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Accurate Joe
Accurate Joe, Owner of Accurate Mechanical.
Category: HVAC
Satisfied Customers: 165
Experience:  25 years experince in HVACR field. I am a master electrican. I own an operate Accurate Mechanical,
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Pilot light on my water heater is lit, but burner does not fire

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This is a propane powered water heater, 40-gal. The problem started
a couple month's ago. We'd discovered the pilot light was out, but the unit would fire up right away upon re-lighting. This has happened maybe four times in this time period. Today the pilot light was lit but the burner wouldn't fire. We have ruled out the gas line to the house as the boiler and other gas appliances are working fine.

Good day and Welcome to Just Answer

It sounds like you have a bad thermocouple.
You need to be competent to work on this type of appliance. Gas is very dangerous. Never work above your skill level.
On your gas valve you have a small copper line that runs to the pilot flame.
Make sure it is at least 40% in flame.
If you have a meter to read millivolts, when under flame it needs to register above 15 millivolts. Here is a link for the thermocouple. Click on blue word. Get the length you need for your application. You can buy them at Lowes.
The thermocouple should cost under 20 dollars.

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Customer: replied 8 years ago.

It appears the problem with the water heater is a little more complicated than I first anticipated. We replaced the thermocoupler as you suggested. Not known to me when I first communicated the problem last evening, the problem is more complicated.
I have checked out the situation myself instead of relying on my significant other for an accurate description. My significant other replaced the thermocoupler but now the flame is going everywhere but to the burner. It seems to be coming out a 1/4" diam aluminium line running from the ignition unit toward the burner. This line appears to be crimped or bent and possibly cracked about 1 1/2 " from the end closest to the burner. There is no flame on the burner at all. Is this fixable with additional parts?

Cut the gas off. Is it possible that in changing the thermocouple, the pilot tube got, took half way off and crimped. That is the pipe to your pilot. You will have to loosen thermocouple and main gas to burner, and pilot tube to slide the whole burner out. At that point you are going to need a new pilot line. You can get at HVAC supply house. Put it back together. I will post in next post a You tube video on changing the thermocouple.

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Customer: replied 8 years ago.

Ok, the saga continues. We have a new thermocouple, pilot line and cleaned burner. We have a good strong pilot flame. Now for more information. Our water heater is vented with an electric fan that is wired to the main valve by four wires. Two of the wires are clipped onto a front "solenoid" and two wires are hooked to what looks like a small "fuse" unit on the side of the main valve. Earlier this afternoon, we thought the problem was fixed. After the burner assembly was re-installed and the water heater re-lit, the fan came on and the burner ignited and we had our first tank full of hot water. The burner shut down and will not re-fire. We started looking at the connections to the fan. It appears that when we pull off the two wires to the "fuse-looking" module on the side of the main valve and re-connect them, the fan comes on for a few seconds and the burner fires, then the whole thing shuts down and we're back to a nice looking pilot light.
That was happening hit or miss. Now when we repeat that process, nothing happens.
Any suggestions? If I haven't described this well enough, maybe I could take a photo.

Your image need to be Jpeg file and just hit the tree and download, or you can post to photo bucket and copy the address from title bar and post.

Give me the Make and model of water heater and the fan device.

Customer: replied 8 years ago.

It is a AO Smith "Powershot" Power vent water heater. The fan assembly
is Fasco Industries brand. I can't find a model number.

Good enough
Customer: replied 8 years ago.

As far as I can tell the water heater is a 50 gal (not 40 gal as earlier stated) and
the model no. is FPS 50 217. There is also another number that may help. That
number is FPS - 50 - J01P040S19. There is also a serial no. MD92-0052902-S19.
Let me know if this doesn't help and I'll look for more numbers.

Double check your numbers, they are not matching.

Customer: replied 8 years ago.

It is the night before Thanksgiving. We got someone over to fix the problem but under other circumstances, I would have waited for you to help us. I know the problem would have been much worse and prolonged had it not been for your expert attention. Thank you again.

Thank you for your time and look forward to hearing from you again.
Happy Thanksgiving and God Bless.

Accurate Joe and other HVAC Specialists are ready to help you
Customer: replied 8 years ago.

Thank you. Happy Thanksgiving and all of us have so much to be thankful for. Have a wonderful day!