InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
Use of un-vented gas heaters in homes:
This article explains possible hazards when using an LP gas or natural gas fueled un-vented heater inside a building and cites safety standards, codes, and research on the dangers of un-vented gas heaters used indoors.
This article series explains how to recognize and fix combustion air defects on heating appliances such as boilers, furnaces, and water heaters. These articles answer most questions about central hot water heating system troubleshooting, inspection, diagnosis, and repairs.
Our photo at page top shows a mobile home oil fired furnace installed in a closet with an airtight door; there was no outside combustion air supply. The heating system could not work properly nor safely in this home.
Safety Concerns with Un-Vented Gas-Fired Heaters in Mobile Homes & Manufactured Homes
Watch out: inadequate combustion air supply to a gas burner (and less often to an oil burner) is very dangerous and can produce potentially fatal carbon monoxide. If you suspect unsafe heating system operation or a carbon monoxide problem be sure everyone leaves the building immediately and then call your local fire department for assistance.
Our photos show an un-vented gas heater used as the primary heat source in a mobile home.
Question: is it safe to use an un-vented heater in a mobile home?
2017/06/05 John said:
Can I install a 36000 non vented heater in a Mobil home?
Reply: not without safety concerns and IAQ questions
You don't specify the kind of heater, its fuel, nor its manufacturer and brand, nor where the heater is to be installed, so it's tough for me to offer exact advice. You also didn't tell me where you live. Your state, province, city or other local code probably regulates your use of an un-vented heater in your home and may or may not outright prohibit them.
Even where standards such as ANSI Z21 designed to improve the safe use of gas heaters apply, you could not use a 36,000 non-vented heater in a mobile home bedroom, so just where the heater is to be installed and how it's to be used are key considerations.
Do Not use an Un-Vented Gas Heater as a Primary Heat Source
Quoting a U.S. ASHRAE position paper:
Unvented combustion appliances should never be used as the primary/sole source of heating. - ASHRAE position paper cited below
There is a risk of lack of oxygen or of fatal carbon monoxide poisoning (CO) when un-vented heaters are used in homes, particularly when the total space is small. The risks are particularly great with older un-vented heating equipment that lacks a built-in oxygen sensor designed to turn off the appliance if the oxygen level drops to an unsafe level. There are also indoor air quality concerns with carbon dioxide (CO2), Nitrous oxide (NO2) and high levels of indoor moisture or water vapor produced by un-vented gas heaters.
Vent-Free Heater Industry Position on Safety of Un-Vented Gas Heaters
The vent-free heating industry defends the use of un-vented gas fired heating appliances made since 1980 and asserts a less frightening position:
Every vent-free gas product marketed in the U.S. today, regardless of the size, shape, appearance, heat output, or price, has been approved by a recognized listing agency. This involves testing to a rigorous safety standard, in this case ANSI Z21.11.2. But what does that mean to the average consumer?
The American National Standards Institute (ANSI) oversees a process for the development and constant upgrades of safety standards for virtually every class of home appliance available in the marketplace. Every manufacturer must submit their designs for testing to the applicable standard by an independent laboratory. ... In 1980, the ANSI standard for these products was revised to require each unit to be equipped with an oxygen detection safety sensor (ODS). The ODS is remarkably equivalent in function/reliability to what a circuit breaker is to electrical current.
The ODS automatically shuts off the unit in the unlikely situation that carbon monoxide is elevating and there is oxygen depleting in the vicinity of the unit (regardless of the CO source). - source: Vent-Free Gas Products Alliance [an industry association] c/o AHRI 2111 Wilson Boulevard, Suite 500 Arlington, VA 22201-3001 www.ventfree.org original source: Ventfree, www.ventfree.org/content/view/42/18/
You will see in that industry-article that the vent-free heaters described are not intended for use as a primary heat source for a mobile home or any other home. Quoting further:
Vent-free gas products provide warmth to a chilly area of the home, enable the family to focus heat in gathering areas while reducing the central heating cost, can convert a wood burning fireplace to a convenient attractive heating site, or can update a home by adding a fully dressed out space heating fireplace. In the event of power outage, there will always be heat available from the non-electrically dependent vent-free gas appliance. - Ventfree op.cit.
Experts Warn about Oxygen Depletion Sensor Reliability for Un-Vented Gas Heaters
Really? Experts do not completely agree that oxygen depletion sensors provide adequate safety for un-vented gas fired heaters.
