Heat demand and wood consumption

STEP 1. What is the heating load of my building?

It's not possible to tell how high the consumption of wood might be, if the average heat demand of the building is unknown. This is called the heating load, most often estimated on the basis of heated room area. Exact data can be derived from a heating load formula according to EN 12831.

Heating load estimation for a given heated room area and building insulation standard. According to the graphic below, a low-energy framehouse with 142 sqm (approx. 1528 sq.ft.) is estimated to have a heating load of 5.8 to 6.5 kW, average approx. 6.1 kW.

STEP 2. How much wood has to be fired every day?

Before you learn anything about consumption of wood, you must know the specific heat demand of the building first. The very most frequently asked question is: "How often will I have to fuel my stove?"

For some people it wouldn't be a problem to stoke their stove with up to 40 kg of wood every day, whereas for busy households and hard working people it would be almost impossible to find time for this. Because the heating load is related to outdoor ambient temperatures between -12 and -16°C (10–3°F), it always refers to the maximal heat demand of the building. However, during the cold winter season, up to 60% of days are usually having an average outdoor temperature between -5°C and 5°C (23–41°F). In this case, the actual demand is around 30-50% below the expected peak value.

Therefore it is advised to distinguish between a normal winter day and the theoretical peak demand. The amount of wood to be fired every day is a natural limit, determining how far it is possible to think of tile stove as a heating source. First, if the stove will be stoked only manually, the maximal heating load of the building should not exceed 9 kW! More heat can be delivered only by continuous burning, using pellets as fuel and a blower for forcing air draught. Combination of wood and pellets in one stove (with BRUNNER pellet module) extends the heating load limit for a stove-based central heating up to approx. 12 kW.

The amount of wood to be fired every day depends on the heating load of your building and the outdoor temperature. The graph includes an active night setback function and hot water demand of four residents. Solar, passive or any other energy incomes are not included! For extreme winter conditions (avg. outdoor temperature -15°C / 5°F), a building with a heating load of 6.1 kW will need 42 kg of wood to be fired every day; for normal winter days with outdoor temperatures between -5°C to 5°C (23–41°F) only 16-28 kg is required.

STEP 3. How high is the annual consumption of wood?

When talking about daily wood consumption, it's a normal thing to ask about annual consumption. The required yearly amount of wood is important especially when using split logs - which must be kept in a dry place - for example, to provide enough room for storage. The same applies when planning a pellet stove, which includes a yearly stock of pellets to be held somewhere inside the building.

Annual wood consumption relating to the heating load of building and outdoor temperature. Domestic hot water supply is not included. When estimating annual wood consumption, it is deciding how long is the cold season. For example, in Munich there are approx. 255 days with an average temperature of 4.1°C. In this case, with a heating load of 6.1 kW, the expectable annual wood consumption is around 4100 kg.