Reuben's Home Inspection Blog

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

October 1st, 2013 | 3 comments

If a water heater backdrafts, it means that potentially hazardous exhaust gases are coming back into the home.  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.  There are many potential causes for a water heater to backdraft, so I’ll cover as many as I can think of.  In many cases, it’s a combination of conditions.

The one guaranteed fix for all of the issues that I’m going to discuss is to install a new powervent water heater that typically vents through the side of the house.  Powervent water heaters won’t backdraft, but they cost more money, they’re more work to install, there’s more that can go wrong with them, and they won’t operate if the power goes out.

Vent Connector Problems

A vent connector is the thing that takes the exhaust gases from the top of the water heater (draft hood) to the vent.  If a vent connector isn’t properly installed, the potential for the water heater to backdraft increases.

One of the most common installation defects with a vent connector is improper pitch; the vent connector need to pitch upwards towards the vent with a minimum slope of 1/4″ per foot, and there should be no sags or dips.

Backpitched Vent Connector

Insufficient rise in the vent connector can contribute to backdrafting.  While the code requires a minimum pitch of 1/4″ per foot, if there is a quick 90 degree turn right at the top of the draft hood and the vent connector only has the minimum pitch, there’s a good chance that it won’t draft properly.  There needs to be more of a rise created in the vent.

Buy a short water heater when I failMost atmospherically vented water heaters come in two shapes – tall and short.  Short water heaters are a couple of inches wider and nearly a foot shorter than tall water heaters.  Replacing a ‘tall’ water heater with a ‘short’ gives the vent connector almost an extra foot of rise, which will increase the potential for the water heater to draft properly.

Another common defect with a vent connector is too long of a horizontal run.

Long vent connector

The longer a vent connector runs horizontally, the greater the potential that it won’t draft properly.  Section 503.10.9 of the Minnesota Fuel Gas Code says that “a vent connector shall be as short as practical and the appliance located as close as practical to the chimney or vent“.  The photo above shows a water heater installed much farther away from the chimney opening than it should be.  That water heater could certainly be moved closer to the chimney.

Another thing that can be done to help with a long horizontal run is to replace a single-wall vent connector with a Type B vent connector.  This will help the vent to heat up faster, which will help with draft.

Mid-efficiency furnaces have an induced draft fan that is designed to pull the exhaust gases out of the heat exchanger for the furnace.   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.  Most water heaters will not draft properly with this configuration.

Induced draft fan on furnace across from water heater

One way to fix this is to re-arrange the common vent so the water heater vent connector is offset from the furnace.

Vent / Chimney Problems

Here’s a common scenario for water heater draft problems: an old 80% efficient furnace is replaced with a new, high efficiency furnace that vents through the side of the house, leaving the water heater vent an orphan.  The illustration below, provided by the Minnesota Department of Commerce, Division of Energy Resources, shows what this looks like.

Orphaned Water heater w text

The relatively small burner on the water heater may not generate enough heat to warm up the chimney or vent, which can lead to backdrafting.  In many cases, a new vent liner will be installed to allow the water heater to vent into a smaller space.

The text in the image above also mentions adding a power vent to the existing water heater.  Tjernlund of White Bear Lake, MN makes such devices; one is a powervent conversion kit for an existing water heater, and another is a chimney stack assist kit for an existing water heater.

An obstructed chimney or vent will almost certainly cause backdrafting. I’ve heard stories of leaves, acorns, and squirrels in chimneys and vents; I once even found a dead bird blocking the draft hood at a water heater.  To help prevent the possibility of that happening, a listed cap should be installed at the vent terminal at the exterior.

Missing Cap

A proper cap will help to prevent debris from falling into the vent, keep rain out,  prevent downdrafts caused by strong outdoor winds, and is required by the Minnesota Fuel Gas Code (section 503.6.6).

Insufficient Combustion or Makeup Air

For a water heater to draft properly, it needs to have plenty of available air.  Some water heaters will be right on the brink of backdrafting even with a proper installation, and all it takes is a single bathroom exhaust fan to pull enough air out of the house to make the water heater backdraft.  Even in a home with a water heater that drafts very well, just throw in a big 600 cfm kitchen hood fan and it’s almost a guarantee that the water heater will backdraft with the fan running.

Insufficient combustion / makeup air is a condition that many plumbers and HVAC folks seem to have a tough time diagnosing.  They get called out to correct a backdrafting water heater and they head right down to the basement to diagnose the problem. They see that the venting is all installed to code, there’s a nice rise in the vent connector before the chimney, there’s a combustion air duct installed, and there are no signs of previous backdrafting. They get the water heater to fire up, and it drafts perfectly for them, so they proclaim there are no problems.

Hmm… did they miss any steps in testing for draft?  ‘Fraid so.

According to Steve Schirber of Cocoon, LLC, BPI standards for testing water heater draft are extremely similar to the steps that I listed in my blog post last week.  I won’t go into all of them, but they involve closing all windows and doors, turning on exhaust fans, and only running the water heater.

If a water heater backdrafts under a worst-case scenario test like this, open a few windows or a door to see what happens.  If the water heater starts drafting properly right after doing that, it’s obviously a problem with insufficient combustion air or makeup air.

