Fireplaces: Studies in Contrasts

A. C. S. (Skip) Hayden is head of Energy Conservation Technology at the Combustion and Carbonization Research Laboratory (CCRL) of CANMET in Ottowa, Canada.

New fireplace designs--specifically advanced-combustion wood fireplaces--offer an alternative. Advanced fireplaces are attractive, comfort-supplying, and cost-effective complements to conventional heating systems, even in tight homes. They can eliminate indoor air quality problems caused by existing fireplaces, in a safe, energy-efficient and environmentally benign way. They are also addressing what has been an extremely challenging weatherization problem.

Myth versus Reality

Fireplaces have long been a staple of North American households. Builders find it difficult to sell a new house without one. Yet the mythological attraction of cozy fireplaces rarely translates into reality. Most fireplaces are difficult to start, smoke, create unpleasant cold drafts, and cause a number of other unseen problems of which the homeowner is often unaware. In most homes, conventional wood-burning fireplaces are between -10% and +10% efficient. They supply little if any heat to the house, particularly with cold outside temperatures.

How Wood Burns

If you take a close look at a burning log, you will notice something strange. In most instances, fire appears only over a portion of the log. At the same time, smoke is coming off, usually from a part of the log remote from the flame itself. This smoke is composed of a complex mix of volatile incomplete combustion products that are being boiled or distilled out of the wood before they can be burned. Without a means of igniting these products and further burning them before they leave the combustion chamber, these incomplete combustion products become creosote which can cause chimney fires, and also turn into particulates, which can be a major source of air pollution as well as indoor air quality problems.

Efficiency

Field trials conducted by the Combustion and Carbonization Research Laboratory (CCRL) of fireplaces in Canadian homes, in conjunction with other combustion equipment, have shown that in all but one case, on cold winter days, use of conventional masonry fireplaces actually resulted in an increase in fossil-fuel consumption for heating. The fireplaces actually had a negative energy efficiency during the tests.

In the exception where a fireplace did reduce fossil-fuel consumption, the fireplace was situated opposite from the house thermostat. Without glass doors, the fireplace's infrared radiation fooled the thermostat into thinking the house temperature was satisfied, while allowing the rest of the house to become quite cold. The owners had just arrived from Great Britain and were used to cold bedrooms, so they thought nothing more about it. The thermostat cutback did save energy, but the fireplace itself was still very inefficient.

Figure 1. Conventional fireplace schematic.

The low efficiencies of conventional fireplaces arise for a number of reasons, the most important of which are:

• High Excess Air. Excess air is the amount of air supplied for fuel combustion, over and above that which is theoretically required. A typical oil or gas furnace will require about 50% excess air for satisfactory performance. Conventional fireplaces, on the other hand, operate at about 1500% excess air, 16 times the theoretical requirement or more than 10 times what a fossil fuel furnace needs. This translates into extremely high mass flows of air through the fireplace and up the chimney when the fire is blazing.

  In any combustion system, the greater the mass flow, the poorer the heat transfer from the combustion gases to the system and the higher the sensible heat loss up the chimney. Any combustion system operating at high excess air levels is inherently inefficient. Table 1 shows the effect of excess air on a wood-burning fireplace burning seasoned wood with a flue gas temperature of 300deg.F. At 100% excess air, the sensible heat loss is 10% and the maximum possible efficiency the system can have is 78%. Looking at a fireplace burning at 1500% excess air, the sensible heat loss up the chimney is a huge 73%. The best efficiency a conventional fireplace can have is only 15%. This does not even consider the other inefficiencies and heat losses described below.

• House Air Requirements. If a fireplace does not have its own dedicated outside air supply and is not perfectly sealed from the house, under high excess-air conditions, it will take up to 1.4 air changes per hour (ACH) of heated house air and exhaust it up the chimney. If a fireplace were run continuously like this over the entire heating season, this would increase the heating requirements of the house by as much as 50%. Luckily fireplaces are used only about 15% of the day, on average.

• Poor Heat Exchange. Conventional fireplaces have minimal means to collect heat from the flame and flue gases and transfer it to the house. Often the only heat exchange is to the firebricks of the combustion chamber; prefabricated fireplaces often have a second wall around the firebox through which air may pass.