What about carbon monoxide risks? CO, a colorless, odorless, tasteless, non-irritating poison, is highly toxic and can cause death or permanent brain and organ damage. CO poisons more people than all other poisons combined. When properly maintained and adjusted, gas heaters produce low amounts of carbon monoxide.
One cause of carbon monoxide poisoning from unvented heaters– incomplete combustion caused by lack of air–has been virtually eliminated in newer heaters by use of Oxygen Depletion Sensors (ODS).
Unfortunately, the ODS does not respond to incomplete combustion caused by improper gas pressure; dust, dirt, or rust on the burner; incorrect placement of artificial logs in a gas fireplace; or disruption of the burner by air currents. CO poisoning from unvented heaters remains a concern. … Can the health hazards of an unvented heater be reduced? The most effective method to reduce the hazards is to discontinue use of the unvented heater by switching to vented gas or electric appliances. - (Greiner T.H. 1998)
The article continues to make suggestions for reducing the risk if you are nevertheless going to use an un-vented gas heater. The article also cites risks from CO2, nitrogen dioxide (NO2), and water vapor.
Field Experience Found Unsafe CO2 Emissions from Heaters that Include an Oxygen Depletion Sensor
Perfect combustion of propane or natural gas would produce just CO2 and water vapor. Is combustion reliably perfect? No.
I have tested four LP gas fueled fireplace heaters in a home in Mexico over the last ten years. All of these heaters carried a U.S. certification label. These were all appliances not designed for use as a primary home heating source. All of them included an oxygen sensor, and all of them, left operating long enough in a closed room or set of rooms, ultimately turned "off" automatically.
However in every instance, hours before the Oxygen sensor turned off the heater, in 100% of the cases, a properly located, installed, and tested carbon monoxide detector (CO) alarm, sounded to indicate unsafe levels of carbon monoxide in the home. A response of home occupants to the CO alarm sound can include simply turning off or removing the battery from the CO alarm, in my OPINION, then risking death of the building's occupants.
Building Code Limitations on Vent Free Gas Heater Use
Here is an example of an LP-gas room heater installation code from a U.S. state (Alabama) discussing use of vent-free gas heaters in a mobile home:
§ 9-17-122. Installation of heaters—used manufactured homes.
The following LP-Gas room heaters may be installed in a used manufactured home as follows:
LP-Gas listed vented room heaters equipped with a 100 percent safety pilot and a vent spill switch or LP-Gas listed unvented room heaters equipped with factory equipped oxygen depletion safety shut-off systems may be installed in a used stationary manufactured home (mobile home) but not in sleeping quarters or bathrooms in the manufactured home (mobile home) when the installation of the heater is not prohibited by the appliance manufacturer and when the input rating of the room heater does not exceed 20 BTU per hour per cubic foot of space and combustion and ventilation air is provided as specified in Section 5.3 of the National Fuel Gas Code, NFPA 54.
All room 27 heaters installed pursuant to this section shall be securely anchored to the wall or floor. (Acts 1994, No. 94-706, p. 1369, § 3.) - Alabama Room Heating Law, ret. 2017/06/05
The hazards of un-vented heaters, particularly in mobile homes, has been known for a long time. In addition to the risk of fatal carbon monoxide (CO) poisoning there are indoor air quality issues including excess moisture and carbon dioxide CO2 and other combustion products.
Manufactured Home Warning on Use of Unvented Heaters
Here is a quote from the Manufactured Housing Research Alliance
Do not use unvented propane, kerosene, or other unvented combustion heaters. About a gallon of water vapor is released into the air for every gallon of fuel consumed. This is a significant source of water vapor that can quickly cause damage.
Some unvented heaters can also increase pollutant levels and contribute to health problems. - "MOISTURE PROBLEMS in MANUFACTURED HOMES", Manufactured Housing Research Alliance
Supporting research on safety of un-vented gas heaters
Alabama Room Heating Law, ret. 2017/06/05 original source: www.alabamapropane.com/uploadedFiles/State/Alabama/Dealer-Supplier/Alabama%20Room%20Heater%20Law.pdf
ANSI Z21.11.2 - retrieved 2017/06/05 original source: http://shop.csa.ca/en/canada/gas-fired-domestic-and-commercial-heating-equipment-and-air-conditioning/ansi-z21112-2016/invt/27017312016 Excerpt:
This Standard applies to newly produced gas-fired unvented room heaters and gas-fired unvented decorative room heaters for connection to the house fuel supply system (see Clause 3), hereinafter referred to as room heaters or appliances, constructed entirely of new, unused parts and materials, having input ratings up to and including 40,000 Btu/hr (11,723 W) except:
a) unvented room heaters suitable for installation in bedrooms have input ratings of 10,000 Btu/hr (2,931 W) or less; and
b) unvented room heaters suitable for installation in bathrooms have input ratings of 6000 Btu/hr (1,758 W) or less.
