. Mitsubishi Ecodan 11.2Kw ASHP, weather compensated flow temp, UFH, MVHR.


   
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Mars
 Mars
(@editor)
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@redbuzzard, wood burning stoves can emit a variety of pollutants into the air, including particulate matter, carbon monoxide, volatile organic compounds (VOCs) and nitrogen oxides.

But we swear by our Chesneys wood burning stoves. We maintain them and only burn seasoned wood, and they have saved our bacon on numerous when the heating’s gone out. They create a cosy environment and spike room temperatures when temperatures outside drop. We are very rural, so we’re OK, but I understand the issues posed by wood burners in towns, cities or large villages.

In terms of carbon dioxide emissions, the CO2 emissions from oil and gas are generally higher than those from wood, especially if the wood is sustainably sourced and burned efficiently.

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(@allyfish)
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We have a 10yr old Bohemia Regent 5kW stove as secondary heating. A fairly cheap sheet steel multifuel burner. It's lit every night in the winter and late autumn/early spring burning seasoned hardwood. Very occasionally we add some ovoid smokeless fuel to it to keep it burning if we're going out. We always burn bright and never let it smoulder. Given the number of power cuts we experience each year I would not be without it. The cost of buying seasoned wood is sky rocketing however, same as all other fuel prices.


   
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