• Location on the House Outside Wall. As previously mentioned, the little heat that most fireplaces can extract from the flame often goes into the bricks and/or fireplace casing. With the fireplace on an outside wall, the most common installation in North America, as much as 50% or more of this heat is conducted directly outside and never reaches the house proper.

• Poor Methods of Heat Transfer to the House Interior. Most fireplaces with convective passages around their casing do not do a very effective job of actually getting the heat to the indoors by natural convection. Therefore, many manufacturers have installed circulating fans. However, many of these fans are quite inefficient in their use of electrical energy, even for those few that do happen to move reasonable quantities of air. In addition, the fans are usually noisy and are not run for any length of time when people are around the fireplace.

  To reduce excess tramp air that a fireplace uses, many fireplaces have glass doors. (Tramp air is air that doesn't take part in the combustion process, but is sucked out of the house and up the chimney by the high burning rate of the fireplace.) These units are not all that tight-fitting and still allow the fireplace to operate at high excess-air levels. They may even increase the burning rate because air may be more forcibly supplied to the fire, increasing overall instantaneous air demands. On the other hand, most fireplace doors use tempered glass to withstand the heat from the fire. Tempered glass does not transmit infrared radiation so that heat from the flame is reduced dramatically with the doors closed. The fireplace doors can serve to reduce the amount of heated house air aspirated up the chimney overnight, at the tail end of the burning cycle.

• Incomplete Combustion. The poor combustion in most fireplaces has two consequences for efficiency. The high excess air is a major impediment to efficient operation, of course, but there is a high level of incomplete combustion products, as volatiles in the wood escape the flame and pass directly up the chimney, resulting in an additional efficiency loss as well as serious pollution problems, both indoors and outdoors.

• Leakage when the Fireplace is not Used. Masonry fireplace chimneys have a large cross-sectional area, using 8212, 12212 and even 12216 tiles. This represents a large leakage area either where heated house air can escape--even when the fireplace is no longer warm, or where downdrafts of cold outside air can enter the house. Dampers nominally serve to close off the chimney, but in most cases they are quite ineffective, if they are even used. Devices such as roof-top dampers can provide a much more positive seal on the off-cycle.

   

People have been trying for years to improve the performance of conventional fireplaces--adding this and changing that--to little or no avail, often at significant cost. Devices such as glass doors, heatilator type heat exchangers, and even using outside air supplies improve efficiency only marginally, to the 10-20% level at best.

Air Requirements

Table 2 presents a summary of the air requirements of various residential combustion equipment, for a typical Canadian house. The house is a bungalow with a full basement, having a total internal volume of 498 m3 (17,500 ft3). The measure of air tightness of a house is most often given in terms of ACH, the air change being the total volume of air present in the house. To get some appreciation of what the number means, 0.3 to 0.5 ACH are considered necessary by many groups to ensure there is no long-term build-up of contaminants for indoor air pollution. Some of the new, tight homes require forced ventilation systems (often using heat recovery ventilators) to achieve this level.

There are large differences in the air requirements of residential wood-burning appliances, ranging from a fireplace with the highest air requirement of any combustion appliance in a house to the negligible levels of airtight woodstoves.

Fireplaces and Indoor Air Quality

After the fireplace is lit, and before the chimney gets hot and begins to draw properly, there is often significant smoke spillage into the house, with the tell-tale result of a darkened mantle.

Furthermore, during this high-burn period, the fireplace causes depressurization, and as a result searches for air within the house. Often the most convenient opening is the chimney of the central furnace or water heater. This can reverse the flow down the chimney of a conventional, naturally aspirating gas appliance, disrupting combustion and bringing the combustion products into the house. High levels of particulate emissions, along with volatile and semi-volatile organics, are produced by the fireplace during this period. These emissions can spill into the house or be released out the chimney to pollute the outdoors.

At the tail end of the burn cycle, a fireplace can be a major source of another indoor pollutant--toxic carbon monoxide (CO). The wood progresses through its burning to a charcoal state, similar in composition to hibachi briquettes. Who would put a charcoal barbecue in their living room? Nobody. It's too dangerous! However, overnight with a fireplace, the draft is low and other exhausting appliances may take their air down the fireplace chimney. Alternatively, the house itself, with its internal stack effect may become a better chimney than the real one during this period. In either case, fireplace combustion products can enter the house, and there is a potential for CO poisoning. People have died this way.

