If you have a gas water heater, you should have a flue pipe connected to it. This flue pipe is used to carry toxic gases, such as carbon monoxide, from the water heater to the outside air. This can be vital, as otherwise these gases would leak back into the home. Blockages or debris in the flue pipe can cause it to stop working, so checking the pipe regularly is vital.
Step 1 - Check Inside the Heater
Check the flue pipe where it is connected to the heater. You should open up the heater using your screwdriver, and then look at the connection. Wipe around the edge of the pipe using a cloth. If it appears oxidized, or there are white stains around the mouth of the flue, this can indicate that it is blocked. You may also notice a bad smell, or have your carbon monoxide alarm sound, if you have one.
Step 2 - Check Outside
You can also assess if the flue pipe is blocked from the outside. Using a ladder, look at the top of the flue. You may find the same oxidization marks, or white stains. You could also see a black, paste-like substance on the outside of the flue. This suggests that the flue is blocked.