Why Is My Water Heater Backdrafting?

I find a fair amount of water heaters in Minneapolis and Saint Paul that backdraft.   Backdrafting happens when the exhaust gases from a fuel-burning appliance,  such as a water heater, spill in to the room rather that rising up and out of the house through the vent.  This is a potential safety hazard that should always be repaired.  

The most common question that I get when I find a backdrafting water heater is "How can this be fixed?"

There are many potentials reasons for a water heater backdrafting, but there are several basic things to check – any one of these conditions could be enough by itself to cause a water heater to backdraft.

A missing rain cap.  Not only do rain caps help to prevent rain from dumping in to the vent, but they help to downdrafts; these are wind currents that can blow exhaust gases back down the vent.  A listed cap is also required by the Minnesota Fuel Gas Code, section 503.6.6.

Missing Cap  

A backpitched vent connector.  A water heater vent connector, often called a 'flue', is required to pitch upwards towards the chimney or vent at a minimum slope of one quarter inch per foot (MFGC 503.10.8).  This is a minimum requirement.  Just meeting this minimum requirement doesn't mean everything will work the way it's supposed to.  The steeper the pitch, the better the draft.

Backpitched Vent Connector

Insufficient combustion air.  I start every home inspection the same way – I close up all of the windows and doors, and I turn on all of the 'motors' at the house that remove air.  These fans include bathroom exhaust fans, kitchen exhaust fans, and clothes dryers.  Besides giving me the opportunity to make sure the fans are properly working, I'm removing a lot of air from the house at one time, which creates a 'worst-case' scenario for the fuel-burning appliances, such as the water heater.  If there isn't a proper combustion air duct installed, the water heater vent may end up acting as an air intake.  This will definitely cause backdrafting.  

A vent connector located directly across from a fan.  Most newer furnaces have an induced draft fan that is designed to pull the exhaust gases out of the heat exchanger.   This fan ends up 'pushing' the exhaust gases up the vent, and if the water heater vent connector is located directly across from where the exhaust gases are directed, the exhaust gases from the water heater get pushed right back down.  The water heater doesn't stand a chance of drafting properly with this configuration.

The water heater isn't venting in to a chimney liner.  When a water heater vent connector enters a chimney separately from the furnace or boiler vent connector, there's a good chance that the vent just pokes in to the chimney and the exhaust gases rise up the annular space between the chimney walls and the metal vent liner.  The heat created by the water heater may not be enough to heat up this huge area.  The water heater vent needs to connect to the metal liner.

Improper Water Heater Venting

An obstructed vent.  This one is a curveball that most home inspectors won't be able to diagnose.  While replacing a water heater once, I found a dead bird sitting on top of the draft hood.  No wonder why the water heater was backdrafting.

If you have a water heater that backdrafts, have it professionally repaired.  Your home inspector may be able to give you several potential reasons for the water heater backdrafting, but an obstructed vent is always a possibility, and without taking stuff apart, there's no way to know for sure.

Reuben Saltzman, Structure Tech Home Inspections - Email - Minneapolis Home Inspections

Facebook Reuben's LinkedIn Page Follow StructureTech on Twitter ASHI Certified Home Inspector - Click To Verify Click to subscibe to Reuben's Blog

Written By

Reuben is a second-generation home inspector with a passion for his work. He grew up remodeling homes and learning about carpentry since he was old enough to hold a hammer. Reuben grew up thinking he was going to be a school teacher because he enjoyed teaching others so much. In a sense, that’s a lot of what home inspections are about, so Reuben truly does what he loves. Sharlene has worked with Structure Tech since 2000 and Reuben has been contributing to her blog since 2008.

Related Posts

Water Heater Backdrafting: What if you can’t reproduce the problem?

"No matter what conditions I produce in the house for fans, vents, open doors, hot water running, furnace on, etc. I can't produce conditions that create a backdraft. Do you...

Water Heater Backdrafting, Part 2 of 2: Why It’s Happening and How To Fix It

Last week I blogged about how to determine if a water heater is backdrafting; that's the easy part. The tough part is determining why it's happening, and fixing it.
1 Response
  1. Erica Smith

    I want to submit 1 guest article in your site: http://www.homesmsprealestateblog.com/, only if you permit me. The post contains 400 words above and totally unique as it is written by my content writers and the article will be on the theme of your site.

    Please contact me at my mail soon. In return, I will place your links in my different finance sites covering debt, mortgage, real estate, credit and insurance matters.

    Erica Smith

Subscribe to Our Newsletter for Market Updates & Mid-Century Modern Listings

Our weekly HomesMSP Update includes current local market information and a curated list of mid-century modern properties for sale, plus posts from an inspector, a lender, a stager, info about neighborhoods, life in the Twin Cities… even recipes!


Blog Categories


Sharon and John Hensrud

About Us

The HomesMSP Team is committed to meeting you where you are and listening… really listening to understand you so we can use our extensive knowledge of the market and local neighborhoods to give you personalized service.