This article describes combination or multi-fuel heating boilers that combine burning wood with oil. We explain how a multi-fuel heating system works and we list the special considerations that such equipment needs such as attention to combustion air supply, draft regulation, combustion chamber design and cleaning, and general safety.
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These articles on chimneys and chimney safety provide detailed suggestions describing how to perform a thorough visual inspection of chimneys for safety and other defects. Chimney inspection methods and chimney repair methods are also discussed. As with most inspection and safety topics, this material may be incomplete.
Combination Wood Burning Boilers, Furnaces, Fireplaces, Woodstoves - Special Considerations
Especially as oil prices have increased dramatically in the past few years and given the 2008 Bush Presidency economic crisis, many homeowners have a renewed interest in alternative heating energy sources.
Where firewood is available and economical, wood fired heating boilers and furnaces, and combination wood and oil heating systems that were first popularized in the 1970's oil embargo crisis have renewed interest.
This article describes wood heat sources and special considerations in operation and safety of combination fuel wood and oil heating boilers and furnaces.
Many of the components on a wood fired warm air heating system such as ductwork and the blower assembly are the same as on oil or gas fired furnaces but the heating system itself is quite different:
The combustion chamber location and construction are constructed to withstand a wood fire
The heat exchanger may be simplified, with fewer passages to reduce soot and creosote clogging
The wood fire burns continuously (rather than like a gas or oil burner that turns on and off in response to temperature controls).
A back up heat source is usually provided either as a separate heating system (oil, electric, or gas). Controls on that heating equipment are designed to keep it turned off when the wood furnace is in operation. The backup heat lets the occupants leave the building without fear of total loss of heat (and frozen pipe damage).
Kerosene heaters: Our photo (left) shows a kerosene heater which an owner was using as their alternative heat source. It did not prevent pipes from freezing in a remote corner of the building when the central heating was inoperative, and we also were concerned about the adequacy of its fire-clearance distances.
Combustion air is regulated to adjust the fire and thus the heat output of the wood furnace. Temperature controls in the occupied space and on the wood burning furnace adjust the combustion air flow in to the combustion chamber as the primary means of controlling the fire size and thus the heat output of the system. We discuss the wood furnace combustion air damper in more detail below.
Firewood: Unless your system uses a processed wood fuel such as a wood pellet stove, heating fuel (firewood) is added by hand by the occupants as needed by opening a combustion chamber door.
Ashes from burned wood are also removed manually by the occupants by opening an ash pit cabinet door.
Pellet stoves feed processed wood pellets automatically from a feed hopper and thus require less frequent attention.
Provide Combustion Air, Ventilation, Cooling Air for the Wood Furnace
Before looking in greater detail at wood fired furnaces and combination wood-oil furnaces, we and Carson Dunlop emphasize this safety note.
Because a wood-fired furnace operates at high temperatures and needs lots of combustion and cooling air it should not be located in a confined space.
Otherwise the risk of fire or improper operation are increased.
Chimney inspection and cleaning will need to be frequent to reduce the risk of a chimney fire as well. Sketch courtesy of Carson Dunlop.
Forced draft wood furnace operation: As the sketch (left) shows, instead of relying on natural draft, a forced-draft combustion wood air furnace uses an electric blower fan to feed air to the wood fire.
Heater controls can turn off the blower fan and on some models adjust the airflow rate as needed.
Wood Furnace combustion chamber details
Wood Furnace combustion chamber cleaning details are shown in the sketches at left and below, courtesy of Carson Dunlop.
Wood Furnace barometric damper inspection
DRAFT REGULATORS, DAMPERS, BOOSTERS are devices used to regulate the draft on oil-fired heating equipment such as furnaces, boilers, or
water heaters.
The barometric damper or draft regulating device we are discussing here is normally used only on oil-fired heating equipment, not on gas-fired equipment. The inspection requirements such as assuring that the damper is level, properly located, and operating freely are provided in our heating section at DRAFT REGULATORS, DAMPERS, BOOSTERS
The equivalent draft control on gas fired heating systems is discussed at Furnace Draft Hood on gas fired equipment.
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Additional technical contributors & reference sources for this article are listed below.
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Thanks to Luke Barnes for suggesting that we add text regarding the hazards of shared chimney flues. USMA - Sept. 2008.
Arlene Puentes, an ASHI member and a licensed home inspector in Kingston, NY, and has served on ASHI national committees as well as HVASHI Chapter President. Ms. Puentes can be contacted at ap@octoberhome.com
Roger Hankey is principal of Hankey and Brown home inspectors, Eden Prairie, MN, technical review by Roger Hankey, prior chairman, Standards Committee, American Society of Home Inspectors - ASHI. 952 829-0044 - hankeyandbrown.com
NFPA 211 - Standards for Chimneys & Fireplaces, NFPA 211: Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances, 2006 Edition (older editions and standards are found at the same bookstore)
NFPA #211-3.1 1988 -
Specific to chimneys, fireplaces, vents and solid fuel burning appliances.
NFPA # 54-7.1 1992 -
Specific to venting of equipment with fan-assisted combustion systems.
GAMA -
Gas Appliance Manufacturers' Association has prepared venting tables for
Category I draft hood equipped central furnaces as well as fan-assisted
combustion system central furnaces.
National Fuel Gas Code, an American National Standard, 4th ed. 1988 (newer edition is available) Secretariats, American Gas Association (AGA), 1515 Wilson Blvd., Arlington VA22209, and National Fire Protection Association (NFPA), Batterymarch Park, Quincy MA 02269. ANSI Z223.1-1988 - NFPA 54-1988. WARNING: be sure to check clearances and other safety guidelines in the latest edition of these standards.
