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Masonry Heaters

A masonry heater utilizes a mass of masonry to store the heat generated from burning a fuel. It then gives out this heat over a long period of time, to heat up the space. Because of its large mass and ability to store heat for some time, such heaters may only be fired intermittently, just long enough to build up the heat in the radiating mass. Because of the need for a large volume of storage medium, mason heaters are generally built on or, at least assembled there. They are made up of a solid-fuelled heating device, some form of firebox, a large masonry mass, and a labyrinth of heat exchange channels. As the heat from combustion passes through these channels, within the masonry structure, it is absorbed by this and then slowly released into the home throughout the day.

As their name suggests, these heaters are made of some form of masonry, such as brick (firebrick), tile, stone, stucco, or a variety of combinations of these materials. This this way, they differ from their modern-day wood-burning counterparts which are more-usually constructed from steel or cast iron. Because of the large mass of material in a masonry heater, its weight needs to be borne in mind and it will, most probably, require some special support to take its weight.

As the fire in a masonry heater will, most-likely generate much higher temperatures than in a metal stove, the emissions from the exhaust gases is significantly reduced. Because of their intermittent firing cycle, these heaters are usually fitted with a damper which is closed when the unit is not being fired. This prevents the heat from the firing chamber escaping up the chimney, leaving it to be radiated from the masonry.

Because of it is a poorer conductor of heat, masonry takes much longer to heat up than metal. However, this means that, once warm, the masonry will give off its heat relatively slowly, radiating it into the room over a much longer period of time than the firing cycle took and doing so at a much lower temperature than a metal stove would. This is because there is a much lower temperature gradient through the walls of a metal stove and hence, the temperature on the outer surface of a stove is much closer to that inside where combustion takes place. Because the heat can flow so quickly through the walls of a metal stove, it cools down rapidly when the fire inside is extinguished. The metal of such a stove is hot only when there is a fire burning inside it and for a short time thereafter. Because a masonry heater’s outside surfaces are cool enough to touch safely, there is no danger of touching its hot surfaces and furniture may be located quite close to the stove itself.

Heat stress is a major concern during the construction of masonry heaters. Differences in temperature inside the masonry core of the heater can result in differential expansion. A skilled heater mason knows how to allow for this stress when designing and constructing the heater, thereby preventing uneven expansion from causing cracking in the exterior.

Masonry heaters take a long time (from 5 hours up to two days) to get up to the right temperature and so are not always practical for taking the chill off a single cool evening or morning, but they are well-suited for long periods of cold weather because they store heat so well and provide dependable, even heat all day and night. Because the radiant heat is given off at a low level a masonry heater is not likely to overheat a home the way a metal stove might in warmer parts of the year like fall or spring.

Modern masonry heaters have done away with the ceramic-tile exterior, replacing the refractory bricks with a heat-resistant plaster and they provide a refractory-glass door which allows you to see the burning fire. These changes are essentially cosmetic as it is still the burning of the firewood which warms the bulk of the stove’s masonry which continues to radiate heat. However, modern designs make use of the knowledge gained in furnace and heater design and sizes of the flue passages of modern masonry heaters have been more exactly calculated than heretofore, resulting in greater efficiency and output and the use of less wood.

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Hydronic Heating Systems

Wood-fuelled hydronic heating systems utilize some form of heater which is usually located in a small outbuilding and is used to heat water which is then pumped to the building it is designed to heat and supply with hot water. In colder climes, the water has antifreeze added  and is then connected by a heat exchanger to the potable water system.

In the system shown below, the control unit which regulates the operation of the boiler and radiators within the house is located in the basement.

A Typical Hydronic Heating System

Units such as this may use other forms of biomass fuel for convenience.

In the USA, the EPA has expressed concern about hydronic units in which users burn green wood, as this generates a lot more smoke. Even worse is when people burn household trash or construction debris, as this not only releases a cocktail of harmful chemicals and pollution but can also be infringing state law.

To improve the situation, the EPA runs a voluntary program for the designers and makers of hydronic heaters with the intention of persuading them to design and build less-polluting hydronic heater systems. Users are urged to buy only those models which qualify for the EPA’s voluntary program. It is estimated that systems which conform to the recommendations of the EPA are about 90% more efficient that older installations.

Hydronic heaters which do not conform to the EPA’s specification are generally much dirtier and less efficient than most other systems used for home heating. The use of poor and inappropriate fuels leads to smoldering fires and the short smokestacks (often no more than six to ten feet high), create heavy smoke which is released close to the ground. This is dangerous for people around the installation as well as causing aggravation and the smoke may persist low down where there is insufficient breeze to remove it.

