Combustion Air Details for Heating Appliances InspectAPedia® -
Combustion air: how to provide adequate combustion air for combustion appliances in newer tighter buildings and in older homes that have been made more airtight to save on energy costs
How to provide adequate combustion air without wasting building energy through air or heat leaks
Adequate combustion air is essential for building safety (avoiding potentially fatal carbon monoxide poisoning) and essential for proper operating of fuel-burning appliances
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This article discusses methods for providing adequate, safe combustion air for fuel-burning appliances in tight buildings - how to provide outside combustion air for heating appliances. Sketch at page top and accompanying text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
This article explains the need for adequate combustion air for fuel burning appliances in buildings, for both safety and for proper equipment operation. Figure 1 (page top and shown in more detail here) notes that by confining the gas furnace in a separate room, adequate air for draft and combustion can be supplied [from outdoors] without adding infiltration to [and cooling] the house. The author notes that
... standard formulas used to compute whether the indoor air supply was sufficient assumed a minimum rate of air infiltration of 0.5 air changes per hour.
In newer, tighter homes and in older retrofitted houses, air for combustion is not as readily accessible. If an appliance does not get enough air for complete combustion, its efficiency drops [increasing heating costs], soot can build up [potentially leading to fatal carbon monoxide poisoning in some cases], condensed water can collect in and corrode the flue [dangerous flue gas leaks]. Moreover, in a tight house, an exhaust fan - for example in the kitchen, greenhouse, or clothes dryer [or a whole house fan or bathroom exhaust fans] - may create a negative air pressure strong enough to draw toxic gases back into the house.
For both safety and energy reasons, then, more designers are deliberately supplying outside air to combustion appliances.
Simple Homeowner Tests for Adequate Combustion Air & Adequate Chimney Draft
An easy test of adequate draft in a gas appliance is to hold a just-blown-out match near the vent hood and see if the smoke is drawn up the flue. This chimney draft test should be performed under worst conditions: in warm weather (the chimney stack pressure will be lower in warm weather), with the house closed up (shut windows and doors, especially the windows and doors feeding the utility room where the appliance is located), and running all of the building's exhaust fans at once.
See Backdrafting Appliances for a detailed description of the test procedure to check for adequate combustion air and adequate draft.
Two Methods for Supplying Combustion Air for Heating Appliances in Tight Buildings
The National Center for Appropriate Technology (NCAT) [and other sources such as the Uniform Mechanical Code and the National Fuel Gas Code] suggest looking for carbon build-up around the burner and looking for flue corrosion. Two approaches are given for supplying outdoor combustion air to fuel-burning (oil, gas, wood, coal) heating equipment:
The heating appliances are fully isolated from the living space with partitions. Figure 1 (page top and shown in more detail here) provides two vents to the outside, one within a foot of the ceiling and one within a foot of the floor. If vertical ducts are used to bring combustion air to the appliance each vent should be sized at one square inch of free vent area per 4000 BTUH of the appliance input rating.
The high and low vents allow heat to dissipate from the enclosure and they allow combusting gases to escape should there be any backdrafts. The double vent arrangement permit a freer flow of combustion air to the heating appliance such as a furnace or gas fired water heater.
In cold climates steps may be necessary to protect plumbing in the outside-air-cooled utility area from freezing, and the specific details of any combustion air system should be checked against local building codes and importantly, against the installation instructions from the equipment's manufacturer.
The figures given above are for gas fired appliances. At oil burner school we were taught that for oil burning appliances we wanted to see one square inch of free vent area per 1000 BTUH of oil fired appliance input rating. Remember this is "free area" so vents that are louvered and screens need to be larger to allow for the effects of that obstruction - DJF.
Confining partitions are left open at the top [saving construction costs] (image at left). The open-topped partition has a duct delivering fresh air to the bottom of the enclosure. The logic is that the basin created by the partitions will trap [the incoming] cold air and minimize its mixing with the conditioned [heated] house air. NCAT provides a booklet with these details.
Direct-Vented Combustion Appliances
Some heating appliances, furnaces, boilers, and water heaters, are designed to isolate the combustion process from the living space entirely, avoiding the need for complex combustion air and venting schemes. Direct-vented combustion appliances are designed and tested to burn fuel and draw combustion air properly even when high winds hinder draft. Typically such systems include two sets of piping or ducts between the appliance and outdoors, one bringing combustion air in directly to the appliance burner, and a second venting combustion air outside. The two vents might appear on some systems as a single larger diameter double-walled pipe containing actually two vents, the smaller located inside the larger.
