Mass Stoves

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We build several types of mass stoves. The choice of stove depends on the individual’s demands regarding needs and design. We will provide design and assist with appropriate placement of foundation, chimney and so on for a small fee.

Mass Stoves

Renewable Energy offers a line of different mass stoves.

The choice of stove and its design depends on the individuals needs.

In all mass stoves the “inside” is built from refractory materials (12-1600 kg) and the “outside” is built conventionally from brickwork or other stone materials (2-3 tons).

Between the inner firebrick lining (the refractory) and the brickwork, a refractory cloth 6-7 mm is placed, this prevents formation of cracks that are caused by the big difference in temperatures – inside temperatures are 500-1000 degrees and outside surface temperatures are 40-55 degrees C. In spite of these precautions it is difficult to avoid hairline cracks, when the stove is most warm. These cracks close when the stove cools down.

The fireplace is normally 30-42 cm wide, 50-70 cm deep and 1 meter high and can contain 12-20 kg of wood, depending on the tree species used – soft or hardwoods.

The fire door is normally 41×41 cm and if there is a baking oven it is 41 cm wide and 21 cm high in the middle.

The doors are made of cast iron and most of the time with ceramic glass – so you can see the fire and the pastry.

Renewable Energy offers a line of different mass stoves. We offer the ” craftsman’s-stove,” the round mass stove and mini mass stoves.

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Craftman's Stoves

Round Mass Stoves

Mini-Mass Stoves

Get help with your construction plans

If you would like us to help with your building plans, etc. please call us on 57 80 45 22 approx. 9-16. If we are at construction sites, for meetings, at the workshop or elsewhere, our voicemail is ready for short messages.

About Mass Stoves

Useful information

Below you will find useful and practical information on the construction and use of mass stoves.

What is a mass stove?
The mass stove is an economical, practical and ecologically sound solution to the problem of home heating.

Several essential matters distinguish the mass stove significantly from conventional stoves.

The interior of the mass stove is designed so that rising smoke is burned yet again and dispersed through long flues throughout the stove. This utilizes almost all of the energy for heating and only a small percentage of the heat goes out the chimney.

Because the smoke is burned a second time in an upper chamber the combustion process is cleaner.

Because of its large mass and extensive heat channelling, the mass stove is the most effective way to utilize energy from wood.

Combustion temperature in the stoves upper chamber is 900 degrees C. This gives a clean combustion utilizing all the energy and making for a minimum of pollution.

Despite the high temperatures in the core of the stove, the stoves surface remains at a constant temperature of 45-55 degrees C, making it child safe and allowing for the placement of furniture close to the stove. Normal problems with very strong radiant heat are totally avoided.

With only one fire a day, consisting of 12-15 kg of firewood, the mass stove can produce 1.5 – 2.5 kw in 24 hours – enough to heat a 125-150 sq.m. home.

A baking oven can be built into the mass stove, and it is possible to connect a hot-water system to the mass stove.

In addition to all this the stove is beautiful, and there are countless design possibilities, so that it can be designed for individual tastes and needs.

What is unique to the mass stove is the dispersal of heat throughout the mass. Rising smoke and heat from the grate flows up and throughout several flues that come down the sides and around the back before exiting out the chimney – this “heat-flow” of the mass stove is called “contra flow.”

The primary air comes from below the fire through the ash door and the grate. From the fireplace the fire and smoke presses through a narrow throat into the upper combustion chamber along with the secondary air.

The secondary air presses up along the inside of the doors and is preheated from the fire. Therefore the air will not be cold when it arrives in the upper combustion chamber.

The upper combustion chamber is essential to “clean combustion.”

There is abundant space and high heat (900 degrees C) to burn the gasses mixed with the secondary air.

From there the heat, fire and smoke are pressed to both sides and forced all the way down on each side through the sidechannels in each side – typical 8×50-70 cm. in cross section and 160-180 cm. high (5-6 sq.m. in each channel).

The heat accumulates in the channels so that the difference from the 900 degrees C in the upper combustion chamber to the 170 degrees C in the chimney dissipates in the stove.

To prevent a high heat loss the damper, placed at the exit to the chimney is closed when the fire has gone out.

When using a wood-burning stove it is important that the living space is properly ventilated. The total combustion uses 8 cubic metres of air pr. kg. wood.

The chimney should inside by minimum Ø20 cm. Traditional build chimneys with an inside measure of 25 x 25 cm are good.

The consumption of wood in a mass stove is around 12-20 kg. of wood over 24 hours, depending of the size and construction of the stove. Typically you only burn one fire in 24 hours. The fire lasts around 1 1/2 hours.

The foundation under a mass stove should be at least 30 cm high standing on firm ground (the pressure is around 1.5 kg/cm2).

Manual of: How to use the Mass Stove

A newly built mass stove must dry for minimum one month with all the doors, cleaning channels and dampers open- in a dry room. In other circumstances this period of time should be longer (if possible use a small fan heater in the fireplace for the last few days).

After that you start to dry it out with a fire, which also takes one month. You begin to light it with ½ kg. of wood the first day.

The next day ½ kg. more (altogether 1 kg). In this way you increase the amount of wood with ½ kg per day until you have reached 14-17 kg of wood, which is the right amount of wood per fire in a mass stove. Read the fire manual below.

