Fireplace Blower by tetheredtoit


									                 Oklahoma Cooperative Extension Service                                                                  BAE-9437

                                                   Fireplaces and Fireplace
Raymond L. Huhnke
Professor and Extension                                                      Oklahoma Cooperative Extension Fact Sheets
Agricultural Engineer                                                            are also available on our website at:
      Over half of the single family homes in the U.S. have at
least one fireplace and about one million new fireplaces are
being installed in new and existing homes annually. In days of
inexpensive home heating fuels, these fireplaces were installed          to be at least 12 feet thick to have the same insulating value
primarily to enhance the appearance and atmosphere of the                of an ordinary wall with 31/2 inches of fiberglass insulation.
home. As conventional heating sources (propane, natural gas,
and electricity) rise in price, many homeowners are taking a             Fireplace Add-on Devices
look at their fireplaces as a means of providing supplemental
                                                                               In response to the desire by many homeowners to increase
heat for the home.
                                                                         the heat output and efficiency of their conventional fireplaces,
      Unfortunately, the standard masonry fireplace is basi-
                                                                         many manufacturers have begun producing fireplace add-on
cally a low-efficiency heating unit with an average efficiency
                                                                         devices designed to fulfill this desire. Most add-on devices will
of between 10 to 15 percent. In comparison, a well-designed
                                                                         help produce or reclaim more useable heat than is possible
airtight wood burning stove will have an average efficiency of
                                                                         without them. However, careful attention must be given to
between 50 to 60 percent. A standard masonry fireplace can
                                                                         exaggerated claims made by manufacturers. Simply install-
also be expected to burn 3 to 10 times as much wood as an
                                                                         ing an add-on will not make a fireplace capable of heating an
airtight stove to generate the same amount of useful home
                                                                         entire house. Also, an ‘’efficiency increase” of 40 to 50 percent
                                                                         will still only make a fireplace about 15 to 20 percent efficient
                                                                         as a heating unit.
Why are Fireplaces Inefficient?
      First of all the fire that is burned in a fireplace is basically
                                                                         Outside Air Inlet
an uncontrolled fire. Unlike most wood stoves which have air                    Conventional fireplaces require large quantities of air
inlet dampers to control the rate of burn and conserve wood,             for combustion. Unless special inlets are installed to provide
most fireplaces have little or no provision for such control.            air directly to the fireplaces from outside, the combustion
Some fireplaces do have dampers but unfortunately they are               air for the fireplace is drawn from the warm air inside the
located up in the top of the fireplace which makes them very             house. Cold outdoor air must then infiltrate into the house
difficult to adjust while the fire is burning. Also, because they        through cracks around doors and windows to replace the
are located at the outlet, their adjustment is more related to           warm air being used to support the fireplace fire. Because
the prevention of smoking than to fire control and efficient             of the dynamics of air movement, this infiltration will usually
wood utilization.                                                        occur at the highest rate in those rooms which are farthest
      A second condition which contributes to low overall ef-            from the fireplace. To prevent this situation from occurring,
ficiency ratings for fireplaces is the fact that a briskly burning       one can install an outside air inlet near or in your fireplace to
fire in a fireplace draws a tremendous amount of air up the              channel outdoor air directly to the fireplace. This will result in
chimney - about 400 cubic feet per minute. This volume of air            a significant reduction in the amount of heated air which is
would cause a complete air change in a 15 by 20 foot room                drawn from the living area for combustion.
every six minutes. Since this air must be replaced, the result                  An outside air inlet may be a necessity in a tightly con-
is infiltration of cold outside air into the house to replace the        structed house. In such a house, infiltration has been reduced
warm air used by the fire. On a very cold night a fireplace may          to a minimum and it’s possible for the air needs of a fireplace
draw more heat out of the house than it gives off, resulting in          to result in a reverse draft in the chimneys of any gas or oil-
a negative efficiency rating.                                            fired heaters or furnaces in the house. This reverse draft will
      And finally, even when a fireplace is not in use, it may be        cause poisonous carbon monoxide fumes to be discharged
drawing heat out of the house through the chimney walls and              into the living areas of the house instead of going up the chim-
then up the chimney. If the damper has been left open an even            ney. Also, in tight homes the fireplace may consume enough
greater amount of heat may escape. If the fireplace is built on          oxygen from the air to cause problems to the occupants. To
an exterior wall a significant amount of heat will be conducted          be safe, a positive source of outside air should be supplied to
directly outdoors through the back of the fireplace whether it           all fireplaces and wood burning stoves. This can be provided
is in use or not. The reason for this is a brick wall would have         by installing an outside air inlet or opening a window when
                                                                         the fireplace or stove is being used.

