Mass stoves




There are different types of mass stoves: small, large, heavy, light, industrial, artisanal, standard, custom, certified, unlicensed, radiant, semi-radiant.

AFeuxDoux mass stoves are handcrafted radiating stoves. They are custom-designed and tailored for your project using the most modern techniques, guaranteeing some of the highest certified performance.

It is in a controlled environment that I wish to explain to you the principle of the mass stove.

A mass stove (also called an accumulation stove or inertia stove) is a heater composed of heavy materials (stone, brick, earthenware) that accumulate the energy of a clean, fast and intense outbreak (from 30 minutes to 1:30) and return the heat at length once the fire is extinguished (from 8 a.m. to 36 hours depending on your desires and needs).

The diffusion of heat is mainly by infrared radiation but also by convection. Mass stoves also have an exchange circuit (channels or chicanes) to recover heat from the fumes before they are evacuated through the duct.

To increase the efficiency of the outbreak, the mass stove uses the principle of post combustion or secondary combustion: the gases emitted by heated wood burn at very high temperatures, the emissions of carbon monoxide, fine particulate matter and other pollutants usually generated by wood combustion are therefore very reduced. Emanations are no longer loaded with harmful and polluting particles thanks to more complete combustion, making this heating system one of the cleanest and most efficient.

Depending on the mass of the stove, outbreaks are spread over one, two or three times a day, or even an outbreak every day, 2 days or 3 days (off-season). The smaller the mass, the greater the number of outbreaks.

The operating principle takes place in three stages:

  1. An effective outbreak
  2. Maximum recovery of the heat produced by the outbreak
  3. Prolonged restitution of this heat

Physically, each stove consists of 3 specific “organs” at each stage:

1. A refractory brick fireplace that is designed to burn almost entirely wood, with a combustion efficiency of more than 99%

4 air inflow injectors in the fireplace (combustion chamber)

4 air inflow injectors in the fireplace (combustion chamber)

2. A circuit of chicanes or refractory brick carnals that guides the “gases” from combustion (we do not speak of smoke circuit, because good combustion guarantees the absence of smoke) while allowing the greater recovery of heat emitted by combustion.

Circuit of refractory brick smoke carnaux of a mass stove

Circuit of refractory brick smoke carnaux

3. A dressing designed to ensure a controlled diffusion of heat in the home

Exposed brick dressing of a mass stove

Exposed brick dressing

Power and efficiency of the stove

The power of mass stoves varies greatly. It depends on the needs of the house and the way of life of its inhabitants.

Power is therefore determined by:

  • The surface to be heated
  • The thermal performance of the house
  • The level of inertia of the house
  • The home’s insulation and inertia ratio
  • The organization of the different rooms of the house: the presence of open or partitioned spaces
  • The number of possible or desired daily outbreaks
  • Busiest living spaces
  • What is the average power of the stove?

To find out, it’s best to calculate it using the following formula:

  • P – b.c.η/t (write the divided with a large bar)


  • P: Power of the stove (kW)
  • b: Wood consumption (kg/day)
  • c: Heat power of the stove (4 kWh/kg)
  • η: Useful stove yield (%/100)
  • t: Outbreak autonomy time (h)

For example, a stove with a useful efficiency of 85%, which heats a building B with 15 kg of wood in a single outbreak per day, has an average power of 2 kW. Indeed, 15 x 0.85 x 4 / 24 – 2.1 kW. With an average stove of 6 kW for the same B building, you risk overheating. Conversely, with a stove with an average power of 1 kW, you will be in thermal discomfort.

Warning: It is important to make the difference between average and maximum power. To know the average power needed to heat your home, the ideal is to contact a thermal design office.

I design stoves with average power between 160W and 16 kW. It is interesting to calculate the price of a stove at kW of average power.

Energy efficiency is the relationship between the actual efficiency of a machine (the useful work it does) and the maximum theoretical efficiency that can be expected of it. Yields always range in value from 0 to 1 (or 0-100%).

