Advice for people building our stoves

Advice for people building our stoves.

Translated by Alex Chernov.

How to select a stove?

You would have to learn and understand some rules, and would have to do certain prep work before you start actual construction. First of all, you have to make a sketch with floor plan showing proposed stove’s location. The stove should ideally heat all the rooms. Location for the stove should be selected in such way that each room would have some area of the stove’s surface exposed. This particular area of the surface should produce enough heat to cover heat loss of the corresponding room. Partitions should be attached to the stove according to the safety rules stated in the ASTM standard and only at the corners or at the locations of stove’s internal walls. You would also have to plan for the proper clearances around the chimney and for the chimney location. In case of a retrofit into an existing building, location of the load bearing beams or trusses, and location of the existing chimney (if applicable) have to be determined in advance as well. If a mirror design of the existing stove is required for the application, you can read the drawings using mirror or flip the drawings using computer software. Location of the chimney connection can be changed using “smoke chamber” technique, considering that height of the ceiling allows for the safe alteration, of course.

About heat output.

Many of my stoves don’t have overall output and output of each face calculated due to the lack of time, as it is only me who does all the work. These calculations can be done by specialists if it is found necessary. For approximate calculations, there is a rule of thumb that states that 1 sq m of the stove’s surface generates in average 500 Wt/h under two firings a day (Top of the stove is not usually counted, because often the top is insulated and therefore releases negligible amount of heat). For example, a stove with dimensions of 1x1x2m high has output of 1000Wt-h at each face, and therefore, 4000Wt-h overall. In other words, in average, this stove generates 4kWt-h every hour through 24-hour period. (For reference, 1kWt = 3450Btu). Heat output of the same stove under a single firing a day can be estimated as 0.6-0.7 of the heat output under two loads per day schedule.

We now receive requests for stove design and construction from USA, Canada, France, Germany, Netherlands, Sweden, Spain and other countries. We often get questions why “output” of our stoves is so low? For example, a Swedish fireplace insert “KEDDY 520” has output of 14kWt, and our stove, for instance, has only about 4kWt? Does our stove has output 3.5 times lower then “KEDDY”? This is not right. Let’s try to find answer to this question. Different books give different ways of expressing output: -

In the book written by Sosnin Y. P. & Buharkin E.N., Strojizdat 1985, pg. 296, output is calculated based the rule of thumb: 520-580 Wt/sq m under two firings a day (without any comments). -

The book written by Kovalevskiy I. I. “Stove works”, Moscow, 1977, pg. 13-14, says that “normal heat output of a stove is an average amount of heat, genereated by the stove during 1 hour under two firings a day” (not stating what is the measurement unit). The book also gives average value of output in kkal/sq m-hour (i.e. multiplied by period of time in hours – hourly output rate). -

Shkolnik A. E. “Heating of low-rise buildings by masonry stoves”, Moscow, 1991, pg. 9, table 2, says that output is measured by Wt/sq m, not stating that it is average hourly output rate for the period of 24 hours.

Kovalevskiy I. I. talks about heat output over some period of time, which in fact is energy or hourly output, which reflect stove’s ability to release heat energy over 24 hours correctly. This is the way of determining heat output of stoves in Russia (kW*h, MJ, kcal, kJ).

1 kW*h =3,6 MJ =860 kcal; 1000 kcal = 1,163 kW*h; 1 MJ =0,278 kW*h =239 kcal.

1 kcal =0,0012 kW*h =0,0042 MJ; 1 kJ=0,239 kcal; 1 kcal =4,18 kJ.