Air Pollution

Aside from being a source or cause of indoor air quality problems, fireplaces can also be a source of significant ambient air pollution. Indeed, CCRL experiments indicate that fireplace particulate emissions can be on the order of 50 grams per hour (g/h), twice the level of conventional dirty wood stoves. Visually, fireplace pollutants are not as obvious as those from wood stoves as they leave the chimney, because they are diluted with the high fireplace excess air levels.

Conventional Solutions

A Band-Aid solution is to attempt to isolate the fireplace from the house. Maybe the best way to do this would be to put it out in the backyard and watch it through your living room window. Another is not to use the fireplace at all, closing it off and sealing the connection to the chimney. Inflatable plugs can do just that, on an effectively permanent basis.

A more logical alternative, though, is to retrofit the fireplace with tight fitting glass doors along with a large combustion air supply directly from the outside to the firebox. Glass doors can cut down somewhat on the maximum air requirements of the fireplace and they also reduce the risk of combustion gas spillage into the house at the tail end of the burn, as well as house heated air loss during this latter period.

These actions seem simple, but are not in practice. It is difficult to find truly tight-fitting glass doors. Moreover, tempered glass, the common material for conventional fireplace doors, is not a good transmitter of infrared radiation, so that direct heat from the flame is prevented from reaching the room. The outside-air supply can also create problems. The size of the hole required to supply a conventional fireplace is very large, often 8 inches in diameter or more. If the outside terminal becomes exposed to significant negative pressure due to eddying wind effects, it is possible that hot combustion products may find the air supply duct is a more convenient exhaust than the existing chimney, with consequent risk of fire. Even if they did do their job, glass doors and outside air reduce problems with the fireplace, but still do nothing for its miserable efficiency.

Another partial solution is to burn an artificial (manufactured) firelog instead of cordwood. Manufactured firelogs, particularly those with a paraffin base, can minimize problems by lowering the high air demand, reducing pollutant emissions by up to 80%, and lessening the chances of combustion gas spillage into the house. Only one log is burned at a time, so burning rates and hence overall air requirements, are much lower than for split wood. In addition, a flame is developed over the whole surface of the artificial log. The volatiles, which, for normal wood, come off of the log remote from the location of the flame, are ignited and burned as they leave the wood surface, resulting in the low pollutants levels. However, artificial logs provide almost no heat and can be costly.

A Technical Revolution

There has been a revolution in wood combustion technology in the past few years, brought about by efforts to reduce the pollutant emissions of wood stoves. This is affecting fireplace designs, with remarkable performance improvements. First, let's look at wood stoves.

Airtight Wood Stoves

A well-designed airtight wood stove can fulfill most of a home's heating needs. Most wood stoves transfer heat primarily as black body radiators by long-wave radiation to solid bodies which they can see. They are most effective in warming up all the solid objects such as furniture, walls, floors and people that are in their line of sight. At the same time, natural convection is set up in the area due to the difference in temperature between the stove surface and the room air, so that heat is moved from the stove to the room and to other areas of the house by virtue of air motion. A few stoves also come with a circulating fan that increases the flow of air over the stove and out into the room, increasing convective heat transfer.

To best take advantage of the efficient heat-transfer mechanisms of a new woodstove, one should make every effort to locate it in a major living area, where occupants spend a large proportion of their time in the heating season, and which has at least reasonably open access to a significant portion of the house. The temperature of the rest of the house can be allowed to fall somewhat, resulting in a reduced overall heat demand. Tests have shown that the net efficiency of a well-located wood stove can be higher than that of a conventional gas or oil furnace. The seasonal efficiency of such an appliance in an intelligent installation can actually be significantly higher than its tested efficiency, because of this intrinsic zoning effect. There is no dilution device on an airtight wood stove. Air requirements for such an appliance are very low. For a stove fired at 2 kg/h, operating at an average 100% excess air, the demand for air is only about 17 m3/h, or 0.03 ACH.

Air Pollution and Conventional Woodstoves

Conventional woodstoves have been high emitters of incomplete combustion products, as have conventional fireplaces. Wood burns in a complex manner, with the incomplete combustion products coming off the wood remote from the location of the flame. In conventional airtight stoves, as represented in Figure 2, including those built even 5 years ago, a large amount of volatile incomplete combustion products (carbon monoxide, hydrocarbons, particulates and creosote) escaped the burning process. As a yardstick, emissions of particulates from conventional airtight stoves average around 25 g/h.

