The Heat-Kit Planning Guide
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A masonry heater allows you to heat your home with wood in a unique way. It's main distinction is the ability to store a large amount of heat. This means that you can rapidly burn a large charge of wood without overheating your house. The heat is stored in the masonry thermal mass, and then slowly radiates into your house for the next 18 to 24 hours. You get a number of benefits, described in more detail below. If you burn wood fairly rapidly, it is a clean fuel. If you try to burn it too slowly, the fire will change from flaming to smoldering combustion. The burning process is incomplete and produces tars. Atmospheric pollution increases dramatically.
This is important if you are planning an energy-efficient house. The average energy demand of your house will be quite low. For most of the time, it may require only 1 to 2 kW of heat. For most conventional woodstoves, this is below their "critical burn rate", or the point where they start to smolder. In other words, woodburning and energy efficient houses don't really suit each other very well, unless you have some way to store heat so that your stove can operate in the "clean" range all of the time.
Masonry heaters are radiant heating systems. Radiant heating differs in several fundamental aspects from conventional convection and forced air heating. Understanding these differences is important if you plan to include a masonry heater in your house design.
A good approach is to ask the question "Why do we heat?". While this may seem trivial, it's important to consider the fact that our body produces its own heat, about 100 watts when we're resting. Strictly speaking, we don't need to heat our bodies at all. Rather, we need to prevent them from losing heat.
We exchange heat with our surroundings through two main mechanisms: radiation and convection. The proportion of each varies depending on how you are dressed and on your environment. In a T-shirt in an average house in the winter, you're losing about 60% by radiation. The exact amount can vary dramatically depending on what surface temperatures your body "sees" in surrounding objects.
A good example: When you are outside in the spring on a calm, sunny day, you can wear a T-shirt even though the air temperature might be 55 or 60 ºF. But step into the shade and you will start shivering within several minutes, even though the air temperature hasn't changed. The presence or absence of the sun changes your radiant environment dramatically. Similarly, even a slight breeze at these temperatures would change your convective environment. The word "draft" in this context immediately conjures up mental images of discomfort.
Radiant heat is electromagnetic energy, the same as visible light. The only difference is that it is in the infrared range of the spectrum, with a longer wavelength. Long wave infrared is more comfortable than short wave infrared. As the surface temperature of a radiating body gets higher, the wavelength of the radiant energy gets shorter, until eventually it is seen as a dull glowing red. This is the piercing heat you feel from a campfire as you get close to the charcoal bed.
With radiant heating systems, the concept of air temperature assumes less importance. This is a key point. Our conventional model of heating and comfort is based on forced air heating systems, where air temperature is all important: you need a thermostat that can regulate the air temperature to within a couple of degrees. To understand what really happens, we need to add another parameter to our model: mean radiant temperature, or MRT. This is simply the average surface temperature of all the walls in the room, including the approximately 100 sq. ft. surface of the masonry heater.
The chart above shows what happens: As the MRT rises, the "comfort band" of air temperatures widens dramatically. This helps to explain why a masonry heater doesn't need a thermostat. It also explains why your glass patio doors need a curtain at night, even though the curtain is not reducing heat loss substantially: The curtains have a higher surface temperature than the cold glass, particularly with a radiant heat source. Other aspects of radiant heating systems
With a conventional forced system, you use the air that you breathe as a heating medium by cycling it through your furnace blower every few minutes.
When you wake up in the morning with a "parched throat", you associate this with dry air. In fact, it is not dry air at all, but dust. The furnace circulates very fine dust throughout your house. Furnace filters only get the larger particles, not the very fine dust that causes the most sensitivity. When you add a humidifier, what you are actually doing is using moisture to help settle the dust out of the air.
Consider this: For a person with respiratory problems, where is the best place to be in the winter? Answer: Arizona, where the air is dry.
The reason there is less air movement with a radiant heating system is that there is a much smaller temperature difference between the air at the ceiling and at the floor. There is less overheating of upstairs bedrooms, and ceiling fans can usually be eliminated. Radiant heating gives you more mileage from a given amount of heat output by targeting it better to where it is needed: in the living space where the radiant panels are located.