Metal inserts and metal stoves, manufactured in Western countries, are essentially heating devises, developed for single load of calculated amount of firewood. They are usually used as additional heat source during the coldest period. To use such metal heating devise as a main heat source means to become a “slave” to it. It is very inconvenient to get up during the night to load the unit, and it is impossible to maintain comfortable atmosphere if occupants of the house have to leave it for some time. Value of heat output of such metal devise expressed on the package cannot give an idea of what area it can heat. I think, expressing area, which a heating devise can heat on its package is simply a salesman’s trick and an unfair practice. Heat energy can only be released during firing of such devise, and size of the heated area will depend on its heat loss. I think that for such devises, output is expressed as a “nominal/max” without taking time for consideration, just for the time in which a calculated load of firewood is burned. In other words, it is output of the firebox (Wt) Qt=ВQdNt /3.6, where Qt is amount of heat, released during combustion of a calculated amount of firewood over one hour. Qd is amount of heat released after complete combustion of 1kg of firewood. Nт is efficiency. Manuals for such devises also tell that they can be used in two modes: maximum output, and slow burn. It explains why two different numbers for efficiency are given. However, it is not known how it is possible to tie hourly heat loss of the heated area with an average hourly output rate from the devises over 24 hours.

Using Russian methods for calculating output, our stove described above can release 24x4=96 kW*h over 24 hour period of time under two loads per day. This calculation, using “rule of thumb” is not tied to amount of burned firewood and output of the firebox. It is an averaged value for heat output for a stove that has been constructed and operated properly. In this case, it is very easy to tie hourly heat loss of the heated area with average hourly output rate from the stove over 24-hour period of time. If we look at definitions in books on physics, power is determined by work done in a given unit of time and is expressed in kWt. At the same time, kWt is described as unit of power that equals energy released in one hour. Power (output) is ability of a system to do work in a given unit if time. If power of the system is determined in kWt, we cannot tell anything about its output without stating amount of time the system worked (if this amount is greater than one unit of time, of course).

For example, if an electric heater has output rated as 2 kWt, it tells us that it is able to release 2 kW*h of heat energy during 1 hour of time. It will accordingly release 1 kW*h during 1/2hour, and 24 kW*h during 12 hours of work. A low-mass wood-burning metal heating system, in contrast, is a single-load heater and cannot keep working for 24 hours without necessity to be reloaded several times during the period of time. It is able to release energy only during the time when fire is burning. In the book written by Miakelia K., Strojizdat 1987, pg. 14, tells about heat requirement of a 1200 sq ft-residential house that equals 3 to 4 kWt of energy that has to be released evenly during a long period of time i.e. it tells about average output of a stove during one hour in kWt that has to produce energy during amount of time greater than one.

About output of sauna stoves and stoves that are typically “over fired” by their function.

Sauna stoves are essentially stoves that are heated to much higher temperatures than ordinary residential stoves by their purpose. All of them have efficiency numbers that are significantly lower than ordinary residential stoves. Sauna stoves have to be fired for long period of time to make sure stones inside the stove and sauna rooms are heated to desirable temperatures. As stove gets heated, difference between temperature of its walls and temperature of the gases decreases i.e. heat transfer rate through the stove’s walls decreases. This leads to increased temperature of the exhaust gases in the chimney, which results in lower efficiency. This is the reason for drop in efficiency in any stove that is “over fired” – fired more often that it was designed for. We have to note that in double-bell stoves such drop is less rapid than in forced-movement systems because the second bell acts as an additional heat exchanger.

Wood-burning boilers of a constant firing type can be related to the same class of stoves as they experience the same conditions. In practice, temperature of sauna stove’s surface can reach up to 320o F (160oC). In this case, output of 1 sq m of the stove’s surface can be more than 1000 Wt-h (exact data is not available). It is possible to utilize excessive heat from exhaust gases by installing an additional heat-exchanging device (single or double bell in itself). In case of installation of such device, a by-pass damper has to be installed to facilitate smooth start-up from a cold stage.

How to read our working drawings.

We use ArchiCAD 5.0 program for creation of working drawings. We have added over 100 specific stove elements into the library of elements including hardware, heat exchangers, specifically cut bricks and other stove elements. However this created library cannot cover all necessary elements due to the fact that our stoves are often highly customized and it is very difficult to create new elements for every new project. Therefore you may notice a slight discrepancy between plans and 3D cross-sections. For example, some library elements red in colour (that usually means they represent red clay brick) can be used in drawings of refractory parts. In this case, such elements are usually shown on a plan as firebrick, but look red in 3D sections.

