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How to Fix Water Heater Backdrafting? – A Step-by-Step Guide

Fact checked by Stephen Conklin

how to fix water heater backdrafting

A water heater that doesn’t vent exhaust gases outside the home is a safety hazard, so homeowners must learn how to fix water heater backdrafting issues. Backdrafting undermines a family’s health and a household’s safety, so resolving the problem is a priority.

You have two choices when addressing or repairing backdrafting issues. A DIY solution can help lower the cost to fix water heater backdraft, while a professional approach can handle more complicated venting problems.

Don’t worry if you’re confused for now. We will share these methods in this article. Let’s start.

Table of Contents

Step-by-Step to Fix Water Heater Backdrafting

What to prepare

Water heater backdrafting solutions vary in resource requirements. DIY methods only require the senses and a little elbow grease. Professional fixes might need more advanced tools and materials.

We will discuss these resources when explaining the different methods of fixing backdrafting from a hot water heater.

1. DIY Solutions

Is a backdrafting water heater dangerous? Yes, it is. And that’s why your immediate action would be to open as many windows and doors as possible to improve ventilation. Alternatively, you can turn on an exhaust fan near the water heater.

Although these fixes can prevent backdraft gases from accumulating in the home, they are temporary. You can still get the backdraft smell once you switch off the exhaust fan or shut the windows and operate the water heater. Hence, you need a more permanent solution.

Step 1: Clean the vent or chimney.


Squirrels and other critters can leave acorns, leaves, twigs, and other objects in the chimney. These objects prevent gas water heater gases from venting effectively into the open.

  1. Climb up the roof and inspect the chimney for any obstructions. You can shine a flashlight to make assessment easier.
  2. Remove debris with a chimney rod to allow flue gases to flow efficiently from the water heater and prevent backdrafting.
  3. Install a chimney cap if it doesn’t have one.

Step 2: Adjust the vent connector’s pitch.


Most furnace backdrafting is due to an improperly installed vent connector. The wrong pitch can prevent flue gases from moving forward and upward. Instead, exhaust returns to the water heater space.

  1. Read the manufacturer’s recommended vent connector pitch or angle. Most observe a quarter of an inch of slope per foot as the minimum.
  2. Check the vent connector for dips or sags.
  3. The exhaust pipe should not have a sharp 90-degree turn next to the water heater’s draft hood.
  4. Remove the vent connector and adjust its slope accordingly.

2. Professional Solutions

Although a professional water heater backdraft repair cost can be prohibitive, the service offers a more permanent and effective solution. Licensed HVAC technicians can also diagnose other problems, saving you from future headaches.

Professionals can address water heater backdrafting issues with the following solutions.

Step 1: Install a backdraft preventer.


Professionals call it a backdraft damper, an actuator-less system that ensures one-way airflow. It allows water heater flue gases to move in one direction (toward the outdoors). It prevents them from moving back inside the house.

Although you can install a chimney backdraft preventer DIY-style, we recommend a professional because the process is a bit technical. Nevertheless, the steps are as follows.

  1. Determine the ideal damper location inside the water heater duct work and mark it with a permanent marker.
  2. Use a torpedo level to ensure a straight line on the duct’s sides.
  3. Align the damper frame with the line markings inside the water heater vent duct, ensuring the damper blades face the airflow.
  4. Secure the damper frame with self-tapping screws.
  5. Apply caulk on the space between the duct and damper frame, and leave for 24 hours.

Here’s a video from Fresh Air Manufacturing Co (FAMCO) showing how to install a damper to prevent water heater backdraft.

Step 2: Replace the vent connector with a Type-B variant.

One potential cause of water heater backdrafting is an extended horizontal vent. Most water heater vents are atmospheric types, relying on the principles of physics to move gases upward and outward. The longer the run, the more likely it is backdrafting.

You can replace a standard vent connector with a Type-B unit. However, the process could be complicated for some folks. Hence, we recommend professionals for this job.


That said, if you want to install the vent yourself, here are the steps:

  1. Plan the Type B vent installation, including its location, clearance, combustion air, slope, and termination area.
  2. Consult local building codes and apply for the necessary permits.
  3. Determine the vent connector’s penetration point through the wall or ceiling.
  4. Create a stable and sturdy ceiling support or wall thimble (or sleeve extension for walls thicker than 6 inches) for the Type B vent.
  5. Assemble the vent pipe sections and secure the connectors, tees, wyes, and elbows.
  6. Add enclosures to any vent section passing through a living space.
  7. Cut a hole in the roof for Type B vent pipe termination.

You can check out Metal-Fab, Inc.’s B-vent installation video for more information about this method.

Step 3: Diagnose Insufficient Combustion Air

If opening the windows or turning on a nearby exhaust fan solves the backdrafting issue, there’s a good chance the problem is due to insufficient combustion air in the water heater space.


Unfortunately, ordinary folks don’t know how to diagnose this concern, let alone fix it. Professionals assess and diagnose the problem systematically.

  • Technicians assess the combustion air duct intake’s integrity, including signs of obstructions.
  • Install a house-specific makeup air duct using local mechanical codes to determine the correct volume of makeup air required for the home.

Causes and Common Signs of Water Heater Backdrafting


Although we already covered some common causes of water heater backdrafting, let’s go over them again.

  • Incorrect vent installation (i.e., downward or horizontal slope, tight turns, long vents, dips, sags, and sudden diameter reduction)
  • Vent obstructions
  • Insufficient combustion air

You can observe the following signs.

  • Warm air and high humidity around the heater’s draft hood
  • Misting or fogging on nearby mirrors and similar items
  • Soot in the draft hood
  • Deformed or melted plastic cap at the heater’s top section
  • Water stains on the heater’s top section
  • Improper vent termination can produce cold air coming down water heater vent

Preventing Water Heater Backdrafting


It’s worth noting that backdrafting is more common in atmospheric-vented water heaters. Therefore, upgrading your device to a power vent will likely solve the problem. However, note that such a system requires regular maintenance and checkup. Otherwise, if its fan fails, there’s a high chance of carbon monoxide poisoning.

Aside from upgrading to a power-vent water heater, here are a few tips to prevent backdrafting:

  • Make sure that the venting system is of the correct size for the water heater.
  • The room with the water heater must always have enough ventilation and no obstruction. Without enough airflow, backdrafting is bound to happen.
  • If there are other electronic appliances in the room, make sure they don’t create negative air pressure that may tamper with the performance of the water heater. Range hoods, furnaces, bath fans, and cloth dryers are some common household appliances that can cause negative air pressure.


Learning how to fix water heater backdrafting can safeguard your family’s health and protect your home against harmful flue gases. DIY solutions exist for budget-conscious families and less serious gas water heater issues (i.e., blocked vents or chimneys and incorrect vent pitch).

Meanwhile, professional solutions focus more on deeply-seated issues (i.e., insufficient make up or combustion air and faulty venting system installation). The cost could be a concern, but the results are worth it.

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