You need the manual for your specific Rheem product, which shows periodic maintenance steps needed and will sometimes show diagrams.
Open following link, click on same-similar product, when page opens, click tab that says Specs/Docs, download Use and Care manual: http://www.rheem.com/Products/tank_water_heaters/gas_water_heaters
The instructions will probably suggest you call service technician do the work.
After all this is a gas appliance, and not really a first-time learning opportunity. One kaboom is a lot more expensive than watching what the service technician does so you can repeat it later.
If cost of parts and labor is higher than $330, then it might be cheaper to install new gas water heater that you buy from local box store.http://waterheatertimer.org/How-to-install-gas-water-heater.html
As a general non-specific guideline for cleaning burner.
Some water heaters have a gasket-seal where pilot and burner tubes enter combustion chamber. You might need this gasket replacement on hand in case old one breaks.
Other combustion covers have tamper-resistant screws that signal a service technician should do the work.
1) Take photos of all parts and areas so you have a record of how it looked for easy reassembly and relighting.
2) Turn off gas to water heater. The gas valve handle should be perpendicular across the gas line to turn off. The gas valve handle is parallel with gas line when gas is ON.
3) Using wrench, loosen tubes that attach to bottom of gas valve control. Notice tightness of each when loosening. The thermocouple is finger tight plus 1/4 turn. The other fittings are snug to prevent gas leak. Note which fittings have sealant on the threads, and then use yellow teflon tape on those fittings when reassembling. Test gas lines for leaks using liquid detergent mixed with water. Soap bubbles indicate gas leak.
4) Lift burner out of combustion chamber.
5) Clean burner and combustion chamber with wire brush, and you can use mild detergent with a bit of water.
6) Use a burner brush to clean out burner tubes, and use air pressure to help remove particulate. Remember, obstruction as small as a cobweb inside the tube can cause the water heater not to operate.
7) Finish up using vacuum cleaner, reassemble parts, and light water heater using instructions printed on side of tank.
Other helpful information available on following link:http://waterheatertimer.org/How-to-install-gas-water-heater.html#clean
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