I have not planned originally to make any of my drawings available for general public. This is the reason why I sometimes used different methods of showing the same element. Beside this, drawings were developed during long period of time, sometimes several years apart, and using different environments. It is the reason why it is often impossible to change an, even a smallest detail in the drawings. It impossible for me to correct all the drawings to correspond to a certain standard as it would take enormous amount of time. Besides that, such discrepancy doesn’t change essence of the project significantly.

We create drawings in the described program using a grid that equals to width of a common Russian brick with a mortar joint. Additionally, each cell (half brick) is divided into 4 parts. All dimensions of bricks, parts of the bricks, dimensions between walls, dimensions of openings etc are modular to ¼ of a cell (1/4 of half of the brick or 1/8 of the full brick). Therefore, all dimensions on the drawings are set and read as 1/8, 1/4, 3/8,1/2, 5/8, 3/4 and 1 of the full brick. This allows to use drawings in different countries where dimensions of bricks differ from Russian 10 to 15% in any dimension. For Russian conditions, we set one brick to be 26x13sm including mortar joint (13x13sm grid) and 7sm high including mortar joint in our drawings. Therefore, real dimensions will be a little smaller.

Sometimes, certain bricks are missing on 3D cross-sections due to specifics of the ArchiCAD program. We draw each brick as a wall in 13x13sm grid. Fat line at the edge of brick represents line of drawing in the brick wall. These fat lines have to be separated to avoid unpredictable representations in 3D views. Besides, if a cross-section is taken at a fat line, this brick becomes invisible at 3D section. I have to be creative to avoid such misrepresentations that takes a lot of time and is at times tricky or not possible at all. Unfortunately, I don’t have an opportunity to correct the CAD program itself to avoid this problem.

Stove construction.

In stoves designed in double-wall construction facing is done using common Russian red clay brick sized 250х120х65 mm. The core or refractory linings in the fireboxes of single-wall stoves are done using Russian refractory brick sized 250х120х65 mm. Mortar joint in facings is considered to be 5mm thick. Total number of bricks required for a stove can be determined using a rule of thumb: number of bricks in the first row is multiplied by the number of rows and then by 0.8 coefficient. This quantity will be sufficient for the project including broken/damaged bricks. Refractory bricks can be count from the drawings. Attention should be paid why selecting refractory bricks. If possible, a product certificate should be reviewed to make sure this particular brick is suitable for your application. Refractory bricks are created for specific chemical/temperature requirements and should be used as directed.

Some of our stoves are designed to be built from red clay brick only. Such stoves are usually designed for periodic usage such as cottage/garden stoves. To increase lifetime of such stoves, it is adviceble to select clay brick of a better quality. I have seen some projects that consider using both refractory and clay bricks in the same comstruction, not separated by an expansion joint. Expansion rate of refractory and clay bricks are different that leads to potential cracking. Therefore we never do it and discourage you from mixing bricks as well.

It is not permitted to build a single-skin stove using refractory bricks. Refractory bricks have higher thermal conductivity and higher rate of expansion that leads to increased fire hazard. AN expension joint of 4-6mm has to be established all around between the refractory core and the facing. Standard packaging cardboard or mineral wool can both be used for such purpose. Mineral/ceramic wool is used around metal parts (doors etc) to allow for expansion as well.

Refractory linings and refractory cores are build using firebrick laid on the edge not paying attention on layout of clay bricks in the facing, and adhering to common bonding rules. Firebrick of different thickness can be used in different applications (2 ½”, 2” or 1 ¼”splits). Height marks for each row whether in core or in the facing are not given. Each next row is laid on top of a previous. It is important to maintain difference between the top and bottom marks given in the drawings. It is also crucial to maintain all height measurements that are given. Height mark for the refractory core is given above the arrows; height of the facing has to be calculated by multiplying number of rows by 7sm (height of a row with mortar joint). The difference in height between the core and the facing is important if the core is supposed to be covered with regular bricks above. It facilitates the necessary expansion joint. In such cases, for example, red filling on the next row represents expansion material in such expansion joint.