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Fireplace Retrofits

Fireplace retrofits are manufactured units which may be installed into an existing wood-burning fireplace, be this of masonry construction or factory built. The aim of the retrofit is to reduce the wood smoke pollution produced by the existing fireplace. Properly installed, fireplace retrofit devices are capable of reducing pollution by about 70%. They burn much cleaner than traditional fireplaces, but still emit considerably more pollution than a certified wood stove.

The US Government’s EPA includes fireplace retrofit units in its Fireplace Partnership Program. This voluntary program for the manufacturers of wood-burning fireplaces and other fireplace units is designed to encourage them to design and make cleaner units which comply with their standards for wood-burning devices. Once they meet these criteria, their products may carry a white tag which enable a purchaser to make an informed purchasing decision.

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Fireplaces

Wood-Burning Fireplaces

 

Wood-burning fireplaces fall into two main categories, the traditional masonry fireplaces which are built of brick or stone, constructed on site by a mason and may form part of the structure of the building; and those which are delivered to the home as a modular unit which has been manufactured in a factory. These latter units are referred to as “low mass” fireplaces as they lack the bulk of the on-site built versions. Low mass fireplaces are grequently used in newly-built homes in the United States as they provide a lower-cost option. In general, fireplaces, whether they be constructed of masonry or are low mass, are not used as the primary source of heating and their function is seen as being primarily for secondary heating and ambiance. Traditional open fireplaces are extremely fuel inefficient, losing over 90 percent of the heat generated by the fire up the chimney.

In the USA, the EPA has set up a voluntary program for manufacturers of wood-burning fireplaces and other fireplace devices in order to encourage them to manufacture cleaner units. Units which comply with the latest standards may carry a white tag which demonstrates their credentials.

The US Government’s EPA Phase2 White Tag

The US Government’s Phase 2 White Tag

 The black line balance point shows the Phase 2 emissions level of 5.1 grams of emissions per kilogram of wood burnt, the target level which the unit must meet to comply with the criteria for Phase 2 certification. This level was established by the EPA using inputs from various stakeholders such as manufacturers of wood-burning units, EPA-accredited testing laboratories, various state air-quality agencies, such as the Northeast States for Coordinated Air Use Management (NESCAUM), and the Western States Air Resources Council (WESTAR).

 

Just below the target figure, in red, is given the emissions level of this particular unit. In the example shown, this is 4.4 grams/kilogram, i.e. 4.4gm of particles are emitted for every kg of wood burnt. The figure was obtained as a result of emission tests carried out by an accredited independent laboratory.

It is considered that the Phase 2 hang tag identifies fireplaces which are approximately 70% cleaner than older models.

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Furnace Design

The Major Two Types of Wood Stove

Designers have taken a new approach to the challenge of meeting the EPA smoke emission limits, these resulting in heaters which utilize a catalytic combustion process. However, in adopting this approach, they have radically redesigned the basic stove structure so that both catalytic and non-catalytic combustors are markedly more effective than the predecessors. Today, most of the stoves available in the marketplace remain non-catalytic but some, more popular high-end stoves utilize catalytic combustion. Inevitably, catalytic stoves are a little more complicated to operate so they are more suited to those householders who like the latest technology and are prepared to maintain their stove carefully so that it continues to operate at its highest efficiency.

Non-catalytic vs. Catalytic

Modern non-catalytic boilers are designed to recover as much heat as possible before sending this up the chimney stack and to burn as much of the non-combusted gases as possible.

The first aim is achieved by passing the air, which is going to be used for combustion, around the combustion chamber, thus preheating it, as shown in the blue, dotted air path in the diagram below. This serves the dual functions of cooling the outside of the stove as the efficient combustion take place at very high temperatures, and providing the air for combustion at a temperature a little above ambient, outside air temperature.

A baffle provided above the combustion chamber is also used as a heat exchanger to recover some of the heat which would otherwise be discharged up the chimney. This baffle, along with other internal components, is subjected to direct contact with the very high temperatures within the firebox and, thus, needs to be replaced from time to time.

Diagram to show the details of a non-catalytic stove

A Non-catalytic Stove

 

Thus, non-catalytic stoves rely on three internal features which create good conditions for ensuring that complete combustion takes place. These are the firebox insulation, pre-heated combustion air which is injected through small holes located just above the fuel in the firebox and a baffle which creates a longer, hotter gas flow path.

