• What is a masonry stove?
• Why is a masonry heater cleaner burning and more energy efficient than ordinary fireplaces and metal wood stoves?
• How is masonry radiant heat different?
• What are the design options?
• What is the history of masonry radiant heat?
• How do I contact Radiant Designs?

Why is a masonry heater cleaner burning and more energy efficient than ordinary fireplaces and metal wood stoves?

To simplify somewhat, we can say that there are three key elements required to achieve a clean burning fire: dry fuel, optimum air mixture, and a temperature sufficient to achieve complete combustion (at least 1,300 degrees F) A metal wood stove dissipates the fire's heat very rapidly through the stove walls, which prevents the temperature in the firebox from reaching the desired levels.* To make matters worse, metal stoves are typically dampened down (which means that the air intake is reduced) in order to produce a lower level of heat for a longer period of time. In this oxygen-starved state a dramatically increased percentage of the toxic hydrocarbons released by the wood escape up the chimney unconsumed - not only is potential heat wasted, but the level of pollutants produced by the stove is radically increased. An open fireplace, on the other hand, pulls in too much room air… this also lowers the combustion temperature and reduces the cleanness of the burn. Masonry stoves have thick-walled fireboxes which prevent the fire's heat from escaping too quickly… it is contained, concentrated, and radiated back into the fire, which allows the temperature in the firebox to rise to the levels required for a clean burn. Another crucial difference in a masonry stove is that the rate of air intake into the firebox is carefully controlled, letting in neither too much nor too little. (Masonry stoves are never meant to be dampened down during firing, and the door is fully opened only to put in fuel or stir the fire.)

Masonry stoves excel not only in combustion efficiency, but they also greatly outperform metal wood stoves and open fireplaces in thermal efficiency. Thermal efficiency is measured in the proportion of the heat produced by the fire that is retained in the dwelling vs. the amount that escapes up the chimney. Since metal stoves and ordinary fireplaces have no heat exchange channels to absorb the heat after the combustion gasses leave the firebox , much of the heat produced by the fire goes straight up the chimney. Even though most fireplaces have considerable mass (which is a requirement for heat storage), because they have such a large opening and pull so much room air up the chimney, some fireplaces actually have negative thermal efficiency - in other words, they pull heat out of the house instead of warming it!

*newer types of metal wood stoves that have firebrick lined fireboxes, and others such as pellet stoves are capable of producing relatively clean burning fires. However, since they lack adequate mass and heat exchangers they still have poor thermal efficiency.

How is radiant heat different?

Although we have all experienced both radiant and convected heat, most people are unaware of the difference. In most cases, what most North Americans are familiar with in their homes is convection heat, where the air itself is heated and circulated through the home. Gas, oil, electric, and metal wood fired heaters all produce short wavelength infrared radiation, or high intensity heat which transmits its heat to the air very quickly. Even without fans, whenever air is heated currents will form spontaneously… the heated air rises to the ceiling, flows across to the coldest walls, drops down to the floor as it cools, then is pulled back to the heat source and reheated. The higher intensity the heat, the faster the air flow. Convection currents tend to roil up dust particles , and if the heater has a sufficiently hot surface (like a metal stove),the dust that lands on it will burn and be reduced to very fine ash…. Which then becomes distributed in the room air by the convection currents. This fine ash forms an air pollutant called pm 10 (particulate matter smaller than 10 microns) that is so small it can penetrate deep into the lungs - a definite health hazard. (Metal stoves also create unhealthy indoor air by depleting oxygen and ionizing the air).

Masonry heaters produce a low intensity (long wavelength) radiant heat which transmits less of its energy to the air , but tends to penetrate solid objects (including the human body) more deeply. Everyone has experienced the wonderful penetrating warmth of the sun on clear winter days when the air temperature is quite cool. If you find a spot where the air is still, you can literally feel warmed to the bone by the long wavelength solar radiation, even if you are surrounded by snow. Like a sun warmed rock that continues to radiate heat long after the sun has set, a masonry stove holds the heat of a fire for many hours after the fire is out, radiating a gentle penetrating warmth.

Design options

Numerous possibilities exist for both the interior configuration and the exterior shape and appearance of masonry stoves. The size and type of structure that will be heated, and also the regional climate will indicate the size (mass) and surface area required for a stove, but these basic requirements can be incorporated into any number of different designs.* Masonry stoves often incorporate heated benches, cooking surfaces, baking or warming ovens….. in the colder regions in Europe it was once common to include a heated sleeping platform!