Main rule for the double skin construction states that the refractory core should have an expansion joint all around to allow core to move freely in all directions without breaking/cracking the facing. Internal walls with width of ¼ of brick can be laid flat or on edge, whatever you prefer. It is necessary to cut some bricks if bricks are shown laid flat and on edge in the same row if maintaining the bond is necessary. Refractory brick with is laid on edge above the firebox door in OIK stoves (height of the row is 124mm or 118mm). In this case, there is a gap between the door and top of the row. The gap is filled with cut firebrick. Stoves with refractory lining and a cook-top also have a row of firebrick directly above the firebox door. These bricks in 3D cross-sections are show in red, which in reality represents firebrick. The reasons for such discrepancy are described in detail at the beginning of the article. Cast iron hardware (doors etc) is installed with 5mm expansion gap all around. The joint is filled with mineral/ceramic wool or any other suitable mineral material. Such expansion joint has to be established around cook-tops as well.

A cook-top is installed into slots cut in the bricks directly below it. The cook-top is set into clay mortar to level it out. Mortar is then scraped out from the gaps between the cook-top and bricks, and the gaps are filled with mineral/ceramic wool. The next row is laid partially on top of the cook-top with a layer of mineral wool laid in between cast iron and mortar. This way, the cook-top will have freedom to move with heat expansion in any direction. If upper bricks are cut instead of lower, the expansion gaps will be filled with mortar that would be impossible to get out.

Firebrick work above firebox doors can be supported several ways: by building an arch, by using a refractory lintel either pre-cast on site or manufactured. Almost all our stoves use standard Russian stove hardware. Size and quantity of hardware is determined by the drawings. List of materials for each model is not provided due to lack of time.

Clay brick facing of sauna stoves should be reinforced with galvanized wire mesh laid every second row. Such wire strip is laid about ½” in from the outside surface. Outside surface of clay brick facing in our stoves can be sealed with special sealer “SUPI SAUNASUOIA” manufactured by a Finnish firm “TIKURILLA”. Only fully cured and dry stove can be sealed. Make sure the bricks are cleaned thoroughly before applying the sealer. To seal the brickwork, start from top at the least exposed wall and work down. Clean-out openings are left in the brickwork for cleaning the stove’s chambers and channels during construction and exploitation. These openings can be plugged with a brick laid on edge or covered using special soot doors available from some Finnish manufacturers. Bell stoves may not require cleaning for years, given that they are used properly.

Clay brick outside facing is laid in clay mortar. Clay for mortar can be dug (the way it was always done in the past). Special techniques exist for determining its suitability, which may be tricky to follow properly for an amateur builder. Although it is possible to dig your own clay and mix a good mortar by adding necessary amount of clean sand, buying a dry clean bagged clay in a pottery/masonry supply store can be preferable for an amateur builder. Dry bagged clay suitable for mortar should be as close to natural state as possible (no additives) and should be fairly sticky. Red Art Clay, manufactured by Resco Co. in USA is a good example. Such clay is mixed with clean brick sand in proportion of 1 part clay to 3-3.5 parts sand. Approximate amount of clay mortar needed to lay 500 bricks is 0.18-0.2m3 (1m3=41ft3). It is recommended to lay refractory brick in special refractory mortar available through most masonry suppliers.

