Living with Fire

loveConsumers are attracted to hearth products for the ambience and thermal comfort they can provide.  When considering home heating, they provide fire viewing ambiance as well as producing significant heat for the home.

Masonry heaters operate at temperatures warmer than the human body and transfer most of their heat as radiant heat, two elements necessary for the human body to feel direct thermal gain. This is the same feeling one gets from direct sunshine. Most home heating systems do not have radiant surfaces warmer than the body and as such do not feel as warm and cozy.

In addition masonry heaters have a very favorable impact on the mean radiant temperature of the living area, a stark contrast to forced air heating systems and products utilizing a high percentage of convection heat transfer. This is an important distinction because when the surfaces of the living area are heated radiantly, thermal comfort can be achieved at lower air temperatures, with less draftiness and less drying of the air.

Comfort Efficiency

A Early Masonry Heater- Image Source: The Book of Masonry Heaters by David Lyle

The word efficiency often comes up when comparing various types of home heating systems. Most of the time however, efficiency discussions involve issues of fuel usage, combustion, environment impact and economic cost, rarely is efficiency discussed in conjunction with issues of comfort.

Thermal comfort is defined by four environmental factors and two personal factors. The six factors are inter-dependent, meaning that when you change one factor the change can have a positive or negative effect on the others.


Choices between manipulating the air temperature factor and the mean radiant temperature factor will have an effect on the comfort efficiency achieved by various home heating systems.

BTU’s in Cordwood

100,000 BTU’s of heat energy in a 5 gallon bucket of cordwood

16 Pounds of Oak = 100,000 BTU.The five gallon bucket in this picture contains 16 pounds of dry (less than 20% moisture content) oak cordwood. When burned in a masonry heater with an 85% efficiency rating, over 100,000 BTU’s will be stored in the soapstone mass of the fireplace and gently heat your home for many hours after the fire has gone out.

Each pound of cordwood has a scientific potential heating value of 8250 BTU’s. Of this amount approximately 10.6% (871 BTU’s) is lost to latent heat in uncondensed water vapor escaping up the chimney during combustion. The water vapor comes from the moisture content in the cordwood (figured at 20%) and the hydrogen in the chemistry of the wood when it burns and combines with oxygen to form water (H2O). When the latent heat loss (871 BTU’s) is subtracted from the scientific potential heating value (8250 BTU’s) the remaining 7379 BTU’s represent the Low Heat Value (LHV) of one pound of cordwood.

The Low Heat Value of 7379 BTU’s would be the “real world” heating potential for a cordwood heating appliance with a 100% efficiency rating. A Tulikivi fireplace with an efficiency rating of 85% has a heating potential of 6272 BTU’s per pound of wood. This means that 16 pounds of dry cordwood burned in a small Tulikivi fireplace would provide 100,354 BTU’s of heat energy for the home. Tulikivi fireplaces with a larger masses can burn, store and then radiate over 300,000 BTU’s of home heating energy from a single firing of approximately 55 pounds of cordwood.

What about the energy content in a 5 gallon bucket of dry SOFTWOOD?

16 Pounds of poplar = 100,000 BTU.
Because softwood is less dense than hardwood it weighs less per unit volume, however, the energy per pound remains the same. A cord of firewood measuring 4 ft x 4 ft x 8 ft (128 cubic/ft) is a very imprecise measure of heat energy (BTU) value. Every species of wood is different in density. For example 16 pounds of poplar cordwood pictured below has approximately 1/3 more volume than the 5-gallon bucket of oak pictured above. Click the following link for a chart comparing the energy value (BTU) of various species of wood.

Radiant Heat and Thermal Comfort

Why is a radiant heater a more comfortable and efficient home heating source?

