Mobile home furnace (C) Daniel FriedmanUnvented Gas Heater Safety Warnings
CO, CO2, NO2 & Moisture Hazards in Manufactured & Mobile Homes

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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.

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Safety Concerns with Un-Vented Gas-Fired Heaters in Mobile Homes & Manufactured Homes

Mobile home furnace (C) Daniel FriedmanWatch 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?

This question and reply were posted originally at COMBUSTION AIR SAFETY in MOBILE HOMES

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 original source: Ventfree,

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


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