Radiant heating is beneficial for many allergy and respiratory related problems, for the reasons outlined above.
In addition, you experience comfort at a lower air temperature. Cooler air next to your exposed skin promotes circulation and your body's own heat production. It feels invigorating and allows you to be more alert. The "sunny spring day" cited earlier demonstrates this effect.
In Europe, radiant heat has been used in medicine for therapeutic purposes for many years, and it is starting to see some use here as well.
Generally speaking, a masonry heater creates a "comfort zone" around itself. The closer you are to the heater, the higher is the MRT that your body "sees" in its environment.
This means that you want the heater located where your family spends most of its time. An open floorplan and a central location work best. Often the masonry structure is designed to serve as a space divider between, say, kitchen and living room. The fact that only minimal clearances to combustible materials are usually required allows for greater flexibility in integrating the system into the home.
Also consider the location of the firebox loading door and its convenience to your firewood supply. Firewood storage inside the house is often not necessary, since the firebox gets loaded all at once.
Many heaters often do double duty as fireplaces, so this function should be considered as well.
Many modern windows come with a special "low-E" coating that acts like a mirror for longwave radiant energy. With a masonry heater in the room, your body sees a reflection of the room's warm surfaces in the window in addition to the cold surface of the glass itself. With large, uncovered glass surfaces such as patio doors, low-E glass will improve comfort levels noticeably with vertical radiant panels such as those in a centrally located masonry heater.
Sustainable construction is starting to come into its own. Many people are beginning to question not only the energy consumption of their houses and their lifestyles, but also wider effects, such as the amount of embodied energy in the house materials, and the amount of pollution that has been generated as a result.
Masonry heating is an ancient technology. Its basic ingredients are earth and fire. While it is relatively new to North America, it dates back several hundred years in many of the colder regions of Europe.
All of the materials in a heater can be recyclable, if desired, and there are heaters in Europe that have been giving faithful service for over a hundred years.
The most important global environmental issue today is global warming and our emissions of greenhouse gases (mainly carbon dioxide). We are risking the well-being of future generations because of our consumption of fossil fuels. Every time that we fill our car with gas we add the equivalent of a hundred pound sack of carbon to the atmosphere in the form of carbon dioxide emissions from our tailpipe.
Hydrocarbons have been sequestered in the earth for millions of years by the actions of plant life on the atmosphere. Through our consumption of fossil fuels we are returning this carbon into the atmosphere at a staggering rate. As North Americans we are the worst culprits, consuming several times the world average per capita. To have an meaningful impact, we need to reduce our fossil fuel use by approximately 70 to 90%.
Our transportation choices are limited - we can drive less, or get a more efficient vehicle. We do, however, have a choice in how we heat our homes. It simply doesn't make sense to use valuable (yet ridiculously underpriced) resources simply to make low grade heat.
Provided that trees are grown on a sustainable basis, there is almost zero contribution to global warming when you burn wood.
Plants use photosynthesis to store solar energy. They accomplish this by converting atmospheric carbon dioxide into stored carbohydrates such as cellulose. When a tree eventually dies, this carbon dioxide is returned to the atmosphere by the action of bacteria and fungi as the wood decomposes. Because this is a closed carbon cycle there is no net impact. When you burn wood, your are heating your house with stored solar energy and simply using a speeded up version of this cycle.
Proper use of wood as a heating fuel requires that it be harvested sustainably. As opposed to clearcutting, proper forestry practice requires thinning and culling to improve the stand. This could, in theory, generate enough fuel to heat a huge number of houses.
Large scale woodburning can lead to air quality problems. The main environmental issue is smoke emissions. Woodsmoke is very similar to cigarette smoke from a health standpoint. It consists of tiny droplets of tar known to regulators as PM-10, or particulate matter smaller than 10 microns in size. Since a blood corpuscle is 6 microns, smoke particles can enter through the lungs directly into the bloodstream. Hence, the health issue.
As mentioned above, emissions from woodburning can vary dramatically depending on how it is burned. For airtight metal stoves, the Unites States Environmental Protection Agency (US-EPA) now requires that all models must be certified to the US-EPA Phase II standard for emissions. This has resulted in clean-burn research and significant improvements in airtight stove emissions from what were very high levels.