We don’t dip bricks in water before laying. Generally speaking, this practice should be avoided if possible. Clay mortar is soft mortar that is set by drying. It allows to lay most bricks successfully without dipping. Dipping may be necessary for soft bricks (reclaimed bricks) laid in cement-based mortars. Clay mortar from joints in the stove that wasn’t fired can be used again by adding necessary amount of water. Mortar that was “fired” cannot be recycled as fired clay changes its properties permanently. Reclaimed bricks that have traces of fired clay have to be cleaned thoroughly to make sure good adhesion is maintained in the new brickwork. Hundreds of liter’s of water are absorbed by dipped bricks. All this water has to be evaporated during curing and drying later. It increases drying time considerably. If the stove is fired while still holding a lot of water, excessive amount of soot can be accumulated inside the core. Such stove may need full cleaning service after drying is completed. It is advisable to dry stoves using electric heating fans or similar devices instead of drying by building fires if brickwork contains a lot of water.

Principles of stoves construction in the System of Free Gas Movement”.

Stoves built by principles of the System of Free Gas Movement are very simple. They are one or two bells (chambers) set up one on top of another or one beside another. Bell is essentially a brick “cap” flipped upside down, where brick walls and/or columns support the ceiling. Two openings are located at the lower part of the bell: one is entrance where hot gases enter the bell, and another is exit, where exhaust gases are living the bell. Upper bell is built by this scheme where exit is exit into the chimney. In lower bell, such exit is an entrance to updraft channel(s) where gases pass from lower bell into the upper. Lower part of the stove consists of a firebox located in a bell according to specific rules. Such rules are listed in the articles: “Stove construction basics”, “Construction of multi-level stoves”, “Boiler construction basics”, “Sauna stoves”, “Pyrolizing boilers” and other available at our web site.

An amateur stove builder usually beings to understand principle of work of the stove after he built his first one. A desire to change, improve something may equally arise in mind of an amateur builder and in mind of a professional who is not yet familiar with our System. However, neither one has full understanding of all reasons for a specific solution. As a rule, any change in the stove’s design affects its performance adversely. It is well know now, that if there are any spots where something is not fully understood, these spots will be potential targets for a mistake. This is the reason why all changes have to be coordinated with me. The alteration will be accepted and used in our work if it is an advantageous change. I will explain what is the problem if the proposed alteration is a mistake.

Foundation

Foundation for a stove has to be done according to existing local Building Codes, maintaining proper depth, thickness, reinforcement and projection. A foundation for specific conditions (high water table, pour soil conditions, extra weight etc) has to be designed by a qualified engineer. It is unacceptable to build a stove on top of foundation consisting of several independent footings as it may lead to uneven setting of the stove and to its cracking. Stove’s foundation should be detached from the foundation of the building for the same reason.

A couple of words about selection of a masonry stove heating system

When someone decides to select wood-burning masonry stove heating as the primary heating system in his/her house, special care has to be taken to ensure the system is designed to fit their requirements and to provide them with desirable comfort level. The client and his/her family have to participate in selecting the system and its components for their house. There is a saying in Russian language that states that a house should be built starting from a stove meaning that a house has to be designed around the masonry stove, not vise versa. This is yet the best approach if masonry heating system is going to be the main heat source.

System of Free Gas Movement allows creating comprehensive stove heating systems for virtually any building with any requirements. Such project is ideally done coordinating efforts of the client, his architect and designer. Such project can have a single stove or multiple and multi-level stoves of different functional purpose. Stoves can be built with a built-in fireplace or without. One of our highly-efficient wood-burning boilers can be used to heat water for hydronic systems, a sauna stove or an outdoor complex can be included in the project etc. It is responsibility of a stove designer to make sure the system is designed for the best performance. If the client insists on a solution that is going to create problems and doesn’t want to listen to the stove designer’s advice, it is better to refuse to build a faulty system then to follow the clients wish blindly. Excellent performance is always appreciated, whereas a faulty system will never satisfy anyone; and it is doesn’t matter that it was the client’s, not the designer’s idea behind a solution that led to a problem that is very difficult or impossible to correct.

Conditional designations.

Igor Kuznetsov. Ekaterinburg, (343) 3077303 e-mail: igor@stove.ru ; http://stove.ru