A.O. Smith Gas Hot Water Heater Flame Sensor Repair

Without warning, a couple days ago our hot water heater became just a cold water storage tank. Our A.O. Smith GCV 40 100 HWH had been functioning perfectly since we acquired it with the house in 2008. Being a gas hot water heater, I had a bit trepidation about messing with it since gas has a way of exploding at the most inconvenient times - like when your face is staring into a burner chamber. I attack electrical problems with near-reckless abandon from having dealt with AC and DC supplies and controls for nearly five decades (I turn 63 in August). Nevertheless, last December when our Trane VX95 gas furnace decided it was time to be a cold air storage container, I sought advice on the Internet for how to exact a repair - and found it. The problem was a dirty flame sensor element. I cleaned it per recommendation and it has work just fine ever since. I wondered, therefore, whether the gas hot water heater might be experiencing the same phenomenon. After all, it was exhibiting the same sort of behavior where the burner would initially ignite and then shut off after a couple seconds. Spoiler alert: It was the sensor.

After turning off the gas supply valve and letting the residual gas in the line dissipate, I removed the three fittings on the bottom of the control mechanism and unplugged the igniter wire. Next, the burner assembly was removed from the burner chamber. The photo shows some metal scale accumulation on the top surface, but an inspection of the bottom of the water tank did not reveal any ominous looking areas. I vacuumed the burner assembly thoroughly and then cleaned everything with acetone. 600 grit wet/dry sandpaper was used to carefully remove all the residue buildup off the flame sensor and other parts so that it looked almost like new prior to being reinstalled.

Everything was replaced, gas turned on, and the pilot light re-lit. I crossed my fingers, clinched an appropriate amount, and turned the temperature control to its normal position. Voila! The burner lit and stayed lit until the water was up to temperature. Yea! I just saved $140 for a replacement burner assembly and, if a technician had to have been called, probably a couple hundred bucks worth of labor. Update: I discovered after writing this that Lowes and Home Depot both sell replacement thermocouple flame sensors for about $10, so if it happens again, I'll just replace the sensor.

I pretty much live by the sage words of Red Green (aka Mr. Duct Tape), of Possum Lodge fame, "If the women don't find you handsome, they should at least find you handy." 

Go to 4:10 in the video

Posted August 10, 2021
(updated from original post on 4/26/2016)