The Masonry Heater Association of North America (MHA) has made a major effort to establish emissions performance of masonry heaters through EPA-audited field testing. As a result, the majority of manufacturers now have emissions numbers for their systems.
Masonry Stove Builders has gone one step further. In partnership with Lopez Quarries Masonry Heaters (Everett, WA), we set up an emissions testing facility, Lopez Labs, and for four years have been conducting an ongoing program of performance testing on a number of different heaters under simulated field conditions.
As a result, we now have the largest database on masonry heating performance testing in North America, with over 100 tests over a complete (24 hr) cycle, and have produced several research papersi.The results are summarized in the Table 1, and compared with US-EPA field tests for other woodburning appliances. The conclusion is quite clear: masonry heaters are by far the cleanest way to burn cordwood and comparable to the cleanest pellet stoves.
The higher initial cost of a masonry heating system can be paid back in a number of ways. Like a masonry fireplace, it adds enjoyment and value to your home - good masonry work is beautiful. Installed cost can actually be very comparable to a conventional masonry fireplace. However, value for money is considerably higher than with a fireplace. With a heater, your investment in bricks and mortar is working for you every day of the heating season as active thermal mass storage and radiant heating panels.
Compared with other wood heating technologies, there is no depreciation or eventual replacement cost. Your fire-tending chores are eliminated, and your maintenance chores will be minimal because your chimney remains clean.
Traditionally, building a good masonry heater has required craftspeople with considerable skill and training. The heater is not only required to perform well in terms of efficiency and emissions, but also to be able to stand up to many thousand cycles of rapid heating and cooling. This severe thermal cycling places tremendous stresses on refractory materials, with which the stovemason must be intimately familiar.
We have been designing and building heaters for 17 years. We are also founding members of the Masonry Heater Association and produce the newsletter (MHA News) for the association. By networking with a very capable group of heater masons all over North America, we're able to benefit from the latest knowledge and field experience.
The Heat-Kit was the first heater core developed in North America and some of its more innovative features can now be found throughout the industry.
We are committed to masonry heater research and development because it is a worthwhile endeavor with consequence. We feel that the results speak for themselves and are the best form of advertising.
We developed the Heat-Kit system in 1985 to better address both our own needs and those of our clients. It is a system of precast refractory components that has allowed us to reduce the amount of expensive on-site time required to build a heater core by 90% . Our main innovation was to retain about 50% of the firebrick construction in the core, including all critical areas such as the firebox. Firebrick are unsurpassed, in our opinion, for taking the heavy abuse of thermal cycling.
For example, we have developed the first all-masonry white bakeoven (i.e., the flames do not pass through the oven) and believe that it is the best oven in the business. At home, we now use one to bake all of our own sourdough bread - R&D can be fun!
A complete system includes a foundation, heater and chimney. The heater consists of a core and a facing. The masonry facing has a minimum thickness of 4" and is part of the active thermal mass. It is typically installed by a local mason, and you have a wide choice of finish options.
The heater core consists of precast refractory components that are assembled on site in conjunction with standard firebricks that are usually obtained locally. The double firebox doors are constructed of high quality cast iron with heatproof glass. All required hardware such as clean outs and dampers is included.
Some of this hardware is installed into the facing, i.e., it will be done by the local mason. In the northeast we offer a delivered and installed price for the core, set up ready for facing.
Gas flow through the Heat-Kit 2 is illustrated below. It is shown with the bakeoven option.
Outside combustion air enters at a lower level through the foundation. It passes through the air damper (1) and then into the firebox (2). Rapid combustion of the fuel charge results in long flames, which are directed around the bakeoven and then burn out in the secondary combustion chamber. The hot gases are cooled in two downdraft heat exchange channels (7) and enter a connecting plenum (8) under the firebox. From there the cooled exhaust gases enter the chimney at floor level. The design of the connecting plenum allows the chimney to be connected anywhere at the back or side of the heater without unbalancing the downdraft channels.