In this drying period water is being pulled out of the building material and water can even run out of the cleaning channels.

It is important that the drying is happening slowly because if too fast the water will “blow” itself out through the brick work.

Now the mass stove is once and for all “fired in “ and all the fireproof material is ready for the daily use.

Suggestions for heating the mass stove

Pull out the damper fully before you light the fire.
Pull out the ash drawer 10-15 cm (there has to be enough air for the fire- which comes through the ash room). After 5-10 min. when the fire has really started you push back the ash drawer app. 3-6 cm. from the front, so that there is a clean burn, but not blazing.
The firewood, which has to be totally dry, is stacked like on the drawing below. The wood is chopped with a 10/10 cm. end. Light types of wood can be used like; pine, larch, fir, birch, alder among others, (need to dry under roof for minimum one year).
Hardwoods like ; beech, elm, oak can also be used, (need to dry under roof for a minimum of two years).
Wood pellets and the like cannot be used because there always has to be air between the wood pieces. If the air circulation is being hindered because of the wood being too tightly stacked it can create an explosion in the oven, because the gasses in such an event can’t get out and are accumulated in the oven.

We also recommend that you find a local timber/fire wood supplier who can supply the right wood (don’t necessarily trust that the wood is dry even if the supplier says so).

Feed 7-8 kg of firewood in the fireplace room as shown here at the drawing. Put paper and thin dry kindling on the top of the fire and light the fire here on top, this way the fire will reach a high temperature faster and the gasses in this phase will burn quicker- this way there will be less smoke from the chimney.
When the fire is half burnt. You fill the rest of the firewood in the fireplace room 6-9 kg. The total of the fire will be 1½-2 hours.
The fireplace, the doors and the handles on the doors get very hot during the fire, so we recommend to use fireplace gloves.
When the fire has burned completely down, push the damper fully in and close the ash drawer.
Now the oven will get warmer and warmer from this one fire.
The next fire is normally lit 22-24 hours after. If it gets colder you can increase the heat further by lighting a fire after minimum 4 hours with ex. 7-9 kg of wood. If it gets warmer f.in. in spring you can fire with the normal amount of 14-17 kg of wood every second or third day. In summer it is of course possible to burn smaller fires from time to time, in case of a cold and damp day.
It is a good idea to prepare the next fire minimum ten hour after the fire is finished, this way the wood will be completely dry for the next fire. Don’t do it earlier, the fireplace might be too hot – and you might make a big mistake – the firewood starts to burn at 275º C!
If there is a bake oven you can swep it clean (the ash can be swept into the gaps of the fireplace) and the oven is ready for use.
You have to get to know your bake oven- put your hand into the bake oven, count the seconds and when you feel it getting too hot it’s a hint that 8-14 seconds is good for baking white bread. A pizza needs it even hotter 6 seconds. We recommend using a roasting thermometer in the bread and meat. Most meat needs 68- 75 degrees C. Bread 95- 99 degrees C.
The idea is that the mass oven is lit every day.

The maintenance of the mass stove

We recommend that the stove is painted with silicate paint or plastered. This can only be done when the oven is completely dried out.
Because of the big changes in temperature in the whole mass of stone we have purposely tried to avoid that the oven gets cracks by placing the fireproof “expansion joint” between the firebrick centre and the outer layer of brick. Even so, minor cracks can occur through some of the joints. This is not dangerous and those cracks can be fixed with a bit of paint or plaster.
Soot on the glass door can be removed easily with a little ash on a damp cloth or on a piece of rolled up newspaper. On the combined ovens where there is a fireplace and bake oven door you must expect that the glass in the bake oven door becomes black because of the massive difference in the heating and cooling on the bake oven glass. Therefore clean the bake oven glass regularly.
Make sure to empty the ash drawer so that the air intake works. If the ash gets up over the grate under the fireplace the grate can buckle and must be replaced. As previously mentioned if the primary air is blocked bad burning will follow.
Very important is to clean the flue channels at least once a year (do it when the stove is cold). Open the cleaning doors and move any ash. Do keep the stove clean inside all the time.

Wood as fire fuelis a renewable energy source. Through its growth (the photosynthesis) the tree becomes the solar energy storage of nature. When the wood is totally dry, it will burn quickly and efficiently when burned correctly. The combustion produces only the amount of CO2, which the tree uses through its growth – that is to say a CO2 neutral combustion.

The consumption of wood in a mass stove is around 12-20 kg of wood per 24 hours – depending on the size and design of the stove.

You only light one fire per 24 hours. The fire lasts around 1 1/2 hours.

The wood must be totally dry and split. Lightweight tree species like spruce, alder and birch dry in 1 year. The heavyweight tree species like beech, oak, elm and fruit trees must dry for 2 years. It is important that the wood is split in pieces under 10×10 cm. in cross section. Keep the wood under cover with plenty of ventilation.

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Mass Stoves

Projects in

All sizes

Mass Stoves

Renewable Energy offers a line of different mass stoves.

Get help with your construction plans

About Mass Stoves

Useful information

Manual of: How to use the Mass Stove

Suggestions for heating the mass stove

The maintenance of the mass stove

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