Thermal Comfort Factors - Image Source: support.caed.asu.edu
Thermal comfort occurs when a satisfactory thermal balance exists between a person and their surrounding environment. This balance is affected by six factors, two relating to the person and four to the environment. Personal factors take into account changes in the body’s heat producing metabolism and changes in the type and amount of clothing worn. Environmental factors are influenced by air temperature, air velocity, mean radiant temperature and relative humidity. The human body is exquisitely sensitive to changes in these factors. So much so that in order to maintain a thermal balance, any significant change in one factor needs to be offset by an adjustment in other factors. For example, if air velocity increases, the resulting “chill” could be offset by increased clothing, increased metabolic activity or increased temperature.

In the home environment, raising and lowering air temperatures is the conventional way to adjust for thermal imbalances. During the summer an air conditioning system can positively affect the balance by lowering air temperature, lowering relative humidity and stimulating air velocity. During the winter, when a home heating system raises the air temperature, the same thermal comfort factors are affected but the results are inconsistent. The positive effect of raising the air temperature must be balanced against the negative effects of lower relative humidity and increased air velocity. It is safe to say adjusting the air temperature to improve the thermal balance is more efficient and comfortable with summer cooling than with winter heating.

Thermography - Images Source: http://thegenesiscenter.com/thermography/
A better method for home heating delivers radiant heat to room surfaces in the same way the sun heats the earth. As these surfaces absorb heat, the mean radiant temperature rises and radiant heat loss from a person’s body diminishes. The air is heated by the room surfaces, its temperature remains steady and even, its humidity stable and its movement minimal. It is more efficient and comfortable to keep the mean radiant temperature and air temperature as close as possible.

The thermal radiation from a masonry heater is warmer than the human body projecting a gentle, penetrating, sunshine like warmth that is absorbed by the body as direct thermal gain. This is the main difference between in-floor radiant heating, operating at less than body temperature, and a masonry heater operating above body temperature. Both promote thermal comfort by reducing body heat loss, but the direct thermal gain of a masonry heater provides and additional thermal comfort boost. This additional boost allows for achieving thermal comfort at lower air temperatures and is particularly useful in large cathedral ceiling rooms as well as rooms with less than adequate insulation. Masonry heaters, with their innate comfort and efficiency, promise even and steady comfort, less draftiness from air movement, less loss of apparent air temperature from low humidity, less loss of body heat to cold room surfaces and a warm coziness from their direct thermal radiation.

Home Heating – differentiating the good from the not so good

Determining the differences between better and worse, when it comes to thermal comfort, is more a matter of intuitive feeling than analytical thinking. Matt Kramer’s article in the April issue of Wine Spectator Magazine regarding differentiating between various qualities of wine draws an interesting parallel between differentiating between the thermal comfort of various heating systems.

Matt Kramer writes that “if you have a lineup of the same type of wine with differing qualities, even a supposedly [inexperienced] wine taster will likely land on the ‘best’ wine in the lineup – or at least one of the best”. He goes on to say that “there’s a school of thought that you have to first learn the ‘grammar’ of wine before you can ‘speak’ it”. However he believes “that you can learn much of what’s really worthwhile about…fine wine itself – simply by being asked to distinguish between better and worse”. Still further he states that “a lot of folks would have you believe otherwise. Not surprisingly, they’re almost always the people who have labored hard to append the initials of some credential to their name, They like jumping through hoops. They believe in tests. What’s conveniently forgotten is that no special talent is needed to identify better wines from worse”.

Matt Kramer’s observations on distinguishing between better wine and worse wine struck a chord with me when I thought about how difficult it has been over the years to explain and quantify the difference in comfort between various types of home heating systems, yet how easy it actually is for users to identify the better from the worse when it come to heating comfort. The comfort of any home heating system is impacted by the way it affects the four environmental factors of thermal comfort – those factors being air temperature, mean radiant temperature, humidity and air velocity. Differences in the impact on these four interrelated thermal comfort factors would easily result in the user distinguishing between better and worse if we could lineup the heating systems and sample them in the same way as one could sample different bottles of wine. Unfortunately lining up and sampling four or five heating systems is not as easy as doing the same with four or five bottles of wine. However, our skin is very sensitive to differences in thermal comfort – probably much more sensitive than our taste sensitivity is to differences in wine. Most people have a very good memory bank when it comes to identifying thermal comfort versus thermal discomfort. I would argue that this thermal comfort memory bank is quite unerring in distinguishing better and worse even though it may not be possible to sample one heating system after another in relatively quick succession.