Beyond the overall size, shape, and type of fire door, there are many options for the exterior finish materials - brick, tile, stucco, soapstone, slate, marble, granite, etc. These materials can be used alone or in combination to create striking original stove exteriors. The stove pictured on the front page (designed and built by Stuart Davies in Ashland, Oregon) uses a combination of stucco and a granite and marble mosaic to create an unusual one-of-a-kind design. In this type of design, the lines between the pieces of stone provide a significant part of the visual interest, and add an accent to the flowing grain and rich coloring that give the stone its natural beauty.

This kind of stove is not an appropriate choice for those looking for a lower cost masonry heater, but if the owner is willing to make the investment in this labor intensive art form, then the options for exterior stove designs really are wide open…. A unique stove can be built to suit anyone's taste.

Over the past few centuries, the design of masonry heaters has been the source of considerable controversy in Europe, and differences in design philosophy persist to this day both in Europe and North America. Without delving into the finer technical aspects of these different viewpoints, it would be fair to say that designers from different schools of thought have produced designs that function very well. Currently in North America there is an effort by the Masonry Heater Association to create a standardized code for the design and construction of masonry heaters to ensure safety and high performance, and thereby to also gain exemption from regulations in some states that prohibit the installation of any new wood burning device. Standardized designs lend themselves well to this process, because a given stove design need only be put through the rigorous and expensive testing process one time, and the test results apply to any copies. Although we fully understand the need for testing procedures and standardized codes that will gain the acceptance of government bureaucrats, and we fully support the MHA in their efforts to gain exemptions for masonry stoves from local regulations which indiscriminately prohibit all wood burning devices, we nonetheless believe in maintaining the rich tradition of design innovation and creativity that masonry wood stoves have long possessed. Wherever possible (i.e., wherever rigid regulations are not in effect), we should encourage this tradition of innovative, one of a kind designs to co-exist with standardized production models.

History

The awareness that the heat from a fire could be stored in rocks surely arose almost simultaneous to the earliest human use of fire…. It seems plausible that it would be a phenomenon that would be quickly discovered when fires had by chance burned in proximity to stone. In his excellent book, The Book Of Masonry Stoves , David Lyle provides a thorough history of the progression from a simple stone lined fire pit, to a vented pit hearth, to the early protofurnaces which demonstrated an understanding of draft (the chimney effect) and the benefit of controlling the intake of air into the fire chamber. He also details early kilns and bake ovens, rock stoves and walk in chimneys, all of which utilized principles which were eventually incorporated into modern masonry stoves.

According to Lyle, three events contributed to a dramatic surge in innovations in stove designs which culminated in the modern masonry stove. One was the growing scarcity of fuel wood throughout most of Europe during the middle ages. Another was an increasing concern about the danger of catastrophic fires in cities and towns caused by sparks from open hearth fires without chimneys. The third event was the advent of "the little ice age", a period of extremely cold winters in Europe between 1550 and 1850. In 1557, seven years after this frigid shift in the climate began, the first patent was issued for a wood conserving stove.

It was a German artist and inventor by the name of Franz Kessler who reportedly was the first, in the early 1600s, to utilize baffles in a stove to force the combustion gasses in a circuitous path between the firebox and the chimney. Thus the modern masonry heat exchanger was born, arguably the most essential element in an efficient masonry stove. In the years that followed, a flood of innovative designs that included heat exchangers in a multitude of configurations was seen, mainly in Central, Northern, and Eastern Europe. By the mid 1800s, the fundamentals of heat exchanger and firebox design had been established…. they have seen no significant change from that era, and proven worthy by the test of time. The improvements that have arrived in recent years have mostly been in hardware, such as airtight firedoors faced with ceramic glass, and building materials like better firebrick, high duty refractory mortar, high temperature, compressible insulation materials for making expansion joints, and cast refractory material reinforced with stainless steel strands.

There are also many new materials available to contemporary designer/builders for creating beautiful new exterior designs. Particularly in Central Europe, the exterior finishing of masonry stoves has been elevated to a recognized art form. Some German stove builders create a form of tile stoves using a technique known as uberschlag to create fantastic heated art pieces: functional sculpture. These sort of stoves, one-of-a-kind designs intended to please the eye as well as warm the home, embody one of the finer aspects of the established tradition of masonry stove design.