In the UK the idea of how to heat a home and how to conserve heat seems to have been muddled up considerably over the years. Perhaps it has been the relatively changing weather within our present interglacial period that has made it unpredictable what building techniques and designs best to use. For example, during the last hundred years, periods of severe cold and heavy winter snow fall (and dryer) was for some decades prevalent in the temperate zones of northern Europe but always followed by warmer and wetter periods too. The population density at the time, political climates and the consequent resource availability also played a factor in choice of homes and heating, and maybe this is why the most popular form of heating  in the UK is the steel stove – regardless of its efficiency. But here I argue that with the recent introduction of the building regulations another heating system is appropriate, that is the mass heater.

A mass heater is a heater with a large density which makes it able to store heat produced by the combustion in a fire chamber. The dense mass, such as masonry, tiles and cob is built with a long and windy duct to ensure that the heat encounters the surface area and enough time to be transferred to the thermal mass ‘battery’.

The different designs vary from big 2 m3 upright columns of bricks (Finland), upright quartered mass heaters for every corner of the room (Sweden) to mass heaters stretched out as a kitchen surface with oven and cooking hobs (Germany, Russia). Many more designs can be found the world over where efficient heating has been necessary or where metal was scarce, and where clay soil has been processed into bricks (burnt or unburnt) or tiles and monolithic earth building materials such as cob. Although such heating has not been used actively, the same heat storage principles are found in the hot desert and arid areas where thick walled dense buildings of earth accumulate the sun’s heat throughout the day to release it slowly at night. The mass heater is therefore built on age old principles fitted to its vernacular and bioregional landscape.

The reappropriation of the mass heater for all new homes

The mass heater has in certain countries over the last 20 years been adopted by the green movement into the low impact and eco home design inventory. For example in Denmark, the Rocket and the Finnish style Mass Heater have been re-appropriated by self-builders and can be found in all ecovillages and many green homes. One of the keys making the mass heater popular has been the increased understanding of heat conservation and thus the application of better and thicker layers of insulation. A mass heater is able to accumulate a large amount of energy, but, as in the desert houses, its immediate heat output is low, and mass heaters are therefore only really able to heat to comfort temperatures in homes that are well insulated.

Now, with the (rather late) implementation of the building Regulations in 2010 and Codes for Sustainable Homes (now integrated into the building regulations) in the UK, insulation, draft proofing, energy efficiency, heat exchangers etc have become more mainstream as they are required elements for all new homes, and here the mass heater comes back into the picture. Especially with renewable biomass fuel gaining momentum the mass heater really is an appropriate technology for the standard 21st century low impact home.

A well built mass heater can need as little as 1 burn session every 24 hours to provide a comfortable steady temperature in a well insulated house. This will of course vary in degrees depending on the size of your home, the level of insulation, the calorific value of your wood, and the choice of mass heater. However, the principles remain the same.

In my opinion this type of heater should be at the very heart of any home design. An open plan building built around the center of a mass heater will ensure that fuel use is minimised as well as keeping your house appropriately heated. A back boiler for radiators (and general hot water use) and under floor heating can also be incorporated to spread the heating more evenly (pricy tho’).

Good mass heater designs will have 2 combustions which means that most wood gasses have been exposed to high enough temperatures as well as oxygen leaving nothing but CO2 and water vapour left at the time the ‘smoke’ leaves the chimney. The combustion level and fuel use is something I think generally neglected in the UK, e.g. in low impact communities where the iron or steel stove is a simple quick solution. This might be in the many cases where building reg’s have not been followed and the dwellings small. However, the amount of less work, resources (and perhaps money) needed is considerable if insulation and thermal mass principles are followed, and investing in them is always worth it regardless how small your home is.

The rocket mass heater

The Rocket Mass Heater, if well built, will have a very clean combustion and its simple design is an economic choice that can be built DIY for about £500 or less. In Denmark this specific mass heater is called a flex oven too because of its malleable design often seen shaped into a bench that one can ‘snuggle up to’. It can go under the floor, heat beds,

incorporate an oven and if one is clever enough, have a cooking surface. If one feels less ‘flexible’, they can also take the shape of more traditional brick and tile mass heaters.

The mass heater is lying there waiting to be discovered and adopted by the UK green movement. With the new imposed insulation requirements, the mass heater fits well at the center of any home design.

If you would like to know more about the rocket mass heater get in touch or sign up to our rocket mass heater course in March 2017.

Sara Tommerup

Natural builder & MSc Architecture



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