I would like to know the age of a Rheem unit S/N 6225F239916151

I can't find any information on that MODEL you listed in your question. It's sort of like the Rheem Air Handler models but notthing comes back. In any case you will NEED the SERIAL number, as that will tell you all you need to know about it. If you can't decipher SN, call RHEEM and they can give you the info on that unit. Sorry I could not be of more help.

you have trash in the valve .shut it down take the valve apart clean it . also maintenance on this unit is needed.seal can be bad also. hope it helps.send pictures of the unit and age and model serial number.

Rheem, Richmond, Paloma tankless computer?
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Add a comment and describe product in detail, with approx age, brand, type of heater, and what you have done so far to keep the unit maintained as shown in product manual.

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

call plumbing supply house with serial #, & model #of your tank. give them the info and if they handle the brand of your tank , they should be able to help.

1) Rheem offers on-line chat, which you rarely see for typical tank-type water heater
http://www.rheem.com/products/tankless_water_heaters/customer_support/
http://www.rheem.com/hrc.aspx

2) Call local service technician, and put that person in your rolodex-wallet for future help.
When technician arrives, ask that person for tips and tricks keeping unit working with regular maintenance.
They will gladly offer yearly service.

3) Read manual several times get accustom to yearly maintenance required for tankless water heaters. Tankless water heaters are maintenance heavy. All costs are energy.

4) Read manual several times and notice word troubleshoot appears many times.
Since tankless are complicated machines, many internal and external factors can affect operation, especially as unit ages.

5) Look at label on unit to identify exact product number. Identify same/similar product at Rheem, and become familiar with Rheem website
http://www.rheem.com/products/tankless_water_heaters/

6) Read other question/answers at fixya about Rheem tankless to become familiar with the tankless era
http://www.fixya.com/tags/rtg_74pvn

7) Install DIY tank-type gas water heater and avoid the whole headache
http://waterheatertimer.org/How-to-install-gas-water-heater.html

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

1) Rheem offers on-line chat, which you rarely see for typical tank-type water heater
http://www.rheem.com/products/tankless_water_heaters/customer_support/
http://www.rheem.com/hrc.aspx

2) Call local service technician, and put that person in your rolodex-wallet for future help.
Ask technician for tips and tricks for keeping unit working and needed maintenance.
They will gladly offer a yearly service.

3) Read manual several times to get accustom to yearly maintenance required for tankless water heaters. Tankless water heaters are maintenance heavy. All costs are energy.

4) Read manual several times and notice word troubleshoot appears many times.
Since tankless are complicated machines, many internal and external factors can affect operation, especially as unit ages.

5) Look at label on unit to identify exact product number. Identify same/similar product at Rheem, and become familiar with Rheem website
http://www.rheem.com/products/tankless_water_heaters/

6) Read other question/answers at fixya about Rheem tankless to become familiar with the tankless era
http://www.fixya.com/tags/rtg_74pvn

7) Install DIY tank-type gas water heater and avoid the whole headache
http://waterheatertimer.org/How-to-install-gas-water-heater.html

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

All costs are energy.
Here's the order of things I would do.

1) Contact Rheem for name of qualified service technician.

2) Call technician who can fix your specific unit, and can educate you about maintenance, and tricks and tips. These guys learn something new each day and are good to have in your rolodex/wallet as tankless unit ages.

3) Download manual, or read manual cover to cover and review maintenance protocol to make sure you're on a schedule for de-liming and other things required for tankless to work.

4) Ask Rheem how you can become familiar with their website pages that show updates and other information that affect your specific unit.

5) Install ordinary tank-type back up that can be put online quickly in case tankless needs parts or frequent maintenance as it ages.
http://www.fixya.com/tags/rtg_74pvn

http://waterheatertimer.org/How-to-install-gas-water-heater.html

The tank may be corroded and you may need a new unit. Check to make sure that the water is not coming from any of the water connections that are higher up on the unit. Also check the drain plug to make sure it's secure. I don't know the age of the water heater but it may be time to replace. Most people don't know that water heaters require maintenace (draining the tank to remove sediment) from time to time.

my 50 gal heater puts out only 20 gal of hot water

Depending on age of Rheem they have a time delay relay. May be causing the problem. The time delay operates the fan. Try turning fan to continuous and see if it stays on.