Greiner T.H., Ph.D., P.E., “Carbon Monoxide Poisoning: Unvented Gas Space Heating Appliances (AEN-204)”, Iowa State University, College of Engineering, College of Agriculture and life Sciences, File: sep98\AEN-204
"Avoid Unvented Gas Heaters", Green Building Advisor
CHIMNNEYS and FIREPLACES CODE [PDF] IRC, International Residential Code, (2003)
IRC, International Residential Code, Excerpt: R1004.4 Unvented gas log heaters. An unvented gas log heater shall not be installed in a factory-built fireplace unless the fireplace system has been specifically tested, listed and labeled for such use in
accordance with UL 127.
Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
soot problems after oil burner puffback
(Jan 6, 2015) mahgy said:
we had a furnace blow back a while ago, still noticing soot/black build up coming out of the heating vents on the walls.. this is a rental property. is this dangerous to our health? landlord does not seem concerned
No one can assess exposure levels of soot nor health risks accurately from a simple e-text, but in general, breathing soot is not healthy. If the heating system is no longer producing soot (which is itself unsafe) then the duct system may need cleaning.
Question: visible clues of adequate combustion air
(June 25, 2015) Anonymous said:
What if there is no visible combustion AIR
Anon you'd need to be more specific. Most building codes and standards discussing combustion air supply include a square-foot area size specification. For example if a heater of input BTUh amount X is in an open space of Z square feet then the area is deemed capable of providing adequate combustion air. Those guidelines presume a standard minimum rate of building air leakage or air exchange rate.
In the article above you can read details at the paragraph titled
Cubic Feet of Room Space as a Measure of Adequacy of Combustion Air: Total Input BTUH / 1000 x 50
Question: article correction
(Jan 14, 2016) Joe said:
With your cubic ft volume for adequate combustion air equation, you use 140 MBH for the total input BTU of a 180 MBH boiler and 40 MBH DHW. Should you be using 220 MBH total for that?
hanks Joe, we've corrected the math in the article above. I appreciate your careful reading. We can use all the editing help we can get.
Question: combustion air source for water heater in a trailer
(Mar 19, 2016) Anonymous said:
does combustion air for mobile home water heaters enter from under the trailer
In some installations it might;
Watch Out: if your water heater is gas or oil fired and lacks adequate combustion air the unit is unsafe and could kill the home's occupants.
21 June 2016 keith preddie said:
can anyone go through the control sequence between louvered openings and the gas fired equipment?
Questions & answers or comments about combustion air requirements and safety hazards for oil and gas fired heating appliances
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Handbook - Fundamentals, 1993, Chapter 15, page 15.9 Air For Combustion.
ASME CSD-1- Controls and Safety Devices for Automatically Fired Boilers, 1992 with addendum 1a 1993. section CG-260 Combustion Air.
BOCA - National Mechanical Code, 1990, article 10, Combustion Air.
NFPA 31 - Installation of Oil Burning Equipment, 1992, section 1-5 Air for Combustion and Ventilation.
NFPA 54: National Fuel Gas Code (2015), addresses heating appliance combstion air ventilation specifications. NFPA 54, ANSI Z223.1 provides minimum safety requirements for the design and installation of fuel gas piping systems in homes and other buildings.
NFPA 85: Boiler and Combustion Systems Hazards Code: NFPA 85 contributes to operating safety and prevents explosions and implosions in boilers with greater than 12.5 MMBTUH, pulverized fuel systems, and heat recovery steam generators.
NFPA 87: Recommended Practice for Fluid Heaters, This recommended practice provides safety guidance for fluid heaters and related equipment to minimize fire and explosion hazards that can endanger the fluid heater, the building, or personnel
SBCCI- Standard Mechanical Code, 1991, section 305 Combustion and Ventilation Air.
