Wood burner/room/log calculation

[rob39]

Hi all
Is their a calculation of which wood burner (heat output) to use for a certain size of room and what volume of logs it would take over a year???? (I think that sound right :headscratch: )

[calvertwood]

Stoves Online (

http://www.stovesonline.co.uk

) is a very good source of information on how much it takes to heat a room.

I seem to remember reading that to fully fuel a full three-bed house (heat and water, all year round) would require about nine acres of coppice in rotation. For my house, which has solar water-heating and gas backup, I think I use about 10m3 a year of logs. Hope that helps.

[oldclaypaws]

We have a typical three bed house and use no heating other than a woodburner, although we dont use it to heat water. A 5KW rated Dursley Highlander woodburner heats the whole lower floor and by just opening the door of the room the heat carries to the rest of the house. We are perhaps more tolerant of cool temperatures than some people, and are comfortable at 60 degree F. Sooner put on a pullover than cook the dogs, who don't like it hot. We use about 6 metres cu per year. I respectfully disagree with the previous poster, I reckon 2 or 3 acres will supply more than enough wood for a modest house, although it'll depend what you have on it, if we turned our Oak on our 5 acres into just logs it would supply our needs for 150 years, by which time it would have grown again and we'd be back where we started. If you had thinner softwoods or open areas, you'd need more, ours is dense hardwood cover and hazel coppice.

[calvertwood]

OCP, you set me off looking for the source of my "nine acres" recollection. I couldn't find it, but I did find this at nativeforestry.co.uk:

A well stocked mixed broadleaved coppice woodland should produce approximately 3 tonnes of air-dried wood per hectare per year. An average three bedroomed house would need 7-9 tonnes of air-dried wood to provide all the heating requirements. The area of coppice woodland would need to be at least 3 hectares in order to be self-sufficient in firewood.

3 hectares is about 7.4 acres.

I guess it also depends on the efficiency of your home. My house has solid stone walls, high ceilings, and is at the top pf a 150-foot ridge in North Yorkshire, facing straight into the gales - so I guess I burn a bit more than some people.

[Night Owl]

The size of woodburner for a given room size obviously depends upon the degree of insulation etc. but the usually quoted figure is 1kW of heat output for every 14 cubic meters of space. There is an online calculator at:

http://www.stovesonline.co.uk/calculator.asp

Very useful reviews of most stove can be found at:

http://www.whatstove.co.uk/reviews

My understanding is that the quoted outputs of stoves can only be used as a rough guide, as it relates to the output at which manufacturers have chosen to have their stoves tested. They may therefore choose a fireing rate at which the stove is most efficient, rather than the maximum possible.

There are many variables which will affect the use of fuel: how the stove is fired, which wood is used, how dry the wood is etc. I have a Clearview Vision 500 which is rated at 8kW and I have kept records of my usage of fuel. Using dry (20% moisture meter reading) hardwood (oak, ash etc), 1m3 of wood burns for about 250 to 300 hours with the stove working fairly hard. My defintion of 1m3 is wood which is reasonably carefully stacked in my woodstore. 1m3 of loosley piled logs would occupy about 0.6 m3 when neatly stacked. I am not sure how these definations relate to the measures used by others - correct me if I am wrong but I believe the Forestry Commission definition of 1m3 (as given on felling licences) would be for the equivalent of a solid cubic meter without any air spaces.

[SimonFisher]

... what volume of logs it would take over a year????

For comparison purposes and to eliminate the differences between species, log sizes and stacking methods, it's better to use the weight of wood at a given level of moisture content rather than volume.

There's a good section on this in the excellent publication "Woodlands, a practical handbook" by Elizabeth Agate (http://www.amazon.co.uk/dp/0946752338) - Two logs of different species but with the same weight and moisture content will have the same value as fuel. Species differ in their density so a larger volume of a light wood such as willow or pine would be required to give the same value as fuel as a dense wood such as oak.

So if you have 20kg of oak logs at 15% moisture content and 20kg of lime logs at the same moisture level, the lime will feel lighter in the hand for the same size log and the 20kg will take up more space, but they'll both have the same fuel value.

[Rankinswood]

But :-

because the two woods have different densities then they will also potentially have different surface areas and coefficients of thermal conductivity which will result in a different rate and burn temperature. In general softwoods like larch or douglas fir will burn fast and at a higher temperature than oak. Softwoods generally contain highly flamable resins used to reinforce the wood (reaction wood) and repair damage unlike hardwoods that use tanin or soap based extractives to combat damage and decay. Another factor that affects burning is whether or not bark is still attached to a log, for example, Douglas Fir bark is intumesant i.e. it is designed to expand rapidly when a forest fire moves through the woods in order to provide better insulation for the underlying live woody tissue and this effect also takes place on bark in the fireplace reducing the log burn surface area and burn rate. Intumescant bark does not burn very well and can form into lumps of clinker reducing airflow to the rest of the fire.

To ensure a really good burn you need to :-

- debark logs
- split logs into smaller sections with a higher surface area
- allow sufficient period of seasoning time and air flow to ensure that the logs are "dry"
- consider mixing soft and hardwoods to control the fire burn rate and temperature

Rankinswood

[Andy M]

When stacking my logs, I try to divide them roughly into autumn wood and winter wood. The winter wood is ash and oak which provide a good hot fire in the woodburner for the very cold days. The autumn wood eg Rowan, hazel is used for those slightly chilly days when you want a fire but not too hot! Works for me.

[rob39]

Great info guys
THANKS

[Terry]

OCP, you set me off looking for the source of my "nine acres" recollection. I couldn't find it, but I did find this at nativeforestry.co.uk:

A well stocked mixed broadleaved coppice woodland should produce approximately 3 tonnes of air-dried wood per hectare per year. An average three bedroomed house would need 7-9 tonnes of air-dried wood to provide all the heating requirements. The area of coppice woodland would need to be at least 3 hectares in order to be self-sufficient in firewood.

3 hectares is about 7.4 acres.

I guess it also depends on the efficiency of your home. My house has solid stone walls, high ceilings, and is at the top pf a 150-foot ridge in North Yorkshire, facing straight into the gales - so I guess I burn a bit more than some people.

Cant give definitive numbers and as mentioned by others there are a lot of variables.
Efficiency of the building envelope is indeed a significant factor.
my stove is rated at 8kw, but is designed to put 70% of that to the boiler for heating the thermal store (which supplies HW and heating for downstairs UFH) so 30% or 2.4kw goes to the room.
The room is approx 100m2 open plan living/dining/kitchen/office and in the middle of winter it gets really hot as we have to run the fire longer than we would like to get sufficient hot water.
The house is super insulated and draft proofed
on average in the winter we burn two small losely stacked builders buckets of wood. Not sure what we use annually but it is significantly less than the 7-9 tonnes