Water Heater Backdrafting
Cause, Solution, Prevention

In this article we are going to talk about water heater backdrafting, the cause, the affects, how to recognize it, how to fix it and what you can do to prevent it.

The main reasons for the backdrafting at a water heater are the poor vent installation and the obstructions found in the vent pipe. This problem affects mainly the atmospheric gas type water heaters that use the natural convection for the removal of the products of combustion, known as the exhaust gases. The problem is even bigger if the house is sealed and the unit is installed in the confined space.

The exhaust gases, which have higher temperature than the surrounding air, tend to rise up through the chimney and out, if everything is ok. Inside the chimney, the heat creates negative pressure or suction, resulting in gas pull and out of the house. What you don’t want to see are the flue gases coming back into the house. This can be dangerous.

Proper venting is a must due to safety, as the exhaust gases contain poisonous gases such as the carbon monoxide, also the moisture which can produce corrosion on the metal parts.

Every atmospheric vented gas water heater is equipped with the draft hood found at the top of each unit, where the venting pipe is attaching the water heater. And this is the place where the backdrafting can be identified.

How to check is the water heater backdrafting

The simplest ways to check for backdrafting is to put hands around the draft hood, mirror or glass. You will be able to feel the warm air coming out while the moist inside the flue gases will make the mirror/glass fog up, if used as the checking tool.

You can also use the lighter or matchstick beside the draft diverter and if there is sufficient draft inside the vents, the flame will be pulled in. Or, suggested by the home inspectors put a smoke or a powder into the draft diverter, and if it spills back into the room, the test for backdrafting is positive.

If it was backdrafting for a long time… check the elements that surround the draft hood, you might see some dark residues also known as soot. Look for the plastic caps on the incoming cold and outgoing hot water pipes. As said, exhaust gases have a high temperature and if they leak under the draft hood, the plastic elements can be affected - can change the shape, discolor or melt.

Backdrafting can also result in the metal corrosion, because the condensate, which is acidic by nature, can appear on the top of the heater, where hot gases get in touch with the cold metal elements.

emergency plumber

What is the cause of backdrafting

There are several reasons for this problem. One of them is the poor vent installation, such as horizontal or downward slope - the vent pipe has been installed with the improper pitch; too many turns (angles – especially sharp and right angles), sudden diameter reduction, sags, dips, long vents…

The solution for the improper pitch is to make sure that the minimum slope is per manufacturer’s instructions or codes – usually ¼” per one foot. Every manual clearly says for a specific model the maximum vent length and the number of elbows (angles) that have to be applied.

In order to prevent foreign body gets into the flue vents, it is recommended to install the vent cap, no matter is the vent termination horizontal or vertical.

If the vent cap is too far down below the peak roof top you must rise the vent pipe at least 12 inches above the roof top, and be careful not to install the cap way too down of the vent pipe.

And the last thoughts on the water heater backdrafting relates to the proper provision provided in the room where the heater is installed, so the sufficient fresh air is delivered to the combustion chamber for the proper gas combustion. The room where the water heater is located should have two openings, upper and lower, and the size should be done per manufacturer’s instructions, so the outside air can freely enter the room and in sufficient amount.


If you want to avoid this problem at its root, then don’t buy the atmospheric vented water heater, but power-vented, which uses the electric fan for removing the gases out. If the atmospheric model is the only option… this article is for you.

Recommendation is to buy a high efficiency water heater, with the efficiency over 90%. This is especially important if converting from the low efficient unit to this type. The high efficient water heater does not vent flue gases through the standard chimney, but the sealed plastic pipes.

Keep in mind that all the venting installation should be done per manufacturer’s instructions and local codes. As said before, backdrafting test is easy, and it is recommended to perform the check after several minutes of running with the windows and doors closed.

Home > How to > Fixing backdrafting issues

water heaters