Axtman, William H., "Combustion Air Requirements: The Forgotten Element in Boiler Rooms", Grayh Gull Associates, retired executive director of the American Boiler Manufacturers Association, National Board Technical Series, Winter 1995 National Board Bulletin. Retrieved 26 January 2015, original source: http://www.nationalboard.org/index.aspx?pageID=164&ID=191
Excerpt: Several safety codes such as the National Fire Protection Association's standards, NFPA 54 - National Fuel Gas code, NFPA 31 - Installation of Oil Burning Equipment, and the American Society of Mechanical Engineers (ASME) CSD-l Controls and Safety Devices for Automatically Fired Boilers have sections covering the requirements for combustion air intakes. In addition, building codes such as the Building Officials and Code Administrators International (BOCA) National Mechanical Code and the Standard Mechanical Code published by the Southern Building Code Congress International (SBCCI) have air requirements for combustion. Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.
Nussbaumer, Thomas. "Combustion and co-combustion of biomass: fundamentals, technologies, and primary measures for emission reduction." Energy & fuels 17, no. 6 (2003): 1510-1521.
Utiskul, Yunyong P., Wu, Neil P., Biteau, Hubert, "Combstion Air Requirements for Power Burner Appliances, Final Report", The Fire Protection Research Foundation, The Fire Protection Research Foundation
One Batterymarch Park
Quincy, MA, USA 02169-7471
http://www.nfpa.org/foundation, retrieved 25 Jan 2015, original source: http://www.nfpa.org/~/media/files/research/research%20foundation/rfcombustionairrequirements.ashx, Executive Summary: Gas-fired appliances require combustion air to properly function. Adequate air is necessary for
supporting combustion of the appliance burner, dilution of flue gas, and proper ventilation of the
space where the appliance is installed. Current standards and model codes outline requirements
and methods to supply the combustion air. One method is to provide outdoor combustion air
through openings or air ducts communicating with the outdoors through natural ventilation.
Most standards require the outdoor opening(s) be prescriptively sized based on the total energy
input rating of the appliance. However, in the United States, the current standards contain no
separate provisions to address the opening size supplying the combustion air for
commercial/industrial sized appliances, which typically have a high energy input rating of
greater than 300 kBtu/hr and are equipped with a power burner unit. As a result, the opening(s)
can be excessively sized when determined based on the current standards.
This research project establishes minimum outdoor combustion air requirements specific to gasfired
appliances utilizing power burners with input ratings no greater than 12.5 MBtu/hr. A
review of the available literature, engineering guidelines, and current standards and model codes
related to combustion air requirements was performed. This report provides an understanding
of the technical basis for the existing provisions for combustion air and their applicability to
power burner appliances. This report also identifies the range of energy input ratings for gasfired
appliances equipped with power burners, and compares the combustion air requirements
specified by a range of appliance manufacturers. A theoretical model for air flow through
openings was developed and the modeling results, together with the data gathered through the
literature review, were used to provide a baseline to establish the theorized combustion air
requirements suitable for power burner appliances.
 Jeff Wilcox, "Evaluating Duct Work, How to Evaluate Furnace Duct Work & Cure Short Cycling or Inadequate Ductwork Problems" Vermont Department for Children and Families, Office of Economic Opportunity, - dcf.vermont.gov/sites/dcf/files/pdf/oeo/WAPManual/AppendixI.pdf retrieved 12/5/2013. Geoff Wilcox
Vermont Office of Economic Opportunity
Weatherization Assistance Program
Reference Material for the original article came from:
1. Saturn Mechanical Systems Field Guide
2. Bacharach (Rudy Leatherman)
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.
R.E. Prescott Company, a producer of other REPCO™ residential and industrial products as well as a provider of design-build engineering services is at 10 Railroad Avenue, Exeter, NH 03833. Tel: 603-722-04321 or 888-786-7482. Trish O'Keefe from Prescott informed us (10/7/2009) that their company had nothing to do with the failed Repco heating boilers discussed at InspectAPedia.com. She wrote:
Our company manufactures & distributes residential water treatment equipment, including our Repco line of conditioners. We have a plumbing & heating supply dept as well. We are mistaken for the Repco Boiler company on a regular basis, most frequently in Oct. and Nov.
Any information you could give me on the other Repco company would be appreciated. We've assumed it is no longer in business, and we'd like to know that for sure. Many callers want to know where to get replacement parts and I would be glad to direct them if I knew...plus there's always a chance they might be interested in a Crown Boiler instead
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
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.)
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: firstname.lastname@example.org. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones