I recently had to replace the thermostat in a 1 year old Whirlpool propane water heater. Since then the water heater burners ignite about every hour and run for 20 minutes even when no water is being used. This is far more often and longer than before the thermostat was replaced. Please help, this is eating up the propane which is very expensive. Thany You

It may be the Thermocouple safety device. Here is some video help.

https://www.google.com/search?q=Attwood+propane+water+heater+won%27t+stay+lit

Check for error codes and service manual on following link:
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

Replace gas control valve
http://waterheatertimer.org/How-to-replace-Honeywell-gas-control-valve.html

Gene:

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If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Try removing the safety cut out (looks like a resistor in a plastic sleeve going to thermostat terminal) . Just disconnect it at both ends and connect the female spade from power wire directly to thermostat male terminal where the cut out was connected and try it. If it works, replace it when you get a chance. That protects system in case of flame jumping up in that area due to problems or spider webs etc. Once it's been tripped, it's no good. If that's not the problem, try cleaning your ends of your electrode, and make sure they are in front of flame when burner ignites, as it also act as a flame sensor. Let me know how you make out.

Sounds like you have a bad thermostat. You can change the thermostat, but if the heater is more than 8 or 10 years old you might be better off to replace the entire heater. Thermostat replacement is easy:
1 turn off the gas 2 drain the heater 3 unscrew pilot, main burner, and thermocouple lines from bottom of thermostat. 4 unhook gas line from thermostat. 5 unscrew thermostat from heater tank. Use pipe sealer when you install new thermostat, and gas line fittings. burner and thermocouple lines seal on a taper seat and do not need sealer. after reassembly check for leaks with soapy water, not a match, light the pilot and turn up the temp. The heater may sweat quite a bit while it is heating up the first time, this is due to the cold water on the inside and the fire on the outside. some water may drip in the bottom of the heater and you might think you have a leak, but let it heat up completly, if the water is still dripping after the tank is hot then you need to find the leak.

try cleaning the top of the pilot light jet if you can get to with small brush then blow it clean with a compressor.

First I would check to be sure your propane tank(s) are not empty and valve is not turned off and that the required gas pressure is being delivered to the unit i.e. pressure line tubing is not kinked or leaking - pressure regulator is operating properly.

Another culprit could be the thermocouple is malfunctioning or is not positioned properly, hence not allowing it to sense the flame.

it sounds more like the ignitor thats messing up, if you are able to remove it and get a new one that would probobly do it for you

First Clean the ignitor assembly and make sure the flame is reaching the flame sensor strongly.

You may need to replace the flame sensor.