Fire Inspector Guidebook, A Correlation of Fire Safety Requirements Contained in the 1987 BOCA National Codes, (newer edition available), Building Officials and Code Administrators International, Inc. (BOCA), Country Club HIlls, IL 60478 312-799-2300 4th ed. Note: this document is reissued every four years. Be sure to obtain the latest edition.
Uniform Mechanical Code - UMC 1991, Sec 913 (a.) Masonry Chimneys,
refers to Chapters 23, 29, and 37 of the Building Code.
New York 1984 Uniform Fire
Prevention and Building Code, Article 10, Heating, Ventilating, and Air Conditioning Requirements
New York 1979 Uniform Fire Prevention & Building Code, The "requirement" for 8" of solid masonry OR for use of a
flue liner was listed in the One and Two Family Dwelling Code for New
York, in 1979, in Chapter 9, Chimneys and Fireplaces, New York 1979
Building and Fire Prevention Code:
NFPA 211 - 3-1.10 - Relining guide for chimneys
NFPA 211 - 3-2 - Construction of Masonry Chimneys
NFPA 211 - 3-3 - Termination Height for chimneys
NFPA 211 - 3-4 - Clearance from Combustible Material
NFPA 54 - 7-1 - Venting of Equipment into chimneys
Brick Institute of America - Flashing Chimneys
Brick Institute of America - Proper Chimney Crowns
Brick Institute of America - Moisture Resistance of Brick
American Gas Association - New Vent Sizing Tables
Chimney Safety Institute of America - Chimney Fires: Causes, Effects, Evaluation
National Chimney Sweep Guild - Yellow Pages of Suppliers
"Top Ten Chimney (and related) Problems Encountered by One Chimney Sweep," Hudson Valley ASHI education seminar, 3 January 2000, contributed by Bob Hansen, ASHI
"Rooftop View Turns to Darkness," Martine Costello, Josh Kovner, New Haven Register, 12 May 1992 p. 11: Catherine Murphy was sunning on a building roof when a chimney collapsed; she fell into and was trapped inside the chimney until rescued by emergency workers.
"Chimneys and Vents," Mark J. Reinmiller, P.E., ASHI Technical Journal, Vol. 1 No. 2 July 1991 p. 34-38.
"Chimney Inspection Procedures & Codes," Donald V. Cohen was to be published in the first volume of the 1994 ASHI Technical Journal by D. Friedman, then editor/publisher of that publication. The production of the ASHI Technical Journal and future editions was cancelled by ASHI President Patrick Porzio. Some of the content of Mr. Cohen's original submission has been included in this more complete chimney inspection article: InspectAPedia.com/chimneys/Chimney_Inspection.htm. Copies of earlier editions of the ASHI Technical Journal are available from ASHI, the American Society of Home Inspectors.
Natural Gas Weekly Update: http://tonto.eia.doe.gov/oog/info/ngw/ngupdate.asp Official Energy Statistics from the U.S. Government
US Energy Administration: Electrical Energy Costs http://www.eia.doe.gov/fuelelectric.html
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Ceramic Roofware, Hans Van Lemmen, Shire Library, 2008, ISBN-13: 978-0747805694 - Brick chimneys, chimney-pots and roof and ridge tiles have been a feature of the roofs of a wide range of buildings since the late Middle Ages. In the first instance this ceramic roofware was functional - to make the roof weatherproof and to provide an outlet for smoke - but it could also be very decorative.
The practical and ornamental aspects of ceramic roofware can still be seen throughout Britain, particularly on buildings of the Victorian and Edwardian periods. Not only do these often have ornate chimneys and roof tiles but they may also feature ornamental sculptures or highly decorative gable ends. This book charts the history of ceramic roofware from the Middle Ages to the present day, highlighting both practical and decorative applications, and giving information about manufacturers and on the styles and techniques of production and decoration.
Hans van Lemmen is an established author on the history of tiles and has lectured on the subject in Britain and elsewhere. He is founder member and presently publications editor of the British Tiles and Architectural Ceramics Society. Available at the InspectAPedia Bookstore.
Chimney & Stack Inspection Guidelines, American Society of Civil Engineers, 2003 - These guidelines address the inspection of chimneys and stacks. Each guideline assists owners in determining what level of inspection is appropriate to a particular chimney and provides common criteria so that all parties involved have a clear understanding of the scope of the inspection and the end product required. Each chimney or stack is a unique structure, subject to both aggressive operating and natural environments, and degradation over time. Such degradation may be managed via a prudent inspection program followed by maintenance work on any equipment or structure determined to be in need of attention. Sample inspection report specifications, sample field inspection data forms, and an example of a developed plan of a concrete chimney are included in the guidelines. This book provides a valuable guidance tool for chimney and stack inspections and also offers a set of references for these particular inspections.
NFPA 211 - Standards for Chimneys & Fireplaces, NFPA 211: Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances, 2006 Edition (older editions and standards are found at the same bookstore)
NFPA 211 - 3-4 - Clearance from Combustible Material
NFPA 54 - 7-1 - Venting of Equipment into chimneys
Brick Institute of America - Flashing Chimneys
Brick Institute of America - Proper Chimney Crowns
Brick Institute of America - Moisture Resistance of Brick
American Gas Association - New Vent Sizing Tables
Chimney Safety Institute of America - Chimney Fires: Causes, Effects, Evaluation
National Chimney Sweep Guild - Yellow Pages of Suppliers
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