The fix for this is to hire a savvy plumber or HVAC contractor with a holistic understanding of houses. If there is already a combustion air duct installed, they’ll make sure the intake is clean at the exterior and the duct is not obstructed.  If that all looks good, they might install a makeup air duct; it’s pretty much identical to a combustion air duct.  They might just guess at the size needed and do a performance test when they’re done, or they might use Minnesota Mechanical Code Section 501.3 to find a formula for the amount of makeup air needed.  Some day I might follow up with a blog post attempting to explain how to use Section 501.3 to figure out a formula for makeup air.  Post update: Here’s a blog post attempting to explain Section 501.3: How to determine if makeup air is required.

That’s everything I can think of.

Related Posts:

Author: Reuben Saltzman, Structure Tech Home Inspections


Water Heater Backdrafting, Part 1 of 2: Why it Matters and What to Look For

September 24th, 2013 | 13 comments

Backdrafting at a water heater is a common defect identified during home inspections, and it’s almost always a condition that the homeowner or occupant is not aware of.  This happens when the exhaust gases from an atmospherically vented water heater spill out into the room, rather than safely leaving the house through the vent.  Exhaust gases contain carbon monoxide and high levels of moisture, so this is always a condition that should be corrected.

An atmospherically vented water heater is a type with a draft hood, which relies on gravity to get the exhaust gases out of the home.  Exhaust gas is hot and therefore lighter than the surrounding air, which is what allows it to rise up the vent and out of the house.  These are very simple devices that require no electricity to operate, however, this isn’t a very reliable draft method.  Today I’m going to share some clues that could indicate a problem with a water heater drafting properly.

Melted plastic at the draft hood.

Melted plastic at draft hood

When the plastic at the top of a water heater is melted, it’s pretty much a guarantee that the water heater has backdrafted.  I’ve heard some folks claim that the plastic melted when the water heater was being installed, because someone got too close to the plastic with a torch while soldering the pipes.  That’s possible if the solder joints are very close to the water heater tank, but very highly unlikely, especially when the part that’s most melted faces the draft hood.

Corrosion at the top of the tank

Corrosion at top of water heater tank

Backdrafting exhaust gases can condense at the top of the water heater tank.  Over time, this corrosive condensate will begin to corrode the top of the tank.  A leaking shutoff valve can also cause corrosion at the top of the tank, but that type of corrosion is easily distinguishable back corrosion caused by backdrafting because the worst area will be located directly below a valve.

Moisture at the top of the tank

Backdrafting Water Heater

If you can see condensation at the top of a water heater tank, you’re basically watching the water heater backdraft.  The photo below shows moisture, rust, and melted plastic at the top of the water heater. Click the photo to see a large version, showing this water heater backdrafting like crazy.

Excessive condensate on the cold water pipes in the water heater room

Condensate at water pipes

After a water heater has backdrafted for a while inside of a utility room, there will probably be heavy condensation on the cold water pipes.  It’s typical to see condensation on cold water pipes during the summer, but a backdrafting water heater will make it dramatically worse.

A marginal or improper installation of the vent connector

Improper vent connector installation

The vent connector at a water heater needs to pitch upwards towards the vent, and should ideally not have any quick turns, especially right at the draft hood.

If you find these types of clues, there might be a problem with the water heater drafting properly.

To test for proper water heater draft, I do the following:

  • Turn down the heat at the furnace or boiler if there is a shared vent
  • Close all windows and doors
  • Close any fireplace dampers, if present
  • Turn on any kitchen and bathroom exhaust fans
  • Turn on the clothes dryer
  • Run hot water at a bath tub or laundry sink long enough to get the water heater to kick on.

This creates a worst-case scenario for the water heater, but it should still draft properly under these conditions.  Before testing the water heater draft, I let the burner run for a little bit; sometimes a water heater will initially backdraft for a minute or two, but will start drafting properly once the vent connector warms up enough.

I have several different tools that I can use to check for proper draft at a water heater if I’m feeling fancy, but there’s really no need.  To check for proper draft, I simply cup my hands around the draft hood without actually touching it.

Testing draft at water heater

If the water heater is backdrafting, I’ll feel warm, moist air coming out of the draft hood.  If I want visual evidence to put in an inspection report, I can hold a mirror up the draft hood and take a picture of the mirror getting fogged up.  The photo below shows a mirror that happened to be sitting on top of a backdrafting water heater during an inspection; I had to take a photo of that, right?

Mirror at backdrafting water heater

Another easy way to get visual evidence of a backdrafting water heater is to hold a camera close enough to the draft hood to allow the lens to get fogged up from the backdrafting flue gases.  Powerful stuff, huh?

Backdrafting water heater with lens fogged


If there happens to be cobwebs hanging down from the draft hood, they can give a nice visual indication of backdrafting. The short video clip below shows a water heater backdrafting for about twenty seconds before the vent warms up.  My camera lens even fogs over for a second when I get it too close to the draft hood.  Don’t mind the friendly cats making noise in the background.

Next week I’ll have a follow-up post on the causes of water heater backdrafting and what can be done about it.

Post update: How to Fix Water Heater Backdrafting

Author: Reuben Saltzman, Structure Tech Home Inspections


Why Is My Water Heater Backdrafting?

August 31st, 2010 | 3 comments

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 prevent 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

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