Gas Water Heater Pilot Light - thermocoupler?

Question

Pilot goes out AFTER it heats the water. That is, I can start the pilot light, and it will stay on (even all night - as i left it in pilot mode), but once the gas turns on and heats the water for awhile (not sure how long), all gas shuts off. I have watched the gas turn on and appears to be heating the water, however, when i come back an hour later - all is off, so either the gas is shutting off during the heating process, or the water gets to temperature and then everything turns off.

I am only familiar with the thermocoupler, which i thought only sensed a cold element therefore preventing gas from flowing with a pilot light out, but when these things fail, can they do so on the hot side? That is, do they sense when an element to too hot and shut off the gas?

much thanks, Sandy

David Niles · Answer

The sealed chamber with the intake on the bottom is a TERRIBLE design and causes problems like this all the time, some of the units are now coming with a wrap around screen for the base of the tank as a first line of dust defense.

Randy F. · Answer

The sole funtion of the thermocouple is to sense the heat from the pilot light. If the pilot goes out the gas valve closes so that gas cannot continue to flow to pilot or open when theres a call for heat. Rarely is the problem the gas valve. It usually involves a bad thermocouple, a change in pilot flame size, position of thermocouple sensor in relation to flame, or simply carbon buildup not allowing enough heat transfer. Check to ensure thermocouple is not carboned up or burned up. Need good positive contact with flame. Brush it off i carboned. If end appears burned/has irregular shape, replace it. Ensure that pilot flame is strong and blue. If it appears lazy or is more yellow in color, clean orifice where gas comes out. If it is still lazy and not blue flame, there is a screw on gas valve that you can adjust to increase gas flow. Light pilot and turn screw in tiny increments as you watch flame until it becomes stronger and blue in color. Another possibility is when the main gas valve opens and closes the pressure change blows out the pilot. This can be rectified with the adjustment of pilot flame, unless it is caused by blockages in the burner tube orifices. Watch as the burners light. Does the burner light within seconds of the gas valve openin? Do the burners light smoothly from front to rear? If not, that could mean that there has to be a buildup of gas before lighting and the increases pressure blows out pilot. In this case burner tubes may need to be pulled and cleaned. If they are clean and no blockages, thermocouple is doing its job, and pilot flame is strong and blue, air inlet on burners may need to be adjusted to slow draft/air flow. If this is happening when there is demand and the flue is hot, it could be caused by the increased draft. When the flue is hot, draft is greater. Air is movin thru burner tubes at a higher speed and pilot isnt in close enough contact with gas flow. Again, either adjusting pilot position in reference to burner, increasing pilot size, or adjusting air flow at burner tubes, should rectify problem. It could also be insufficient draft, but this is usually accompanied by the odor in the room when combustion occurs which you didnt mention, and with a three story flue isnt likely. The cleaning and thermocouple replacement is simple. The rest is best done by a heating tech. Its a carefully balance system that can change over time and easily corrected by a professional. If you make too many changes and it doesnt fix problem, it could cost you more to have it brought back into balance. I offered what I know so that you are armed against the guys that come out and just start replacing parts until it works. Only other thing I can think of is posession by evil spirits! Do you know a good Catholic Priest?

Steve Babiak · Answer

Even though the problem is now solved, just wanted to point out one other possible factor. I see DIY things as I go through places, and some DIY types don't realize that there should be a drip leg on the gas supply, right where the water heater or heating system gets gas. Normally a tee is used and the the drip part gets capped on the end. Without this, crud tends to accumulate in places from the gas supply, and could affect the pilot supply tubing.

Randy F. · Answer

Ive been out of the heating business for twenty years and havent seen AOSmith water heaters in Alaska( doesnt mean they arent up here!)I did a bit of research and see that they have a a model with a combustion air blower. If its a sealed combustion chamber it must be this model and the blower pressurizes the chamber. In that even it is critical that the air/gas ratio be maintained for clean, efficient combustion. For that reason I wouldnt drill any holes as that will change this balance.

