A blocked water heater vent can be a serious problem. Gas hot water heaters burn fossil fuels that generate, carbon monoxide, a dangerous and deadly gas.
Before you assume that your vent is blocked you want to make sure you check the vent draft. See the article 'Checking Water Heater Vent Draft' for instructions on what to do. Not sure if this is your problem? See 'Repairing Gas Hot Water Heaters' for a listing of all the gas water heater topics.
Any sign of a poor draft may mean an obstruction. There are a couple of things you can do to try and figure out if you have a blockage. Some or all of them may not work in every situation.
Warning!! Shut the Gas Off To Your Water Heater If Your Vent or Chimney is Blocked! The Fumes Can Be Dangerous!!
Locating the Vent
The water heater vent is a section of metal pipe, usually 3" in diameter that runs from the top of the hot water heater to the chimney. It may be connected to a larger metal pipe that also vents the furnace. You only have a vent pipe on a gas water heater, electric water heaters do not need to be vented. This pipe is connected to he chimney located nearby.
Removing the Vent
Before you can check the vent, you need to shut down the water heater. See the article 'Shutting Down a Gas Water Heater' for instructions.
The flue pipe is held together by screws. You can take apart a section by removing the screws that hold it together. Remove enough of the pipe to see the full length of it. and check for blockages. Make sure the water heater is turned off before you do this.
Have you checked all the pipe that goes from your water heater to the chimney. Did you see a blockage? Remove the blockage using a piece of stiff wire. The vent pipe needs to be completely free of obstructions the full length of the pipe.
If the pipe is clear, you can put everything back together using the same screws that you removed. You do not need any sealer for the fittings on a metal vent pipe. However, the vent pipe should be pitched upward toward the chimney. If it is not pitched properly, you will need to make adjustments. See the article 'Repairing Water Heater Vent Pipes' for more information.
No blockage, then your problem is else where. See the article 'Blocked Chimney Flues' for more information.