Backdrafting is a potentially fatal health hazard connected with the presence of heating appliances that use atmospheric draft to exhaust toxic combustion gasses to the home exterior. These heating devices are identified by the presence of a draft hood (also called a “draft diverter”). These devices depend on thermal buoyancy (hot air rises) to propel hot exhaust gases up and out of the appliance combustion exhaust vent to the home exterior.

Backdrafting happens when combustion exhaust gasses reverse flow and are pulled out of the combustion exhaust vent and back into the living space, typically by low air pressure in the home interior.

Low air pressure is created when exhaust fans in a laundry room, kitchen, or bathroom (or some combination of these acting together) expel indoor air from a tightly-built home. If a separate source of makeup air is not supplied, a heating appliance combustion exhaust vent may supply makeup air containing the toxic products of combustion and significant amounts of moisture.

Watch for:

Warped or melted escutcheon plates

Warped or melted plastic escutcheon plates located where the water pipes penetrate the top of the water heater are a strong sign of backdrafting.

Tank top corrosion

Backdrafting exhaust gases can condense at the top of the water heater tank.  Over time, the corrosive condensate can corrode the top of the tank.  Corrosion of this type will be located near the draft hood.

Tank top moisture

Moisture visible on the top of the water heater- unless it is the result of a plumbing fixture leak- will be condensate and a sign of active or very recent backdrafting.

Condensation on cold water pipes

Heavy, extended periods of backdrafting will result in the accumulation of  condensation on cold water supply pipes.  Condensation visible on cold water pipes during the summer is not unusual, but a backdrafting water heater will cause heavier accumulations than environmental conditions alone.

Poor vent connector installation

Vent connectors should slope upwards away from a heating appliance at least ¼“ per foot, and should ideally not have small-radius bends, especially right at the draft hood.

When two heating appliances (say, a furnace and a water heater) vent connectors should not be installed directly opposite each other. The furnace vent connector should connect beneath the water heater vent connector.


Testing procedure when backdrafting is suspected:

  • Turn down the heat at the furnace or boiler if they are a share a common vent with the water heater;
  • Close all windows and doors;
  • Close any fireplace damper;
  • Turn on any kitchen, laundry room and bathroom exhaust fans;
  • Turn on the clothes dryer;
  • Run hot water at a bathtub or laundry sink long enough to get the water heater to kick on.

This replicates a worst-case scenario, but a water heater should still draft properly under these conditions.  Before testing the water heater draft, allow the burner to run for a little while; sometimes a water heater will initially backdraft for a short time, but will start drafting properly once the vent connector has warmed.

Visual confirmation for a photo can be had by:

  1. Holding a lighter to the draft hood and photographing the flame slant, also by
  2. Photographing the vent temperature as measured by a digital thermometer.