I too have an issue where our Rheem power vent water heater will not stay lit. I cannot see that it has a thermocouple though. I can only see the "glowplug" with two wires. The glow plug is fine. I can also here the vacuum switch operating. I suspect the solenoid on the gas manifold is closing because it is not detecting a flame. Thermocouple....but where is it on a Rheem 21WP50?

Check the pilot thermocouple. You may be able to find one at a home center.

Copy following link to troubleshoot by type of gas control valve:
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html
http://www.thetankatwaterheaterrescue.com/forums/forum3/

Rheem does not post service manuals ... wonder if that's why huge majority of water heater problem on fixya are about Rheem/Richmond tank and tankless water heaters

Gene
h

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

Open a nearby window to let in plenty of air and see if problem continues.
Problem might be something simple like vent backflow blowing out pilot, or more complicated like bad gas control, or combination of problems listed below.
Suspects include:
Faulty gas control thermostat.
Air in gas line.
Low gas pressure.
Hot attic air.
Insufficient air, or dirty air intake.
Dirty combustion parts, and tubes.
Thermocouple not sitting in flame.
Thermocouple tightened too much. Finger tight + 1/4 turn. No joint compound or teflon tape.
Bad venting.
Backflow of air down vent pipe caused by tight house and/or other vent fans in house.

Open following link for resources, illustrations and reading:
http://waterheatertimer.org/How-to-install-gas-water-heater.html#clean
http://waterheatertimer.org/pdf/Bosch_Checking_the_draft.pdf
http://waterheatertimer.org/pdf/Pilot-light-will-not-stay-lit.pdf

If pilot lights and flame is robust and clear blue, and pilot goes out later, then some suspects can be excluded: air in gas line, low gas pressure, thermocouple is overtightened (finger-tight + 1/4 turn), thermocouple not positioned correctly in the flame, dirty pilot tube.
If flame is yellowish and small: then clean pilot tube and combustion parts.
http://waterheatertimer.org/How-to-install-gas-water-heater.html#clean
Remaining suspects can include: air in gas line, low gas pressure, bad gas control valve, tight house so household vent fans pull air down water heater vent that blows out pilot, hot attic with inadequate ventilation, water dripping down vent.

Open following links to read troubleshoot resources.
http://waterheatertimer.org/pdf/Pilot-light-will-not-stay-lit.pdf
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot

Burner parts need yearly cleaning, air intake needs periodic cleaning, burner parts cannot be exposed to trace amounts of chemical without damaging parts, no water should be dripping down from vent, vent should be checked for updraft, ample combustion air is required especially in closed basements, tight houses, and hot attic, bleed gas line to remove air, check that gas control thermostat is functioning.
Open following links for pilot light help:
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot
http://waterheatertimer.org/pdf/Pilot-light-will-not-stay-lit.pdf

Hi,
Thermocouple is getting weak... the extra heat from the main burner will keep it lit but after it goes out the thermocouple will then shut down..
Replace the thermocouple...

heatman101

Richmond-Ruud-Rheem
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot
http://waterheatertimer.org/pdf/Pilot-light-will-not-stay-lit.pdf

It sounds like you've read the manual.
And cleaned the burner parts according to manual.
You replaced thermal reset, but system is still tripping, so either the gas valve and tubing are bad, the thermocouple is bad, or there is a problem with gas pressure, or gas line.

Post question on pro water heater forum:
http://www.thetankatwaterheaterrescue.com/forums/forum3/

I think you can use any standard thermocouple of the right length. These are available at most hardware stores.Thanks

The most common reason a pilot light will not stay lit is a bad thermocouple. How old is the water heater? If the pilot light will light, but then go out when you release the safety button, that is a sure sign the thermocouple is bad. You can buy a replacement thermocouple at your local home improvement store. There is a small chance that the control valve has gone bad, but that is highly unlikely.

Most likely it's time to replace the thermocouple, the safety device that makes sure the pilot is lit. New thermocouples are cheap and you can find them at most hardware stores and the big-box places (Home Depot, etc.)

Here is a site with basic information to get you going.