How to Check Your Appliances for Backdrafting

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How to inspect your heating appliances for a dangerous condition known as backdrafting.

A dangerous condition known as “backdrafting” results when the flow of combustion gases in the flue of a fuel-fired appliance is reversed. The normal flow is to the outside of the home, but when backdrafting occurs the combustion byproducts are drawn into the living space. Many fuel-fired water heaters and boilers use household air for combustion and lack an induced draft, which makes them especially vulnerable to backdrafting when indoor air pressure becomes unusually low.

There are several things the homeowner can do to determine if they home is experiencing backdrafting and the way to correct the problem.

A diagram of backdrafting

Some symptoms of backdrafting include the following:

• Smoke odors in the house

• Stale, stuffy air inside the home

• If the burner flames are mostly yellow instead of clear blue on natural gas-fired equipment, such as the furnace, water heater, or other natural gas appliances.

• If the pilot light on your gas furnace, water heater, or other appliances continually goes out.

Check the following items around your fireplace:

• Smoke stains on the outside of your fireplace or below the mantle.

• If the chimney height is marginal, check the draft with a match or smoke candle.

• The presence of a smoke guard could be a sign of a past problem. A smoke guard does not shield smoke, it is a piece of metal about 4 inches deep and the width of the firebox opening that is installed across the top of the firebox to reduce the size of the opening to improve the flue to firebox ratio.

Black metal smoke guard across the top of the fireplace opening

Causes for Backdrafting

Natural gas water heaters, oil or gas-fired boilers, and furnaces are designed to exhaust the byproducts of combustion to the outdoors through a metal flue or chimney. The hot gases rise through the flue and exit the home due to their lower density. Under normal conditions the pressure inside the home is slightly higher than the outdoor pressure, this is known as positive pressure. This pressure differential can be overcome by unusually low indoor air pressure, negative pressure, caused by the exhaust of air to the outside by exhaust fans, fireplaces and clothes dryers. When this happens, combustion gases can be drawn back into the house and may aggravate respiratory conditions or be fatal due to increased levels of carbon monoxide (CO). Improperly installed flues or flue blockages can also cause backdrafting.


Use caution when working around any fuel-fired appliances. If you are unsure of your results or you can identify a potential problem, contact a licensed plumber or HVAC technician.

• Release smoke or powder into the draft diverter to see whether it gets sucked into the duct or is blown back into the room. A smoke pencil or a smoke match can be used to safely simulate smoke.

Draft Diverter – Typical to gas-fired Hot Water Heaters

Smoke test on a hot water heater flue

Barometric Damper – Installed on Horizontal flues of Hot Water Boilers

• You can hold a lighter beside the draft diverter to see whether there is sufficient draft to pull the flame in the direction of the flue.

• Combustion gases that are pulled into a house may leave a dark residue on the top of the water heater. The presence of soot is an indication of backdrafting, although its absence does not guarantee that backdrafting has not happened.

• A carbon monoxide analyzer can be used to test for backdrafting of that gas. Inspectors should be properly trained to use these before they attempt to use one during an actual inspection, primarily to avoid false negatives.

While performing the tests turn on all devices that vent air to the outdoors in order to simulate the worst-case scenario. Such devices may be dryers, or bathroom and kitchen fans.

Types of Fuel-fired Appliances

• Atmospheric Draft

Most backdrafting is the result of this type of water heater. Combustion gases rise through the ventilation duct and draws in dilution air at the draft diverter by convection. The convection of the hot gases might not be strong enough to counter the negative pressure in the home.


• Induced Draft

This system incorporates a blower that creates a mechanical draft. The potential for backdrafting is reduced because the induced draft is usually strong enough to overcome any competing pull from an indoor air-pressure drop.

• Sealed Combustion

The combustion and venting systems are completely sealed off from household air. Combustion air is drawn in from the outdoors through a PVC or metal pipe that is designed for that purpose. The potential for backdrafting is nearly eliminated because the rate of ventilation is not influenced by indoor air pressure, and the vented gas has no pathway into the home.

• Water Heater Location

The installation of fuel-fired water heaters in particular household locations can increase the chances of backdrafting. According to the 2006 edition of the International Residential Code (IRC):

Fuel-fired water heaters shall not be installed in a room used as a storage closet. Water heaters located in a bedroom or bathroom shall be installed in a sealed enclosure so that combustion air will not be taken from the living space.

Mechanical Connections

Inspect the connections at all joints on metal flues on your furnace, boiler, and hot water heater. Look for rust, soot, or other discoloration where fittings are connected to straight vent pieces.

Additional Reading

Health House



Preventing Problems with Combustion Equipment


1 comment

Francina Marie Parks
Posted on Oct 3, 2011