Division of Agricultural Sciences and Natural Resources • Oklahoma State University
      An outside air inlet can be constructed from 26 gauge
or heavier sheet metal. The inlet grill(s) may be located in
the wall alongside the fireplace, in the hearth, or preferably
within the firebox on the floor or sidewall (Figure 1). Inlet grills
located in the firebox should be as far forward as possible
and 11/2 inches above the fireplace floor so no ashes can
spill into them. The duct should be a minimum of 3 inches in
diameter, and preferably should have a cross-sectional area
at least 50 percent that of the chimney flue. The duct should
be screened at the outside opening to keep out insects and
rodents. It should also have a tight, well-fitting damper at the
fireplace outlet to allow closing off when not in use.

                                                                                Figure 3. Radiant grates are designed to hold the burning
                                                                                logs in such a manner as to expose more of the glowing
                                                                                coals and direct the fire's radiation outward

                                                                                Heat Chambers
                                                                                     Heat chambers are designed to be installed in the top
                                                                                of the fireplace opening to capture some of the heat which
                                                                                normally goes up the chimney. Many of these units come
Figure 1. Outside air inlets should be located close to                         equipped with small electric blowers to increase the amount
the fireplace opening.                                                          of air circulation. While there is no question these units do
                                                                                increase the amount of heat output of a fireplace, the actual
     If the fireplace has an ash pit or dump, outside air can
                                                                                percentage increase is quite small. Also in addition to the
often be ducted through it to the firebox. Adaptor units are
                                                                                cost of firewood one must now add the cost of the unit and
commercially available to transform the existing ash pit open-
                                                                                electricity to run it to determine any savings of the overall
ing into a dampered, protected inlet (Figure 2).
                                                                                heating bill.
Fireplace Grates
                                                                                Glass Doors
      Recently there has been a great deal of interest in
                                                                                      Glass doors can be an effective way to reduce the amount
redesigning fireplace grates in an attempt to increase the
                                                                                of heated air that is drawn out of the house and up the chim-
amount of heat radiated outward from the fire. In most cases
                                                                                ney. Estimates range from a reduction of 25 to 60 percent.
the grates are designed to hold the burning logs in such a
                                                                                Glass doors also make it possible to gain some control over
manner as to expose more of the glowing coals and direct the
                                                                                the rate the fire will burn because of air inlet dampers which
fire’s radiation outward (Figure 3). The actual value of these
                                                                                allow one to regulate to some degree the amount of oxygen
grates is questionable. Tests conducted by Dr. Jay Shelton,
                                                                                entering the fireplace. The overall effectiveness of glass doors
author of “The Woodburners Encyclopedia” have shown little
                                                                                is dependent on a good tight fit, both against the fireplace
or no difference from conventional grates when similar wood
                                                                                and around the doors themselves. If either location is poorly
and fueling rates are used.
                                                                                fitted, excessive air will get through which will reduce the ef-
                                                                                fectiveness of the unit.
                                                                                      A major disadvantage of glass doors is the glass will
                                                                                reduce the amount of direct radiation from the fire into the
                                                                                room by as much as 50 percent. This, of course, results in
                                                                                a lower heat output for the fireplace. Because of this, glass
                                                                                doors tend to be most beneficial when they are left opened
                                                                                while the fire is burning briskly and then closed when the fire
                                                                                is almost out or at night to prevent furnace-heated air from
                                                                                being drawn up the chimney. They also can serve very well
                                                                                as spark screens.

