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General: Calculating how much wood I will burn

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      CommentAuthorNovy Mlyn
    • CommentTimeAug 6th 2007 edited
     
    Hello,

    Does anyone know where I can find a formula to work out how much wood I will need for the winter?

    I have calculated how much heat I will need in KWH, and have a supply of dry wood, but I don't know how much I am likely to burn. I'm a metric girl, by the way. There are american pages which use pounds and british heat units.

    I think: 1 kw is 3.413 BTU, a stere is a cubic metre of wood equal to .276 chords of wood.

    I've found this formula:
    http://www.aboutbioenergy.info/Calculator1NN.html
    but it gives the result in gigajules.

    If we needed 31kw to heat the house, 10 cubic metres of wood would last for 30 days?

    (background: we have a large house with thick walls which is currently wood heated (individual stoves not central). We are looking to invest in a heat pump, but I need to check that this is not going to be massively more costly than staying with wood.)

    Thanks if you can help!
    •  
      CommentAuthornigel
    • CommentTimeAug 6th 2007
     
    You would get about 3000KwH from a tonne of wood.

    IF you need 31Kw to heat your house and you heated it continuously you would need about 7 tonnes a month.

    Have you considered wood chip,wood pellet or a log boiler if you have timber available?

    Heat pumps are expensive to install £1000-£1500 per KwH and are best used in very highly insulated buildings which are maintained at a steady temperature.
  3.  
    I think it is futile to try calculating such things precisely. Best thing is to buy a lorry load and see how long it lasts. Then buy another lorry load.

    We have two woodstoves. One is alight pretty much all the time through the winter, the other is lit most evenings. We buy oak offcuts from a sawmill in a lorry load of about 9 tonnes. This lasts us more than a winter but less than two winters.
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      CommentAuthorOlly
    • CommentTimeAug 6th 2007
     
    Going from a sustainable source of biomass to a heat pump isn't really very green, the CO2 emissions from your home are going to increase substantially, and as they use electricity they're not particularly cheap to run. (In the UK that is)

    I agree with Biff, in that any calcualtion is going to be a rough estimate at best. Even if you could model your house perfectly you're always going to have countless other factors (e.g. weather, wood moisture content) to worry about.

    However, it is worth trying to calculate a ball park figure.

    I forget the source of my figures (although it may have been the National Energy Foundation Website), but in the past I have used 3972kWh/tonne for wood chip and 4667kWh/tonne for pellet, assuming 25% and 10% moisture content respectively. Those may be worth double checking.

    What is your estimated winter heating demand, you say you have calculated it in kWh?

    As I'm not quite sure what calculation you're trying to do, this may (or may not) help.
    1kWh is equal to 1000 joules per second for an hour, so 3.6 megajoules
    Therefore 1 kWh = 0.0036 gigajoules
    1 Gigajoule = 278kWh

    Olly
    • CommentAuthortony
    • CommentTimeAug 6th 2007
     
    Were you to insulate your home well enough you would not need to burn any. Using the sun is good for making hot water.
    •  
      CommentAuthorNovy Mlyn
    • CommentTimeAug 8th 2007
     
    Tony, we tried the no heating thing once, but the house remained below zero! However, the house is well insulated. The weather here has been between 25-35 degrees c this summer, yet the ground floor of the house has been a comfortable cool temperature, and only slightly too warm upstairs on extremely hot nights.

    Olly, I calculated the heating requirement by calculating times for heat transfer on the basis of average indoor and outdoor temperatures. The house is very large, but will be training & accommodation space as well as domestic. A frightening 31kWh demand. We will couple the heat pump with a generator, either water or wind powered. At the moment I am thinking that the heat pump will keep the house at a basic temperature with stoves in individual rooms for aesthetics & top up heat. Our cost for electricity is about 10p per kWh, though this will be reduced once we have the heat pump fitted.

    Bifvernon, thanks for your estimates. I'm trying to make rough calculations to ensure that we don't simply freeze in the winter!

    I am now awaiting a system design from a Mitsubishi rep as well as Stealth (http://www.srtuk.com).

    Would wrapping our house in hay bales help?
    • CommentAuthorbens
    • CommentTimeAug 12th 2007
     
    I have no idea about how to calculate this but have you looked at Thermal Stores as a solution>? we are looking at this for our heating and hot water requirements with solar and a wood fired stove with back boiler. We are looking at a Clearview 750 with baffle boiler. The thermal store idea came from advice on this forum.

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