Backdrafting Water Heaters

Backdrafting is very common in new homes, especially western Shawnee and South JOCO. The tighter the house the more susceptible it is to backdrafting. The main issue is from an imbalanced HVAC system which creates a negative pressure in the entire basement of the house.

Take a minute to think about how a gas water heater works, the only thing keeping the exhaust out of your home is the atmospheric pressure of heat rising through the flue.

Most gas water heaters fall under the category of Atmospheric venting appliances. This means they operate without a fan to push the exhaust outside of your home. They work based on the principle that heat rises. However todays homes are more efficient than ever before and various environments or conditions can cause your water heater to vent inside your home.

Negative Pressure

It is far more likely that a water heater will backdraft in the Summer months than it is in the Winter. We tend to redirect all of the conditioned air to the upper portion of the house. Many systems will have dampers that will redirect more air from the basement to the upper portion of the house.

If you redirect all of the conditioned air to the upper portion of the house but are pulling most of the intake air from the basement a negative pressure environment will occur. This can be identified by doors that pull shut on their own and the feeling of air rushing down to the basement.

If you do notice that air seems to be fighting to get down to the basement there are a few steps you can take to resolve the issue. The first is to identify the cold air return duct work and ensure there are no massive leaks or gaps in the system.

Next, if your basement if finished it may have an additional cold air return added by the contractor. These returns are often right off the blower and pull way more air than they should. This will result in the upstairs cold air register to pull less air than it should for an efficient system.

There are a couple of dummy fixes to resolve the issue of backdrafting. Adding combustion passage louvers from not only the basement to mechanical room but also from the basement to the first floor will usually prevent the backdraft. It can also help to keep a supply air register opened in the basement or mechanical room to help balance the system.

Another important step is to properly size and configure the venting, nearly all of JOCO is not sized or configured in accordance with Fuel Gas chart sizing requirements. Below are a few examples of proper vent configuration.


Example #1 – Improper Sized Vent

This picture represents one of the most common venting issues we run into. This was an 18 year old house that was done incorrectly from the time of new construction. The water heater installed was a 40,000 BTU water heater with a 3″ single wall vent. The installation on the left does not meet the Fuel Gas Code requirements. As a result this water heater was backdrafting into this house for several years.

Example #2 – Incorrect venting installation

This is another water heater that was incorrectly sized upon installation during new construction. It was also not correctly addressed when the homeowner had a new high efficiency furnace installed. This signs of backdrafting are the rust on top of the tank. The branch the used to connect the furnace was removed and a 4″ vent was installed. The new tank also has a 1′ rise off the top of the tank which allows the vent to work safely and efficiently.


Example # 3 – Multiple Water Heater Problems


Another Example of an incorrect installation on a new construction home. These water heaters have multiple violations of the Fuel Gas Code, something I find on nearly every home that has multiple water heaters. Pictured below is the top of both the left and right water heater showing evidence of backdrafting.



This is the finished product with a properly installed and sized venting system. The old water heaters both used a 3″ vent off the top of the tank with no connector rise. The two 40,000 BTU water heaters then vented into a 4″ line which is significantly undersized for the combines 80,000 BTU’s.  The new installation has a connector rise, features 4″ vents coming off the individual tanks, then goes up to 5″ when the vents combine.


What to look out for 

The best way to prevent the issue is to watch the system year round. The problem is usually made worse in the summer time. We tend to redirect the conditioned air to the upper portion of the house and close all vents into the basement. It’s extremely important to restrict the amount of air that is drawn from the basement if the supply is limited.

Sealing up the return air venting system is the first and most important step you can take. Second making sure you have combustion air passage louvers into the mechanical room and in many cases even going from the first floor to the basement is critical in some homes.