Catalytic Stoves

Catalytic stoves are at the forefront of technology and, in catalytic combustion, the smoke resulting from combustion passes through a coated ceramic honeycomb located inside the stove, just above the actual site of combustion and smoke gases and particles ignite there and burn. The figure below shows a typical catalytic stove.

A Catalytic Stove

 

Shown on this diagram, just to the right of the catalytic element is the lever-operated catalyst bypass damper which all catalytic stoves must have. This opened for starting the stove and during reloading. Over time, the catalytic honeycomb will degrade and need replacing, but how long it lasts depends upon how careful the stove user is when operating their heater. With careful use, the catalyst may last more than six seasons but if the stove is over-fired, inappropriate fuel (like garbage and treated wood) is used or the regular cleaning schedule and maintenance are not kept up, the catalyst could break down in as little as two years. The EPA insists that garbage should never be burned in a wood stove or fireplace.

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Fireplace Inserts

Although most wood-burning fireplace inserts are basically designed to increase a fireplace’s efficiency, they also add a lot to a room’s ambiance by providing something to sit close to which is attractive to look at and harks back to an earlier period.

While conventional wood burning stoves are often designed to be free standing, an insert is something like a stove module, a fireproof box, enclosed in steel or cast iron with a window of insulated glass. This creates a closed combustion system but one in which the flame may still be seen. If properly installed, fireplace inserts can convert a traditional fireplace into a much more efficient space heater.

Many inserts have a fan which blows the hot air into the room through vents on the front of the unit. They do this in order to recover the heat which would otherwise be vented up the chimney and, instead transfer this to air which may be circulated around the room. This yields a mix of radiant and convected heat, and drastically reduces heat loss, translating into higher efficiency and, hence, lower running costs.

Inserts are generally designed to appear like a regular firebox with a glass door mounted on the front face. There are also models which have glass doors on three or even all four sides when the fireplace/chimney design allows. The sealed glass door provides all the visual appeal of an open fire with none of disadvantages, such as the smoke and soot. It also removes the danger from sparks leaping out from the grate, potentially causing a fire or the risk of injury.

Another benefit resulting from the sealing of the unit is the improved combustion efficiency which reduces the buildup of unburnt fuel and gases and their deposition in the flue as creosote and tar. As well as the efficiency gain, this also reduces the chances of a chimney fire and results in a cleaner chimney/flue which requires cleaning less frequently.

Some units offer an air wash system which keeps the glass doors free of soot. In these, a stream of clean air is directed across the glass to keep it free of any deposits which might otherwise build up.

The benefit of installing a wood burning fireplace insert in a home which already has a functioning chimney is considerable. The fire places which has previously provided the focal point in a room is maintained and the ambiance of the room maintained. In addition, recessed fireplace spaces only need simple fire guard to keep children and pets out of harm’s way.

Wood burning fireplace inserts are generally more expensive than a comparable wood stove and because of the need to match up the pre-built unit with the existing fireplace, call for s specialist installer to do the job, both to make sure that they look good, perform well and are safely installed.

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Pellet Stoves

Pellet Stoves

While pellet stoves may be broadly similar in appearance to wood stoves, they function by using a renewable fuel produced from ground, dried wood and other biomass wastes which are compressed into pellets. The major difference lies in how the fuel is introduced into the furnace. The user loads the pellets into a hopper which then feeds these automatically into the stove. Because of this mode of operation, most pellet stoves require electricity to operate them. They are available on the market either as free-standing stoves or fireplace inserts. Pellet stoves operate far more efficiently than either open fires or wood burning stoves. Some manufacturers produce stoves with back burners, enabling them to provide a source of hot water. The initial costs for pellet systems are still higher than with comparable fossil fuel systems, but in some areas, there may be grants available to help with the capital costs.

Two types of pellet delivery systems are in use, one feeding the pellets into the top of the furnace and the other into the bottom

In the case of a top-fed pellet stove, the fuel is fed into the combustion chamber from a hopper located at the top of the stove. Ash then gathers at the bottom of the stove.

With a bottom-fed stove, the fuel enters from the bottom of the combustion chamber and, in many designs, automatically pushes the ash into an ash pan. With this setup, maintenance is simpler as the larger capacity of the ash pan means that it needs to be emptied less frequently.

The diagram below illustrates one form of pellet stove.

