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. 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.

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.