Typical Canadian home heat demands over most of the heating season are equivalent to such stoves being fired at air flow rates of 1-2 kg/h. Most woodstoves have been oversized for their installation--they supply heat continuously, not in an on-off fashion like furnaces. In order not to overheat the house, air controls on stoves are usually cut back, with dramatic increases in pollutant emissions of incomplete combustion products.

Advanced Combustion Woodstoves

Concern over the pollutants from conventional wood stoves resulted in emissions standards (based on particulates) being set in the United States (EPA 1990) and in Canada (CSA B415). This has led to dramatic performance improvements and emission reductions.

New, advanced-combustion woodstoves are meeting the emissions standards. In order to ensure clean, efficient combustion in the firing range required, major changes to the combustion design of wood stoves were needed. New designs give better combustion and have lower heat outputs, yielding a more useful range of operation. New designs employ advanced combustion techniques or catalysts to reduce the amount of incomplete combustion products and increase efficiency.

In the United States, most manufacturers initially concentrated on reducing emissions by using catalytic converters, similar to those found in automobiles. Such equipment performed well in the laboratory (around 2 g/h) but real-life performance was generally poor, with emissions often in the 9-16 g/h range, due to internal leakage, warpage of the bypass, or failed catalysts. Recent catalytic designs have been more successful, but there is still concern about catalyst longevity. Another potential problem is that the catalyst itself provides resistance to flue gas flow, resulting in flue gas spillage or poor combustion performance under marginal draft conditions.

Canadian and some U.S. manufacturers have concentrated on improving the combustion performance of the appliance itself. From the outside, the new designs appear to be similar to those of the past, but internally they are dramatically different. They have complex advanced combustion systems, with turbulent and preheated primary and secondary air, firebricked combustion zones, and insulated baffles. The result is two simultaneous combustion zones. The first is the conventional flame of wood burning, while the second, immediately above, is an intense bluish turbulent flame which burns off the volatiles, resulting in a complex flame and reducing the pollution considerably.

The Canadian advanced-combustion wood stoves now in the marketplace show an 80% reduction in emissions of incomplete combustion products with a 10-20% gain in efficiency, relative to stoves of a few years ago (see Figure 3). Such appliances can be an effective complement to conventional heating systems in many regions of the country; they offer the potential to displace 60%-70% of the fossil fuel used for central heating in these regions, with a similar reduction in overall CO2 emissions. They are also ideally suited for use in electrically heated homes, easily displacing 70% of the electricity used for space heating.

The Preferred Option

Suddenly we now have a real solution to the conventional fireplace with its many attendant problems and inefficiencies. Advanced-wood-combustion designs which use preheated primary and secondary combustion air along with well-insulated combustion zones, are beginning to be utilized to produce what can be called an advanced combustion fireplace. Such a unit can be built-in like a zero-clearance fireplace, or retrofitted into an existing fireplace cavity.

The new fireplace has truly air-tight, gasketed doors, a special glass window made from a pyro-ceramic to transmit the infrared radiation from the flame to the room and a hot air sweeping of the window to allow clear viewing. With the two combustion zones in plain sight, the result is a unique, riveting, chaotic flame which is far more attractive and hypnotically interesting than any flame burning in a traditional fireplace.

The advanced fireplace has an insulated outer casing to prevent heat loss out the side wall of the house, good heat exchange to take heat from the flue gases, and an effective squirrel cage circulating fan to supply this heat to the house (see Figure 4).

Because of the intense combustion patterns developed, the need for excess air level is low, so efficiency is high. The requirement for house air is also minimized to about 0.04 ACH. There is very little interaction with the house air, so the chances of releasing combustion pollutants to the indoors or in causing other combustion appliances to spill are minimal. Even at this low air rate, provision can be made to supply air from the outside directly to the appliance. However, because all air passes through a tortuous path within the unit to preheat the air before it is released for combustion in the firebox, there is no possibility of the combustion gases reversing and taking this route as an exhaust, unlike the supply for conventional fireplaces.

Most importantly, the emissions of incomplete combustion products of the advanced combustion fireplaces are reduced ten-fold from a conventional fireplace. Potential for chimney fires is almost non-existent, due to the low levels of incomplete combustion products and creosote generated.