Sandy Bars · Answer

SOLVED! friggin dirty vent - that is, no oxygen! i was stuck on the clue that the pilot would stay on, and light easily, but also noticed that the water was not very hot: luke warm, which meant to me, the heating cycle was being turned off before it reached temperature. Not enough oxygen? the pilot assembly is completely enclosed, accept for a 3" diameter hole which leads to outside air. This hole is protected by a fine-mesh screen. I scrubbed the screen (remotely, with a wire brush attached to rod - actually my shotgun cleaning rod!) and the flame doubled in size! unfortunately the screen is soldered on and does not come out - it cant be replaced, and cleaning it is a pain, so my clean job was prolly not very good. been watching all afternoon, setting different heat temperatures, and it turns on and off correctly. i do like the comments about carbon build up on the thermocouple and about its position. after watching the assembly work for a couple of hours, both these scenarios explain the problems i am (was?) having, and if this fix doenst work that will be next. super thanks for all your support! The internet (aka - the weener-tubes) saves the day! PS - can i drill holes through the metal plate that the gas line go through, perhaps next to the little glass window to allow more ventilation? cheers!

Rob K. · Answer

I purchased a used water heater from a plumber who told me that the previous owner had the same problem. The water heater was still under warranty, but the guy wanted a new one. The plumber gave me the guy's name so I could call the warranty # and say I was him and get the new parts. I was told by the plumber that it was one of two items and were easy to replace. I called the warranty number before I installed the water heater. They told me to vacuum the bottom real good and to call back if it didn't fix the problem. I vacuumed it with a shop vac and installed it. It worked fine. This was on a house I purchased that the plumbing and water heater had been stolen prior to my pirchase. When I had my furnace guy come check out the problem, he looked at the water heater also. He did a test to see if the gas valve was bad. I can't remember exactly what he did, but I think it involved turning the dial while tapping the valve with a hammer. He said that it tested fine. It hasn't been a problem since. I would vacuum it first. If you need a gas valve, they are about $80 plus labor. If the unit is only 4-5 years old, is it still under warranty?

Sandy Bars · Answer

thanks for reply: so in my case then , the thermocouple is good bc i can get the pilot to light, and if fact, left it on all night in pilot mode and it was lit the next morning. i then cranked up the heat, burner assembly ignited, and when i returned an hour later - all was off including the pilot light - not sure if the water got to temperature. why would the gas shut off mid-burn? or why would the pilot not remain lit after a heating cycle? thanks for the replies, its fun to learn something new!

Steve Babiak · Answer

You will not smell gas when the thermocouple "isn't working". The thermocouple generates a small amount of electricity when heated; that electricity enables the control valve to allow gas flow to the main burner. When the thermocouple "opens", no voltage is generated, and no gas goes to the main burner. The pilot tubing just might have some accumulated debris, so as suggested in another post you might want to clean this. I had something similar on a stove where the pilots wouldn't stay lit due to insufficient flow in the pilot tubing on the stove. The heavy flow to the main valve is a path of less resistance to the gas, so clogged pilot tubing could allow for less gas to flow when the main burner lights.

Sandy Bars · Answer

thanks for the quick reply - its only about 4-5 years old (AO smith) and i dont think its a wind thing bc the vent goes to the top floor of a 3-story building. However, all the roof pipes are rusted to heck - and i mean its a mess up there, so maybe debris fell in and plugged the vent so maybe no oxygen? the area around the pilot light is sealed with a metal plate and a small piece of glass for viewing the pilot light. What exactly does the thermcoupler turn off if it doesn't sense heat? If the thermocoupler isn't working shouldn't i smell gas? thanks!

Pat L. · Answer

yep Don't know the age of your unit, but mine was a basic NG '03 model. Mine wouldn't let the pilot stay lit??? fixed it with a 18" generic thermocouple $7.95. S I think your problem is different. However, as a PM when you get the burner out check the pilot tubing and clean both the tubing and orifice using an air compressor. Also check the main water heater control valve, it's the valve with the thermostat on, that maybe what is malfunctioning causing it to shut down. My guess its probably this. Another culprit may be the water-heater exhaust vent on the roof. Downdrafts could cause the pilot light to go out. Make sure the vent cap isn't missing or rusted through on the roof, it will cause drafts and wind to blow out the pilot. Also the newer water heaters require more volume of air due to the addition of a heat sensitive diode to the thermocouple so you may need to ensure good air flow esp if it has a captive outside air feed. good luck