Defining thermal comfort is subjective and will vary from person to person. That being said, there are qualitative generalizations that can be made regarding a home heating system’s impact on the four thermal comfort factors.

  1. Air Velocity – heating systems that have less air movement have a better quality of thermal comfort. Heating systems that depend on forced movement of air or promote convection will have a diminished quality of thermal comfort.
  2. Humidity – heating systems that maintain a moderate relative humidity (around 50%) have a better quality of thermal comfort. Heating systems that focus on only raising air temperature usually cause the relative humidity to fall well below the 50% mark with a corresponding negative impact on thermal comfort.
  3. Temperatures – heating systems which maintain relatively equal air temperatures and the mean radiant temperatures have a better quality of thermal comfort. Heating systems that produce high air temperatures with little impact on the mean radiant temperature will have a diminished quality of thermal comfort.

The radiant heat from a mass masonry heater causes very little change in the air velocity or the humidity; in addition it helps to equalize the air temperature and the mean radiant temperature. It is not surprising that users can sense the improved quality of a Mass masonry heater’s radiant thermal comfort. In addition this thermal comfort is usually delivered at lower air temperatures resulting in higher comfort efficiency and lower economic costs.

© Mid-Atlantic Masonry Heat Inc. 2013
Revision 3/29/2013

Comfort Efficiency

The word efficiency often comes up when comparing various types of home heating systems. Most of the time however, efficiency discussions involve issues of fuel usage, combustion, environment impact and economic cost, rarely is efficiency discussed in conjunction with issues of comfort.

Thermal comfort is defined by four environmental factors and two personal factors. The six factors are inter-dependent, meaning that when you change one factor the change can have a positive or negative effect on the others. For example, when the air temperature factor is increased by the circulation of a forced air heating system, there is also an increase in the air velocity factor which causes some degree of thermal discomfort.

Another negative impact from the forced circulation of heated air comes from the fact that the relative humidity factor is decreased resulting in a lowering of the apparent air temperature. On the other hand if the mean radiant temperature factor is increased the negative impact on the air velocity and relative humidity factors is much less pronounced and the impact on the air temperature factor is positive.

Choices between manipulating the air temperature factor and the mean radiant temperature factor will have an effect on the comfort efficiency achieved by various home heating systems.

© Mid-Atlantic Masonry Heat Inc. 2013

Revision 1/11/2013

The Path to Thermal Comfort

Achieving thermal comfort should be the goal of any home heating plan. However, this goal is often compromised by overlooking the fact that there are multiple factors which influence thermal comfort.  Instead, a single factor – air temperature – is allowed to become the Alpha and Omega applied to the task of home heating.  The jigsaw puzzle below graphically demonstrates the interrelatedness of the four environmental factors (blue) and the two personal factors (pink).

CompletePuzzelPicture

Heating the air is a fast and relatively easy way to add heat to the home. However, when only the air temperature is manipulated the following unintended consequences occur.  First, the air in the home becomes stratified – hot at the ceiling cold at the floor.  Second, heating the air does not increase the radiant temperature. Instead the disparity between air and surface temperatures in the home increases.  This means a person’s body continues to radiate heat to the cool surfaces in the home even though the air temperature is warmer. The result is a cold and clammy feeling.  Third, the stratification of air and the cool surfaces result in convection and increased air velocity (draftiness). Fourth, heating the air causes the relative humidity to drop lowering the apparent air temperature. Lower humidity dries out your throat and skin and gives rise to static electricity.  

While radiantly heating surfaces in the home is slower it does not cause the unintended consequences that come with convective heat transfer.  Radiantly heating surfaces in the home stabilizes the air temperature at a comfortable level, does not cause draftiness and does not dry out the air. Thermal comfort is increased resulting in a warm and cozy home environment.

© Mid-Atlantic Masonry Heat Inc. 2012
Revision 3/20/2012