Mass-flow through the system decreases as excess air and firing rates decrease, so efficiency can reach 78% (see Table 1). With the outside casing insulated to prevent heat loss to the outside, and efficient squirrel-cage fans blowing air around the convective passage to be heated and supplied to the house, the efficiency of use can approach 70%.

Because fireplaces are usually located in a major living area, with an open view to other regions of the house, these advanced design fireplaces can become extremely effective space-heating systems, with seasonal efficiencies which can surpass their laboratory-tested efficiencies, if utilized properly. These units are also ideally suited for retrofit into fireplaces in baseboard electrically-heated homes, easily displacing the majority of the electricity required for space heating.

Because of the much lower volume of flue gas products, an existing masonry chimney should be relined with a stainless steel liner, to ensure good draft and no condensation of combustion products. A totally new installation should use one of the high temperature super chimneys, designed specifically for wood burning appliances. To ensure this performance, one should get a new wood burning fireplace which meets the emissions criteria of either EPA 1990 or CSA B415. Only these types of advanced combustion fireplaces may be installed in Canada' R-2000 housing.

Pellet Fireplaces and Masonry Heaters

Pelletized fuels, which are about the size of cigarette filters, and are made from wood and other biomass wastes, can also be used in efficient, clean-burning fireplaces and other space-heating systems similar in concept to the advanced wood stoves and fireplace. They usually have higher capital costs than advanced-combustion fireplaces, but some can be side-vented which avoids the cost of a chimney. The cost of pelletized fuel is usually significantly higher per unit of energy as compared to cordwood. The ease of handling and automated feed may be a compensating factor.

Masonry heaters are another type of fireplace that have long been common in Northern Europe, but are rarely seen in North America. Wood is burned (ideally cleanly) at a high rate for about a two-hour period in a masonry firebox, while the flue gases pass through massive masonry in a complex path to remove and store much of the heat. The masonry subsequently releases the heat to the house slowly over a long period, as much as 22 hours. The small but vigorous North American industry has made significant strides in this area in recent years. Recent work indicates that underfire air leads to poor combustion, inefficiencies and fairly high emissions; also, significant heat loss can occur unless the heater is only installed on inside walls. These and a number of other guidelines are being developed in Canada, based on laboratory and field trials, to let alternative fireplace design be properly utilized as a clean-burning, energy-efficient heat source.

Gas-Burning Fireplaces

In the past few years, natural gas- and propane-fired fireplaces have seen dramatic increases in sales, due to their convenience and cleaner burning characteristics. One dilemma is that gas usually burns so cleanly that it has a transparent blue flame, with little visual attraction to the homeowner. To counteract this, significant effort has been expended to produce yellow gas flames that more closely resemble a wood-burning fireplace. This is usually achieved at the expense of completeness of combustion, as yellow in a flame indicates the presence of soot particles.

Gas Logs

The cheapest way to convert an existing fireplace to gas is to merely install what are known as gas logs. Basically, these are solid ceramic logs placed among gas burners to give the burning feeling. But gas logs have some serious problems. If the fireplace chimney is not relined, the chance of flue-gas condensation and chimney degradation is high due to the high-moisture fuel, low burning rate, and low temperatures. If the fireplace is on an outside wall, there is a good chance that chimney draft will be inadequate, the house will be a better chimney than the chimney itself, and combustion products will be brought directly into the house, causing indoor air quality problems. Finally, these logs will not supply any real energy to the house, and could be considered a waste of a premium fuel. Gas logs are not appropriate for today's new or renovated housing. (A further extension of the gas log concept is the unvented fireplace, which exhausts its combustion products directly into the house.)

Gas Fireplaces

Gas fireplaces can offer the potential for good, efficient performance, but this is not realized with many pieces of equipment, in spite of what might be written on the sales literature. Until recently, there was no reasonable test standard by which the efficiency of gas fireplaces could be determined. The Canadian Gas Association has been developing a seasonal efficiency standard for gas fireplaces, which is in its final draft form. The goal is to accurately represent the performance of gas fireplaces as they would normally be installed in Canadian housing.

When appliances are tested to this standard, dramatic differences have been seen for various technologies, ranging from less than 10% to over 70% efficiency, although most had been claiming 80% efficiency for their product.

