July, 2012
Inspection News and Views from the American Society of Home Inspectors

FVIR ((Flammable Vapor Ignition-Resistant) Water Heaters


Illustration © Carson Dunlop

Natural gas and propane (LP) fired water heaters have caused fires in some homes where flammable liquids such as gasoline have been poorly handled near the water heater. Gas spills can be ignited by the water heater burner. As a result, design improvements have been made to residential water heaters to reduce this problem. All water heaters now produced in North America are Flammable Vapor Ignition-Resistant (FVIR) types.


The ANSI Standard was introduced in July 2003. Existing water heaters that did not meet the standard do not have to be replaced, but all new heaters do have this feature.

Older water heater without flammable vapor ignition-resistance.
Photo © Carson Dunlop.

Rollout in the U.S. began July 1, 2003, with 30-, 40- and 50-gallon atmospherically vented water heaters. Phase 2 began July 1, 2004, with 30-, 40- and 50-gallon power-vented heaters, and Phase 3 covered the remaining models starting July 1, 2005.

Common gas FVIR water heaters, in use.
Photo © Carson Dunlop.

Types of water heaters included: The standard applies to 30-to-50 gallon tanks with natural gas or LP (propane) burners and firing rates up to 75,000 BTUs/hr.

The principle: Water heaters that meet this standard are designed to prevent the burner from igniting vapors from gasoline and other flammable liquids near the water heater. This is particularly applicable in garages, for example.

Water heaters in garages: Water heaters with this feature may not need to be 18 inches above the floor in the garage, depending on the authority having jurisdiction. Crash barriers still are needed if the heater is in a location where it can be hit by a car.

Illustration © Carson Dunlop.

Two key elements

1. A thermal sensor, thermal cutoff switch (TCO) or high-temperature limit switch.
This is also called a thermal release device (TRD). Some can be reset manually and others are self-resetting once normal temperatures are restored.

2. A flame arrestor screen or plate at the bottom allows combustion air to come in, but does not allow flame out through this screen should any vapors ignite. The flame-arrestor screens may be stainless steel or ceramic, for example. Some units have a removable/washable screen to prevent lint, dust and oil from clogging the flame-arrestor screen. The industry refers to this issue as LDO, which stands for Lint, Dust and Oil.

Some also have a flammable sensor that prevents ignition if flammable vapors are detected in the burner chamber. This system will burn off small amounts of flammable vapors in the burner chamber. If the concentration of flammable vapors is too high, the burner will not light.

Lower components of a FVIR water heater.
Photo © Carson Dunlop.

Common problems/conditions

1. A flame-arrestor screen obstructed by lint may starve the burner for combustion air, preventing ignition.

Cause: Clothes dryers are common source of lint. In a garage, lint may accumulate naturally and may be affected by leaf blowers, lawnmowers, etc., blowing dust and debris around in or near the garage.

The high-temperature limit TCO) shuts down because of inadequate combustion air and there is no hot water for the home.

2. If the room or closet with the water heater is under negative pressure, you may get back-drafting, which will cause the high-temperature limit switch (thermocouple) to shut down the unit.

Causes: Negative pressure may be caused by other combustion appliances or clothes dryers, for example. Inadequate draft also may be caused by vent obstructions or other venting problems.

The high-temperature limit (TCO) shuts down because of inadequate combustion air and, again, there is no hot water for the home.

3. Heaters in garages or attics with high ambient temperatures may cause the high-temperature limit (TCO) to trip. This is not a common problem.


If you can tell that the water heater is an FVIR type, you can let the client know that this is a safety enhancement. A good indicator will be a sealed burner access and a piezo-starter or igniter. Most will have some type of labeling as well. Your inspection procedure includes making sure there is hot water. If not, you don't need to troubleshoot the issue, just report it.

Garage installations: If you see a water heater in a garage that is not 18 inches above the floor, it may be all right. Look for an indication that it is an FVIR-type heater.

Lint from clothes dryers: FVIR water heaters near clothes dryers may be subject to lint problems. The area should be kept clean of lint and a disconnected dryer vent should be reconnected immediately. Lint seems to be the biggest issue with these burners.

Don't open burner: Do not try to open the burner because the gasket can be broken. This could destroy the safety feature by allowing the flame out. If you can't see the flame because of a dirty or scorched glass at the inspection port, note in your report that the flame cannot be seen.

About ASHI@Home
The ASHI@Home education system is a comprehensive distance-learning program developed with ASHI by one of the most respected names in training and professional home inspection — Carson Dunlop. This program goes far beyond an introduction to home inspection, providing career training that prepares you for success. Also, individual modules are approved for ASHI CE credits. Choose the printed version or the ASHI online learning program. Call 800-268-7070, Ext. 251 to learn more.