A shutoff damper (9) in the chimney is closed once the fire is completely out, after about two hours. This interrupts the chimney draft and prevents the large amount of energy now stored in the masonry from bleeding out through the flue.
We now offer the largest firebox available in a contraflow heater. Accepted design practice has previously limited contraflow fireboxes to a width of 18 inches because of long term durability issues. These are related to thermal cycling of the firebox refractories. We are able to safely increase this limit for several reasons. Our 4 year combustion testing program at Lopez Labs has shown conclusively that maximum efficiency and emissions performance results, contrary to conventional wisdom, with overfire air supplies and larger fuel sizing. This combination of parameters slows the burn rate enough to lower refractory stresses in the firebox, where it matters the most. The Heat-Kit system uses two layer, 5 inch thick firebrick construction in the firebox. The inner 2½ inch layer is field replaceable, should that ever become necessary. Our 17 year track record indicates that this is unlikely, however.
The firebox upgrade is a low cost option. We now recommend it for all applications except where design dictates a compact footprint. Heat storage eliminates the oversizing issue since a partial fuel load can always be used. With a full load, peak heat output is approximately 20% higher than with an 18 inch firebox. With a partial load, the slightly lower surface temperature and larger surface area are desirable from a radiant heating point of view.
Available with white bakeoven
Please refer to the more detailed description in the next section. We offer the only white bakeoven available with a contraflow heater. Allan Scott, the well-known California baker and brick oven builder, states that only the sealed chamber design of a white masonry oven offers the ideal moisture retaining properties that make a perfect bread crust. A larger oven is optionally available with the 22 inch firebox.
Best masonry heater performance is achieved with the right relationship between thermal mass and burn cycle. Traditional masonry heaters in Europe were used as room heaters, and it is only with their relatively recent transfer to North America that they have been applied to modern whole house designs. As a result, most North American heaters are larger than European models, raising new design issues.
As heater walls get thicker, the surface temperature drops and the storage time and the thermal lag time increase. An optimum design needs to strike the correct balance between these opposing forces. Storage times in excess of 18-24 hours are not desirable, since there will be too much time lag between firing the heater and reaching maximum output, in effect requiring you to predict tomorrow's weather. Similarly, a 140 to 150°F surface temperature provides a reasonable heat output and the drop-off with lower temperatures is fairly steep. For example, at 150°F a 7.5° drop results in a 21% drop in heat output of a typical 10 sq. m. (surface area) heater from 5.80kW to 4.57kW.
Our precast refractory heat exchangers use special 1¼ inch thinwall construction, allowing you to build up to the ideal 5½ to 6 inch final thickness in the heater sidewalls using standard 4 inch masonry units. This gives you a wider range of finish options without incurring the performance penalties of thick sidewalls. It is interesting to note that the traditional contraflow heater in Finland is built from 5½ inch thick bricks with no liner (Finnish building codes do not require a liner).
Firebox doors have a machined fit
We design and manufacture our own firebox and bakeoven doors. They are of substantial cast iron construction with heatproof German Robax ceramic glass. We obtain a gasketless fit in our firebox double doors by first stress relieving and annealing the gray iron castings and then machining all mating surfaces flat. In a pressure test simulating actual use, Heat-Kit doors had substantially less air leakage than a selection of comparable gasketed doors.
Designing custom systems is our specialty, and a number of modification options are already designed into our standard modular parts. For example, heated benches are a simple add-on. Or, if the back surface of the heater is against a wall, we can modify it into a warm air source. We can not only sell you the components and install them, but also have the engineering expertise to best tailor a system to your needs.
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The diagram on this page gives typical dimensions for heater, chimney and optional benches. A good place to start with the layout is the location of the chimney, since there will usually be some constraints such as passing by floor joists or roof trusses.
Clearances to combustibles are given in the table below. Wing walls (partition walls) may touch heater by using a metal stud and cement board:
Clearances to Combustibles
The standard heater has a 22" wide firebox and is approximately 52" wide with the facing. All finished dimensions are nominal, since they can vary with different types of bricks or stone. Tile or plaster facings are approximately the same dimensions, since they are installed over a 4" backing.
This page last updated on January 15, 2001