                                                                                Curved Tube Convection Heaters
                                                                                     A tube heater is a series of C-shaped steel tubes which
                                                                                are designed to add a convective component to the heat
                                                                                output of the fireplace. The principle behind this design is
Figure 2. Ash pit openings may be modified into a very
                                                                                quite simple. When a fire is built over the lower part of the
practical outside air inlet.

tubes, heated air in this area expands, causing an upward                    Blower-equipped tube heaters come with a manifold fas-
flow through the tubes (Figure 4). The simplest tube heater             tened to the lower tubes with a flexible metal hose leading to
designs rely solely on the natural flow of heated air to move           a remote fan and motor, or with both motor and fan mounted
warm air into the room. More complicated designs included               directly on the front of the heater tubes. Both styles require a
small 100-200 cfm blowers to ensure a positive airflow. Many            nearby electrical outlet. Blower-equipped tube heaters, when
of the units, both blower equipped and non-blower, come with,           used with a glass door, can be expected to double heat output
or are designed to be used with, glass door enclosures.                 of a fireplace - raising the heating efficiency from 10 percent
     To be effective the non-blower models must be positioned           to possibly 20 percent.
so that the upper ends of the tubes are flush with the fireplace             When buying a tube heater, pay close attention to the
opening and within 2" of the top of the fireplace (Figure 5).           quality of construction. Look for units with heavy gauge or
Such placement will force the heated air far enough into the            stainless steel pipes. Lightweight units will burn out in less
room to prevent it from being sucked back into the fireplace            than a year’s time, usually before one recovers an investment.
with the combustion air. A non-blower unit, when used with              Look for units which allow replacement of individual pipes.
a glass door, can increase the heat output of a fireplace by            Since the center pipes tend to burn out first this will allow one
about 50 percent - giving an overall heating efficiency of 15           to replace a single pipe rather than a whole unit. Ask about a
percent as compared to 10 percent without it.                           guarantee against burn out. On blower equipped units, turn
                                                                        on the fan to check the noise level and air output. Blowers
                                                                        above 100 cfm tend to be noisy; while ones below 60 cfm are

                                                                        Grate Heat Circulator Units
                                                                             Grate heat circulator units capitalize on the fact that
                                                                        heat from the interior of the fire is far greater than that of the
                                                                        flames. These units use the normally untapped coal heat to
                                                                        increase efficiency and heat production from the fireplace.
                                                                        The fire is built directly on top of the units as with a normal fire
                                                                        grate. A fan forces cold air through the inlet duct and into the
                                                                        interior of the device where it is warmed as it passes through
                                                                        a series of baffles. The warmed air is then blown out into the
                                                                        room (Figure 6). These units do not require a roaring fire to
                                                                        produce additional heat. Even after the fire has burned down
Figure 4. Curved tube convection heaters draw warm air                  to a bed of coals, the device will generate significant useable
into the heating tubes and circulated warm air back into                heat by pulling heat out of the coals. Many units come with
the living area.                                                        an adaptor panel to accommodate glass doors and are most
                                                                        effective when used with glass doors.

                                                                        Pre-fabricated Convective Fireplaces
                                                                             The most efficient fireplace on the market today is the
                                                                        pre-fabricated metal convective fireplace. Manufactured under
                                                                        such names as Heatilator, Majestic, El Fuego, etc., these units
                                                                        have net efficiencies in the 35 to 45 percent range. These
                                                                        units are of a double metal wall construction with a space in
                                                                        between through which air can circulate (Figure 7). Because
                                                                        of their double wall construction the exterior surface of these

Figure 5. To be effective non-blower models must be
positioned so that the upper ends of the tubes are flush
with the fireplace opening and within 2" of the top of the
fireplace.                                                              Figure 6. Grate heat circular units have hollow tubes and
                                                                        a fan which forces room air through the grates and blows
                                                                        warmed air back into the living area.

                                                                                                          of surface available for heat transfer. Those models which
                                                                                                          have extensive baffle systems will naturally provide greater
                                                                                                          heat output than those with few or no baffles.
                                                                                                               As with any pre-fabricated metal wood-burning appliance
                                                                                                          a major concern should be the potential for burn out. Prime
                                                                                                          consideration when purchasing one of these units should be
                                                                                                          given to quality of construction and the thickness of material
                                                                                                          used. Always check the manufacturer’s reputation and his
                                                                                                          warranty against burn out.