A Typical Pellet-stove Installation

A Cut-away Diagram of a Pellet Stove

A pellet stove, such as the one illustrated above, consists of a hopper to store the fuel for burning, a screw feed mechanism, furnace element or , combustion chamber, electronic controls, and suction and convection fans. The overall heat output is controlled by an electronic circuit which uses a  thermostat as its primary sensor and this regulates how much fuel (i.e. how many pellets) is fed into the heating chamber. The control device provides a great measure of control and. unlike with a wood burning stove, the heat output may be adjusted to suit the needs of the heated building. A typically hopper takes about 25Kg of pellets and, depending on heat demanded will last some two to five. Any ash drops down into an ash pan located below the combustion chamber and, because of the high efficiency and combustion temperatures, this should only require emptying occasionally (typically less than five times a year). With reasonable quality fuel, the ash deposits will represent less than two percent of the volume of fuel of fuel burnt.

Having a window through which the combustion is visible, a pellet-fed boiler can make an attractive addition to a living space.

Pellet boilers are available for systems where water is used to deliver the heat and these are suitable for replacing existing gas or oil central heating appliance. Because of the advances in electronic controls, many systems are in excess of ninety percent efficient. As with modern oil and gas systems, they are capable of operating totally automatically but do require some occasional maintenance such as emptying the ash pan every 3 months or so and cleaning of the burner once a year. In many installations, fuel suppliers carry out this work under a maintenance contract.

Pellet boilers are suitable for applications which call for variable heat loads as they are capable of being timer controlled. Systems are available to deliver heat outputs ranging in size from 15kW – 500kW and are means that they are capable of meeting the needs of both domestic and commercial installations, such as houses, schools, offices and factories. Some systems are available which can run on both wood pellets and wood chips, making them very flexible in terms of the choice of fuel..

Systems utilizing pellet stoves are probably the simplest to operate and maintain of all biomass-fuelled installations. As the duel is supplied in a convenient form, it frees the user from chopping logs, humping them around, no splitting and stacking and waiting for the fuel to dry out properly. Pellet stoves produce very little smoke and creosote when used correctly and, as the latter is the main cause of chimney fire, are safer than other forms of timber-fed heating systems.

The wood pellets used for fuel are normally made from sawdust or wood chips using a combination of heat and pressure and the use of premium-grade pellets helps to reduce the volume of ash produced.

Storing the Fuel Pellets

Because pellets have a much lower energy density than conventional fossil fuels such as heating oil, they require more storage space. A house with a total heat demand (water & space) of 26,880 kWh/year, would require 10.6 cubic metres of space to fuel store the  pellets, compared with 40.6 cubic metres for wood chips but only and 3.3 cubic metres for heating oil.

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Wood Stoves

Modern wood stoves are much safer and much more efficient than their earlier counterparts. When properly installed and maintained, they produce virtually no smoke, just minimal ash, and require much less firewood. The older uncertified stoves release some 15 to 30 grams of smoke per hour; those meeting, EPA-standards (Environmental Protection Agency Standards) produce only around 2 to 7 grams of smoke per hour. When buying a new stove, you should be sure to check for the EPA certification label which will be located on the back of the stove somewhere.

How to Choose the Right Size and Model of Stove

Wood stoves are available in an enormous range of different sizes and can be chosen to suit your requirements exactly, be it heating a single room or your entire home.

Small stove may be adequate for heating a family room or a seasonal cottage, whereas, in a . larger homes which has a pre-existing central-heating system, you could use a small stove for heating just a living room or a lounge. By taking some load off the centralized system, this can help reduce overall fuel consumption while conserving energy and saving you money but still providing the level of comfort you’re seeking.

A medium-sized stove should be adequate to heat a small house or a larger one which has been well insulated and is energy efficient. It should also be adequate to heat a small cottage during cold spells.

Large stoves are more suitable for large houses, those with poor thermal insulation and those in geographical areas with harsher winters.

When setting up a new installation, you will need to investigate the specifications of the equipment carefully and the suppliers of this should be able to advise you of the specification needed. A key determinant will be the size of your home so you will need to go along equipped with a floor plan of your home. Given this, a competent retailer will be able to help you choose the wood stove, fireplace insert, or other hearth product which will be suitable for the space you want to heat.

Burning Cleaner, Saving Money and Energy

You should be on the lookout for cleaner burning wood stoves, as the will reduce your monthly fuel bill, as well as protecting your health. In the USA, the EPA cannot support the use of non-vented gas stoves or fireplace inserts due to indoor air quality concerns). You can compare the cost of heating your home with wood, electricity, natural gas, oil, or coal. You can also see how using a cleaner burning hearth device to supplement your existing heating system can reduce your overall home heating cost.

