Hot water tank lights on - Rheem 40 Gallon FVR NATGAS Low WATER HEATER 6YR 22V40SF

The water heater may be cracked and have to be replaced. Look for a leak from the tank. Always fill the tank before lighting the pilot light if you don't there is a possibility that you could damage the tank.

Check your propane tanks (are they full?), make sure the valve is on, then check to make sure gas is making its way to the hot water tank, if the pilot lights but won't stay lit your flame sensor is faulty.

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What are you expecting to happen?

I assume the water heater and water inside it is cold (otherwise you would not be lighting it). You light the pilot. The main burner comes on and the pilot lights it. The main burner should stay on until the tank is hot, When the tank gets hot the main burner should shut off but the pilot stay on. If you draw hot water, cold water will run in and cool the tank. If yu do nothing the tank will eventually just cools off. then the main burner should come back on to reheat the tank.

Hiliary
sounds like the primary coil in side is cracked and is mixing hot water.. with the cold water in tank need to check and replace as necessary

You have tank-type gas water heater with standing pilot light.

Instructions for lighting pilot light are usually on front of water heater.
Since water heaters vary widely, if you want specific information without guessing, then add a comment with model number that appears on label on side of tank, and also describe the gas control valve on front of unit.
This information will let expert located manual and look up specific information plus troubleshoot steps for that model.

1) Your water heater needs to be replaced.
It cannot be repaired.
Tanks rust out after anode rod is depleted unless anode is inspected and replaced 1-3 years.
Water softener causes anode to deplete quicker, so inspect every 2 years.
Also stray household current can cause tank to rust out. Put bare copper ground wire between hot and cold pipes on new heater.

2) First thing is protect yourself from tank rupture where water heater bursts open and water runs until it's shut off. Ruptured tanks are big topic in homeowner insurance industry today because they cause major damage inside house.

- Turn off heat source so tank is not under pressure:
Gas: Rotate dial to pilot so pilot light stays lit, but burner does not turn on
Electric: turn off circuit breaker

- Turn off water going into tank using shut off above tank.
- Open hot side to bathtub to relieve pressure from tank.
- Check label on side of tank to get tank specifications for replacement.

3) What to do until replacement.
You can get by for a while on leaking tank by turning it ON only when needed.
Make sure tank is full of water before turning heat source back on.
This will buy time until replacement.
Electric tank heats 21 gallons water per hour.
Gas water heater 41 gallons per hour
Shower uses 6-9 gallons hot water, bath uses 12-15 gallon hot water.

4) Additional reading so next tank lasts longer
http://waterheatertimer.org/Replace-anode-rod.html

http://waterheatertimer.org/9-ways-to-save-with-water-heater.html

http://waterheatertimer.org/Water-at-bottom-of-water-heater.html

Tanks are marked with Hot and Cold.
1) If water connections are on top of tank, look for red marking or word HOT that shows where hot water pipe connects. Look for blue marking or word COLD that shows cold water connection.
2) If water connections are on side of tank, cold water connects to bottom one, and hot connects to top.

3) If hot and cold are reversed, the tank will not work.
Pipes on top of tank: cold water enters tank and is sent to bottom of tank in a long tube called the dip tube. This means the cold is on the bottom since hot water rises to the top. The hot water exits top of tank.
Pipes on side of tank: You want cold water to enter bottom, so hot water naturally rises to the top outlet.

gas regulator gone bad

The brown flakes are most likely rust from either the tank or associated metal piping. They could also be from the anode used in hot water tanks to combat electrolysis. Does the water have a Sulphur" or "rotten egg" smell to it? Are you on a well? There shouldn't be so much, as to plug up the valves. Turn off your heat source to the hot water tank. At the bottom of the tank is a faucet. Hook a garden hose to it, run it out side where the hose lays lower than the faucet if possible and flush the tank untill you have clear water again. You may want to do this a couple of times to be sure you get rid of any sediment in the tank. When you are done flushing, make sure you have the air out of the tank by opening a hot water faucet until it runs clear with no "spurts" of air. Shut the faucet off and turn the gas back on, light the pilot light and see if the problem persists. Also have your water tested. Sears does this for free -just take a sample in and they will tell you what the hardness is.