A useful efficiency of 85% means that 85% of the energy of your logs has become useful for heating you. The higher the yield, the less wood is consumed.

Useful performance is the result of several factors:

  • good combustion efficiency (above 99%) thanks to the refractory brick fireplace that ensures a near-complete combustion of wood.
  • a good absorption efficiency: that is, the ability of the stove to absorb as much heat as possible emitted by combustion.

For example, a last-generation cast iron stove has a very good combustion efficiency, but an average absorption efficiency: the smoke coming out of the chimney is more than 200 or even 400 degrees Celsius. This means that you heat the outside air at 200 or 400 degrees Celsius.

Responsiveness and radiant surface

The level of responsiveness of each stove adapts to the needs of the project.

During the first fire of the heating season, the stoves put, depending on their design, between 1 and 12 hours to be hot and thus emit a radiation that brings comfort.

For other outbreaks, the surface temperatures of the stove reach their maximum temperatures between 3 and 12 hours depending on the thickness and scatterivity of the materials that make up the walls. It is also possible to make certain surfaces of the same stove more or less reactive by playing on these same parameters.

The stoves are very flexible stoves:

  • In the 100% cooker position, the air in the house heats up very quickly thanks to the convective effect of the cooking plates,
  • When the air is hot, we gradually move into the 100% mass stove position to increase the radiation of the stove and decrease the convection.

In case of prolonged absence during the winter, a heater must take over the stove to keep the house off-freeze (at a minimum) or heat it while you are away. Otherwise, it takes one to two days to regain a good level of comfort in your home. This point is especially important for houses that benefit from a lot of inertia, such as stone houses.

As for the quality of the radiation, it depends on the radiating visible surface of the stove: the larger the exposed surface area, the lower the surface temperatures, the more pleasant the radiation. Conversely, a small stove, with a small visible surface area, will have higher surface temperatures than a large stove, and its radiation will be less gentle.

This is the hot surface of the stove. The larger the exposed surface, the more scattered the heat and the lower the surface temperatures. For indication, a 5 kW stove with an apparent surface area of 8 m2 will have an average temperature of 65oC, while another stove of the same power with an apparent surface of 15 m2 will have an average temperature of about 45oC. Here are images taken with a thermal imaging camera at regular intervals on a 4.5 kW stove with an apparent surface area of about 10 m2. Surface temperatures are slightly below the predicted theoretical surface temperatures. Indeed, the measurements were taken on the stove in operation but in the process of drying.

A stove with exposed large surfaces is not necessarily imposing. The surfaces are distributed by making them useful: benches, worktops, warm and comfortable walls to the touch in winter, and cool and pleasant in summer. The advantage of the bespoke stove is that it can integrate large-area stoves into all environments.

Other technical themes

  • What is the total weight of the device?

The ability of a mass stove to heat an entire house all day with one or two fires a day depends on its ability to build up heat. A 5-tonne stove accumulates more energy than an 800 kg stove. It is sometimes interesting to calculate the price of the stove per kilo.

  • How many kilos of wood does the fireplace burn per hour?

The faster the wood is burned, the faster the flapper of the stove, which allows the heat of the fire to be trapped, is closed. We design fireplaces that burn up to 25 kg/h. In other words, if your home needs to burn 25 kg of wood a day, one hour of outbreak is enough.

However, if for you the pleasure of watching the fire for a long time is a priority, we design the fireplace so that you can prolong the outbreak while enjoying a high yield.

  • How many kilos of wood can be loaded and burned in the fireplace at once?

The more wood the home accepts, the less you need to recharge the fireplace during the day. If the thermal comfort of your home is achieved with a wood consumption of 25 kg per day, you must realize:

  • 3 fires of 8 kg if your fireplace accepts up to 8 kg,
  • 2 fires of 12.5 kg if your fireplace accepts a maximum of 12.5kg,
  • or a single 25 kg fire if your fireplace allows it.

We design outbreaks based on the number of outbreaks you want to do per day. Our homes accept wood loads of between 1.2 and 40 kg per outbreak.