When you learn that a water heater is backdrafting, it may not be self-evident what that means.  I must admit that home inspectors can be guilty of using overly-technical language at times, and in the spirit of clarification, here is a description of water heater backdrafting and what it means to you.

The basics

The draft diverter (blue arrow) is the funnel shaped piece at the top of the water heater that is designed to allow room-temperature air into the exhaust venting.  If there is a problem with the exhaust venting system, the water heater exhaust gases will follow the path of least resistance, and spill out at the draft diverter.

Home inspectors will typically put their inspection mirror next to the draft diver after allowing the water heater to warm up (~ 1 minute).  Condensation on the mirror (red arrow) will confirm if the water heater is backdrafting, because the moisture is a product of combustion and is present in the water heater exhaust gases.

If the water heater is located within, or adjacent to living areas, the exhaust gases entering the building are a health hazard.  Among the products of combustion can be carbon monoxide, and exposure can lead to carbon monoxide poisoning and death.

The causes

Unfortunately, there are a myriad of reasons why the water heater exhaust venting may be backdrafting, and a licensed plumbing or HVAC contractor should be consulted to investigate the condition and make the necessary repairs.  Additionally, a water heater may backdraft at certain times and not others.  Here are just a few examples of what can cause backdrafting:

  • Venting made from outdated materials: such as “single-wall” metal venting or old masonry chimneys
  • Venting that does not have the proper slope: all components of exhaust venting must have a continuous upwards slope
  • Venting that is too long: sometimes just the length of a vent connector may be too long
  • Venting that is damaged, disconnected, or clogged: birds will sometimes make a nest if a proper cap is not installed
  • Environmental conditions: such as windy days, open windows, and even over-sized kitchen exhaust fans


Water heater backdrafting can allow harmful gases in the living areas and should be addressed by a licensed contractor as soon as possible.  Although the signs of carbon monoxide poisoning can be subtle, the condition is a life-threatening medical emergency.

JMC Inspections: Service, Experience, Professionalism since 1982

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