Editors Note: The following article is reprinted (with permission) from
Home Energy Magazine, which is
an energy publication for professionals. It is somewhat technical, but very
readable and accurate.
As a member of the Industry for 17 years, I personally think that inefficient,
open fireplaces SHOULD be against building codes. A building Inspector would
refuse to approve your house if you cut a one square foot hole in the wall
and let your heated air escape, but that's exactly what a fireplace does.
We must move on from early "Americana" , and just as we rid ourselves
of the gas guzzlers, get rid of OPEN fireplaces that waste our resources.
email your comments to webmaster@hearth.com
Fireplaces: Studies in Contrasts-- [ Back to
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To ensure that fireplaces do not cause problems
and major heat losses, the most obvious solution is to seal them up and
not use them. This is usually not acceptable.
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
In summary, advanced-combustion wood-burning fireplaces
or direct vent gas-fired fireplaces offer efficient, safe alternatives to
the outmoded and incompatible conventional fireplace, while allowing viewing
of what can be an even more attractive and interesting flame.
Excess air % Sensible heat loss % Maximum
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
Air Requirements
Cubic meters Air changes
Appliance per hour per
hour
___________________________________________________________________
Conventional oil furnace 260 0.52
with barometric
Mid-efficiency oil furnace 37 0.07
Conventional gas furnace 194 0.39
with draft hood
Condensing gas furnace 29 0.06
Conventional wood fireplace 680 1.4
EPA 1990-type wood stove 17 0.03
Advanced combustion fireplace 23 0.04