Combustion Air for Air Tight Woodstoves
Typical airtight woodstoves require only 10-25 cfm of combustion air - much less than an open fireplace (50 to 150 cfm or more) or to older non-airtight woodstove. But in tighter homes it may be necessary to provide combustion air or a draft inducer fan even for these appliances. Just as modern energy codes provide a vent to supply outside air to open fireplaces, outside combustion air can be supplied to an airtight woodstove through a floor vent or a wall register that is ducted in turn to outdoors - a method that adds to cold air infiltration into the building.
Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
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Additional technical contributors & reference sources for this article are listed below.
Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.
"Introducing Supplemental Combustion Air to Gas-Fired Home Appliances", National Center for Appropriate Technology (NCAT), $4.00, from NCAT, PO Box 3838, Butte MT 59702.
Solar Age Magazine was the official publication of the American Solar Energy Society. The contemporary solar energy magazine associated with the Society is Solar Today. "Established in 1954, the nonprofit American Solar Energy Society (ASES) is the nation's leading association of solar professionals & advocates. Our mission is to inspire an era of energy innovation and speed the transition to a sustainable energy economy. We advance education, research and policy. Leading for more than 50 years.
ASES leads national efforts to increase the use of solar energy, energy efficiency and other sustainable technologies in the U.S. We publish the award-winning SOLAR TODAY magazine, organize and present the ASES National Solar Conference and lead the ASES National Solar Tour – the largest grassroots solar event in the world."
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
Excerpts with updates and annotations expanding the original Best Practices Guide text can be found in the online review and book summary at BEST CONSTRUCTION PRACTICES GUIDE and also at DECK & PORCH CONSTRUCTION, at INDOOR AIR QUALITY IMPROVEMENT GUIDE, and in other articles found at InspectAPedia.com such as HOUSEWRAP AIR & VAPOR BARRIERS, SOUND CONTROL in BUILDINGS, and other topics.
Passive Solar Design Handbook Volume I, the Passive Solar Handbook Introduction to Passive Solar Concepts, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v1.pdf
Passive Solar Design Handbook Volume II, the Passive Solar Handbook Comprehensive Planning Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v2.pdf [This is a large PDF file that can take a while to load]
Passive Solar Handbook Volume III, the Passive Solar Handbook Programming Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v3.pdf
"Passive Solar Home Design", U.S. Department of Energy, describes using a home's windows, walls, and floors to collect and store solar energy for winter heating and also rejecting solar heat in warm weather.
"Solar Water Heaters", U.S. Department of Energy article on solar domestic water heaters to generate domestic hot water in buildings, explains how solar water heaters work. Solar heat for swimming pools is also discussed.
"Heat-Transfer Fluids for Solar Water Heating Systems", U.S. DOE, describes the types of fluids selected to transfer heat between the solar collector and the hot water in storage tanks in a building. These include air, water, water with glycol antifreeze mixtures (needed when using solar hot water systems in freezing climates), hydrocarbon oils, and refrigerants or silicones for heat transfer.
"Solar Water Heating System Freeze Protection", U.S. DOE,using antifreeze mixture in solar water heaters (or other freeze-resistant heat transfer fluids), as well as piping to permit draining the solar collector and piping system.
"Solar Air Heating" U.S. DOE also referred to as "Ventilation Preheating" in which solar systems use air for absorbing and transferring solar energy or heat to a building
"Solar Liquid Heating" U.S. DOE, systems using liquid (typically water) in flat plate solar collectors to collect solar energy in the form of heat for transfer into a building for space heating or hot water heating. The term "solar liquid" is used for accuracy, rather than "solar water" because the water may contain an antifreeze or other chemicals.
Field Controls provides instructions for the installation of LP and Natural Gas spill sensor switches, for example for their Gas Spillage Sensing Kit Model GSK-3, GSK-4, GSK-250M switches. Contact your heating service technician directly, or contact Field controls at fieldcontrols.com for more information. These switch models include a manual reset switch. Field Controls, Kingston NC 28504 - Tel 252-522-3031.
Tjernlund Products provides instructions for the installation and use of their controls, including the WHKE Millivolt Interlock Kit for use with their UC1 Universal Control, MAC1E or MAC4E auxiliary controls for gas fired equipment. This document also describes Tjernlund's recommended combustion air safety check which we recommended in this article. Contact Tjernlund Products at tjernlund.com or at 800-255-4208.
Bachrach Corporation, www.bachrach-training.com provides education for HVAC technicians. We found their web pages hanging during loading -01/2009. Readers may want to contact the company directly at: bacharach-inc.com or at 800-736-4666.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
"Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
Installation Guide for Residential Hydronic Heating Systems
Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
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