                                                                                                          Free-Standing Fireplaces
                                                                                                                Free-standing fireplaces are another type of fireplace
                                                                                                          which are available. These units are made of metal and are
                                                                                                          designed to stand alone much like a woodburning stove.
                                                                                                          Since they throw off heat in all directions, they are more ef-
                                                                                                          ficient than a conventional fireplace. They are, however, less
                                                                                                          efficient than the pre-fabricated convective fireplace because
                                                                                                          in most cases, they lack the ability to adequately control air
Figure 7. Typical Pre-fabricated Convective Fireplace.                                                    flow and provide air circulation.

units remains cool even with a very hot fire. This results in
these units being classified as “zero-clearance fireplaces”                                               Replacing Fireplaces with Wood Stoves
which means they can safely be installed in direct contact                                                      Several stoves have been developed to increase the
with wood walls and floors. In most cases the pre-fabricated                                              efficiency of the fireplace. Some controlled draft wood stoves
convective fireplace comes as complete fireplace assembly.                                                are built to fit inside existing fireplaces. Others are designed
However, some manufacturers do make models which are                                                      to sit in front of the fireplace with a piece of sheet metal to
designed to insert into existing masonry fireplaces.                                                      cover the fireplace opening. These installations eliminate the
      Air circulation in these units is generally achieved by the                                         need for another chimney.
use of small blowers. Some models do, however, rely on natural                                                  If the stove is designed to fit inside the fireplace,
convection to circulate the heat. In most instances, the blower                                           the manufacturer’s instructions for installation should be
equipped models will do a better job of distributing heat.                                                followed.
      Some manufacturers also produce models which can                                                          There are several ways to insert the stovepipe into the
be connected to duct systems to circulate air through the                                                 chimney for stoves that sit in front of the fireplace. One way
house. When purchasing such a model, pay close attention                                                  is to remove or wire open the damper and insert the stove-
to the manufacturer’s recommendations for clearance to                                                    pipe through a snug fitting metal sheet placed in the damper
combustibles for the duct pipes. Because of the potential for                                             opening. A non-combustible material can be used to create
high temperatures in the circulating air, in most cases 2" to                                             a seal at the connection.
3" clearances are required. It is not advisable to attach such                                                  Be sure the stovepipe can be easily removed for cleaning.
a unit to existing duct work used by a gas or oil fired heater.                                           If the stove is too tall for the stovepipe to be inserted directly
      For maximum efficiency, the pre-fabricated convective                                               into the fireplace, or if obtaining heat from the stovepipe,
fireplace must be equipped with an outside air inlet and glass                                            the connection can be made above the fireplace. In some
doors. The fireplace should always be operated with the glass                                             instances creosote may run down toward the fireplace into
doors open while the fire is burning briskly, and closed when                                             the sealing material or past the damper and accumulate in
the fire is almost out or at night, to prevent furnace-heated air                                         the original fireplace opening.
from being drawn up the chimney.                                                                                Check frequently for creosote accumulation. The
      A major factor in the increased efficiency of certain                                               material used to create the seal may need to be replaced
models of pre-fabricated convective fireplaces is the amount                                              periodically.

Oklahoma State University, in compliance with Title VI and VII of the Civil Rights Act of 1964, Executive Order 11246 as amended, Title IX of the Education Amendments of 1972, Americans
with Disabilities Act of 1990, and other federal laws and regulations, does not discriminate on the basis of race, color, national origin, gender, age, religion, disability, or status as a veteran in
any of its policies, practices, or procedures. This includes but is not limited to admissions, employment, financial aid, and educational services.

Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Robert E. Whitson, Director of Cooperative Ex-
tension Service, Oklahoma State University, Stillwater, Oklahoma. This publication is printed and issued by Oklahoma State University as authorized by the Vice President, Dean, and Director
of the Division of Agricultural Sciences and Natural Resources and has been prepared and distributed at a cost of 20 cents per copy. 0803


To top