The US Emissions Limits for Wood-Burning Stoves

The standards of performance for new wood stoves issued by the EPA in 1988 brought about a dramatic change in the internal design of wood stoves. The EPA’s mandatory smoke emission limits for wood stoves are 7.5 grams of smoke per hour (g/h) for non-catalytic stoves and 4.1 g/h for catalytic stoves. (Some states in the US have set even tighter limits and wood stoves which offered for sale in the state of Washington are required to meet limits of 4.5 g/h for non-catalytic stoves and 2.5 g/h for catalytic stoves.).

These regulations have pushed manufacturers to improve their combustion technologies over the years, and, today, now some of the newer stoves on offer have certified emissions in the range 1 to 4 g/h. If you’re comparing different offerings with a view to purchasing a new stove find the EPA white label attached to the back of the stove and look at the g/h (grams per hour) rating The lower the g/h rating, the cleaner and more efficient your wood stove will be.

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Introducing Woodburning Heaters

Modern ovens and fire places which are designed to use wood as primary fuel, today provide much more in comparison to their earlier counterparts. Today we do not need to concern ourselves about smoke or dust which might build up inside our houses and many fire places today may operate using electric controls. In addition, they’re very versatile and you will be able to install the devices in virtually any small home without them crowding out you and your family. Today’s fireplaces are designed to save energy and, if you combine these with gas central heating, they will enable you to use less gas

Diagram showing emissions for old versus EPA- approved wood burning stoves

A Comparion between Old and EPA Wood-burning Stoves

The older wood-burning ovens which you’d just throw wood into are basically little more than container stoves and many of these were very heavy, being produced from cast iron with a hot plate on which you could do your cooking on top of the stove itself. Today, many wood ovens have electric blowers and are designed to reduce smoke and the need for a poker to keep the fire bright. Wood burning ovens today can be real energy savers. The sophistication comes at an price in terms of the initial installation but saves money in the long term. Previously, you could buy wood burning ovens for roughly $29, nowadays however you’ll pay a touch more than that to install a wood burning stove in your own home. Fire places are certainly not cheap either, but you will find some item for roughly $200 or $300. To help you start to see the benefits of today’s wood burning ovens and fire places we will discuss the various issues, such as cost verses the expense incurred in heating bills. We can also look at a few specific models to help you see just what the market offers.

Basically, there is a number of types of wood-burning devices which people use to heat their home, either as a primary source of heat, as supplementary heating, or for ambiance and these may be described under the seven different headings:

  • Wood Stoves
  • Pellet Stoves
  • Fireplace Inserts
  • Fireplaces
  • Fireplace Retrofits
  • Hydronic Heaters
  • Masonry Heaters

These will bedescribed in the various pages of this site.

Wood Stoves are generally standalone devices which are made of cast iron or masonry, many featuring a range of devices to facilitate their use but may be as simple as their predecessors of years gone by.

Pellet Stoves may be similar in appearance to wood stoves but, instead of using raw timber, they burn a renewable fuel made from ground, dried wood and possibly other biomass wastes which are compressed into pellets. They operate by having the pellets into a hopper to feed automatically into the stove itself. Because they require this feeding mechanism, most pellet stoves require electricity to operate.

Fireplace inserts are simply free-standing wood stoves and pretty-well the same in both function and performance but they are designed to be fitted within the firebox of an already-existing masonry or metal fireplace. The certified installer will need to make sure that the flue liner in a masonry chimney is installed correctly. If a fireplace is factory built (or “zero-clearance”), then an insert must be used which was specifically designed and tested for the unit it is be assembled into. This will make it more efficient and also less polluting.

A fireplace retrofit is simply a unit which is installed into an existing wood-burning fireplace cavity, either of factory-built or masonry construction. The main reason for such a retrofit is to reduce wood-smoke pollution from the existing fireplace. When these are installed and operated properly, such devices should reduce smoke pollution by approximately seventy percent.

The term Hydronic is little more than a technical term to describe heating or cooling systems which use water (hence hydro-)as the heat-transfer medium. The commonest examples of such systems are hot water or  radiator installations. These have been used in large-scale commercial buildings such as offices and high-rise accommodation for many years. Such systems may include both a chilled and a heated water loop, which is able to provide both air conditioning and heating.

A masonry heater is an installation which is built onsite or assembled their and is often a solid-fuelled heating device, made up of a firebox, a large masonry mass, and an arrangement of heat exchange channels. Energy from stores heat from fires within its masonry structure,is stored in the masonry mass and released slowly into the home throughout the day. In the US, masonry heaters currently do not need EPA certification.

In the UK, electric night storage heaters work in this way, although they store up the heat generated from electricity which is obtained at a cheap night rate and release this during the day.

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