How do I light my Rheem water heater if my peizo ignitor switch isn't working

The three most common reasons for this happening to you are 1) a defective gas valve, 2) a rusty thermopile, and/or 3) the automatic shut-off not working as it should.
So, the most common reason for many people having this problem stems from having a gas valve that is not working as it should. When your gas control valve is faulty, it will mean that your burner is not going to be igniting as it should-or at all.

Something that worked for me one time was to turn the Temp all the way down then all the way to Max a couple of times..gradually.
Why it worked I don't know. Got a couple more years out of it though. Good luck!

Best to you.

Rheem fault code 11 - No ignition. Check to make sure there is gas to the unit and that the pressure is correct. Ensure the ignitor is operational before calling for service. Rheem fault code 16 - This is an over temperature warning which could indicate a possible clogged heat exchanger.
https://www.horizonservice.net/how-to-troubleshoot-tankless-water-heater-error-codes/
https://www.youtube.com/watch?v=QsmIyYexRzM

Sounds like your ignitor is shot. You may have to replace it.

Rheem does not post service manuals, but other manufacturers do.
Copy following link for troubleshoot by type of gas control valve.
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

Also, scroll up on same page, on right side for heading title 'Pressure switch' for troubleshooting the pressure switch.

Don't forget to check anode rod to protect steel tank from rusting.
http://waterheatertimer.org/Replace-anode-rod.html
http://waterheatertimer.org/How-to-maintain-water-heater.html

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

Ether the ignitor has low voltage or the gas pressure is to high and if the gas line has been worked on lately and there ia air in the line.

1) Rheem does not show that exact model number, since they list current products only.
http://www.rheem.com/products/tank_water_heaters/gas_water_heaters

2) Rheem serial number shows year of manufacture on second 2 digits. So C06 was made in 2006.
Today's manual might not have troubleshoot steps that match older product.

3) 21vp50 is 50 gallon power vent water heater.
Open following link to identify product.
New models have Honeywell electronic gas valve with LCD flashing error codes.
http://www.rheem.com/product.aspx?id=C24E3F52-A6A4-4BC7-AF5B-0DF23A7A0263
http://waterheatertimer.org/gas-water-heater-timers.html#Rheem

4) Sample manual for newer power vent models shows error codes and troubleshoot steps:
http://globalimageserver.com/fetchDocument.aspx?id=a92c7d0e-8a95-48ba-8810-cd714f33d47a

5) Earlier Fixya answer suggests where to buy ignitor for 21vp50:
PART #HSI51409A
http://www.fixya.com/support/t3641891-rheem_21vp50_1_igniter_stopped

6) Your manual should show troubleshoot steps and expanded parts list.
Always save manuals like gold, and save searching later for a manual at Fixya.
If we can't find the manual, then our answers are limited to suggestions and guesses, since we're not on-site and can't perform troubleshoot steps for you.

7) Post your question on professional water heater forum
http://www.thetankatwaterheaterrescue.com/forums/forum3/

8) Call Rheem and they can possibly suggest which local service technician will work on that model:
2600 Gunter Park Drive East
Montgomery, Alabama 36109-1413
Phone: 1-800-432-8373.

9) Call a local plumber.

10) Install inexpensive home-center brand gas water heater yourself:
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, expert speaks with you over phone while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

it may be the thermal coupling that is bad for the pilot light. if it goes bad the flame will go out. replace thermal coupling. so if the pilot light is out then it will not heat water. or turn burner on.

Hi .. if this is a brand new heater, then you probably need to have the gas line bled. This means getting all the air out of the gas line feeding the burner. This can be done by constantly trying to light the burner or poilot . or it can be bled down by a HVAC tech.

sounds like one of the saftey relays might be going bad. or the ingniter might be cracked or need cleaning

If it is an electric water heater, there is no pilot light. You could have bad elements or bad thermostats. Be careful working on an electric water heater, as most use 208-220 volts!

If it is a gas water heater, it could be a bad thermocouple, ignitor, gas valve.

My advice to you is to call a qualified service company to come repair your water heater.