water-heater-vent-pic2Checking a water heaters vent draft is an important safety concern. The fumes from a hot water heater contain carbon monoxide and can make you sick or even be lethel at certain levels. Making sure that the water heater is venting properly can give you peace of mind. The efficiency of the water heater is also a factor.

Not sure if the vent is your problem?  See the article, 'Troubleshooting a Gas Water Heater' for a complete listing of all the gas water heater topics.

The information below discusses how to check to see if you water heater vent is working properly. There is also some information on how the vent should be installed. If it is not working or not installed correctly, you are going to want to correct the situation.

How a Water Heater Vent Works

The burner is on the bottom of your hot water tank. The burner draws air from the room to burn. In a small room, grilles need to be installed to provide enough air flow. Once the gas is burned it is vented up through the center of the tank to the flue. A properly sized chimney will create a draft that sucks the fumes up and out into the atmosphere.

There is an air space around the fume hood that also allows the heated combustion gases to draw air. The fume hood is held up by short pieces of metal that fit inside of slots on the top of the water heater.

WARNING!! An Improperly Vented Water Heater Can be Dangerous!!! Shut Off the Gas To Your Water Heater WARNING!!!

Checking the Vent Draft

Setting Up the Test

As a rule you should test the vent draft under worst case conditions. What this means is that when other exhausting devices are running, such as a bathroom exhaust fan and there are no open windows, the water heater should still vent properly.

To properly check the vent draft you want to first close all of the windows and doors in the house. You should also close the damper to the fireplace if you have one. Then you want to turn on exhaust fans in the bathroom and kitchen. The dryer should be running also. This may be extreme, but it replicates conditions when everything is being used at the same time.

Turn the furnace down so that it does not run while you are performing the test. Run enough hot water to get the water heater burner to ignite. The burner should run for a few minutes before you perform the test.

Performing a Low Tech Draft Test

When the burner is on for a few minutes you should be able to feel air being drawn into the fume hood. Put your hand next to the vent hood without touching it. Hold your hand sideways and cup it slightly. You should never be able to feel air pushing out of the fume hood. This is called a 'down draft' or 'back draft' and could be serious, see below.

You should be able to feel the air being drawn into the vent hood with your hand. If you can't feel any air being drawn in or you feel pushing back there is a draft problem.

Another low tech way to check the draft would be to hold a match or lighter near the fume hood. The water heater burner should be running for several minutes before you perform this test. The airflow should draw the flame into the hood. If the flame sputters or goes out it could be a sign of trouble. Another thing to look for is soot around the fittings. Any sign of black soot means that the smoke is not going up the chimney.

If there is any evidence that the vent is not working properly, you are going to need to correct the problem. See the articles 'Fixing Blocked Water Heater Vents' and 'Unblocking Chimney Flues' for information on how to remove blockages from the vent and chimney.

Back Draft - What It Means

A back draft as the name implies means that instead of heading toward the chimney air is being pushed back into the room where the water heater is. This is a dangerous condition, since the exhaust from the water heater contains dangerous fumes.

If the test above fails, and you have checked tom make sure there is no blockage, try shutting off one or more of the fans and/or opening a window to see if the draft improves. If it does, you may need to provide some 'make up' air for the water heater. This is usually a duct that provides air from the outside. When the water heater needs additional air, the duct will provide it.

Installing this type of ducting is beyond the scope of this article. There are two pages you can check for more information, 'Water Heater BackDrafting - Part 1' and 'Water Heater Backdrafting - Part 2'.

Checking the Vent Installation

The fume hood should be sitting on the tank level. The vertical legs should be straight and inserted in the slots provided. The fume hood should be directly over the round opening in the top of the hot water heater. If it is crooked or off to one side it should be fixed.

The vent should be pitched upward toward the opening in the chimey. The vent should be pitched at least 1/4" per foot in an upward direction. It should not have an elbow directly above the opening in the water heater. It should not have long horizonal runs. The water heater should be as close as possible to the chimney.

If you vent is not installed in a manner that allows the free flow for the exhaust it will need to be corrected. Reduce if necessary the number and location of the elbows and make sure the ducts are pitched correctly.


Carbon monoxide is a dangerous gas and extreme caution should be used to make sure it is being vented from the hot water heater. Make sure you don't have any obstructions blocking the ven or chimney flue.