Canadian provinces have taken the position that since a gas fireplace can be a significant energy user in the home, its efficiency will be regulated to a minimum level, a level which will be raised over time.

Until the standard is finalized and the regulation adopted, real seasonal performance numbers will not generally be available. However, it appears that by far the best performers are direct-vent fireplaces, with radiation-transparent pyro-ceramic glass, good heat transfer to the house, an insulated outer casing and an effective venting system to ensure safe removal of the combustion products.

Key Points

• Conventional wood-burning fireplaces are extremely inefficient. They can lead to indoor air quality problems, and can result in life-threatening situations. Hence they are incompatible with modern housing, or with housing which has undergone energy retrofit/renovation.

• Advanced-combustion wood-burning fireplaces have low pollutant emissions and can have seasonal efficiencies greater than conventional gas or oil furnaces.

• Only advanced-combustion wood-burning fireplaces meeting the EPA 1990 or CSA B415 performance standards should be installed in new or renovated housing.

• Gas firelogs are inefficient, can result in chimney degradation, and can cause severe indoor air quality problems. They should not be installed in today's housing.

• There is a dramatic range in seasonal efficiencies for gas-fired fireplaces, based on the new CGA standard. It appears that direct- vent equipment with pyro-ceramic glass doors and an insulated casing is the only type that should be considered for efficient energy use and good performance.

   

Barnett, S.G. and Hayden A.C.S.; In--Home Evaluation of Emission Characteristics of High Technology Non-Catalytic Woodstoves; Final Report on Contract 23440-9-9230/01-SQ; June 1990.

Barnett, S.G. and Roholt, R.; In--Home Performance of Advanced Technology Woodstoves during the 1988-89 Heating Season in Glens Falls, New York; AWMA Paper 90-80.6; Air & Waste Management Association Annual Meeting; Pittsburgh, PA, June 1990.

Hayden, A.C.S., Braaten, R.W. and Brown, T.D.; Oil Conservation in Home Heating; ASME Journal of Engineering for Power, Vol. 99, Series A, No.3; July 1977.

Hayden, A.C.S. and Braaten, R.W; Effects of Firing Rate and Design on Domestic Wood Stove Performance; Proceedings, Combustion Emissions from Solid Fuel Appliances, Air Pollution Control Association; Louisville, KT, March 1982.

Hayden, A.C.S.; Residential Combustion Appliances - Venting and Indoor Air Quality; Environmental Progress, V.7N.4, pp 257-261; November 1988.

Hayden, A.C.S. and Braaten, R.W.; Efficient Residential Oil Heating Systems-a Manual for Servicemen, Designers and Builders; SP 88-1E; Energy, Mines and Resources Canada; Ottawa, 1988.

Hayden, A.C.S. and Braaten, R.W.; Variations in Emissions of Wood Burning Appliances Fired with Different Fuel Types, AWMA Paper 89-95.4; Air & Waste Management Association; June 1989.

Hayden, A.C.S. and Braaten, R.W.; Retrofitting Residential Heating Systems to Improve Efficiency and Reduce Emissions; AWMA Paper 91-64.2; Annual Meeting, Air & Waste Management Association; Vancouver, June 1991.

Macintyre, K.R.; A Creative Approach to Efficiency Testing and Efficiency Rating of Gas Fireplaces; AWMA Paper 94-FA150.03; Annual Meeting, Air & Waste Management Association; Cincinnati, OH, June 1994.

Senf, Norbert; Recent Laboratory and Field Testing of Masonry Heater and Masonry Fireplace Emissions; AWMA Paper A99; Annual Meeting, Air & Waste Management Association; Cincinnati, OH, June 1994.

Table 1. Effect of Excess Air on Fireplace System Efficiency

_____________________________________________________________________________ 100 10 78 500 29 59 1,000 48 40 1,500 73 15 _____________________________________________________________________________ Assumptions --seasoned wood at 17% moisture --flue gas temperature of 300deg.F --no loss due to incomplete combustion products

Figure 1:

Figure 1. Conventional fireplace schematic.

Figure 2. Conventional woodstove schematic.

Figure 3. Pollutant emissions for different wood-combustion technologies.

Figure 4. Schematic of an advanced-combustion high-efficiency fireplace.

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