Rheem 21vp40 1

http://waterheatertimer.org/pdf/How-to-adjust-temperature-on-Rheem-tankless-water-heater.pdf

To adjust the thermostat, turn off the water heater. Then remove the access panel(s) and any insulation. The access panels are above the drain valve. Take a small flat blade screwdriver and rotate thermostat dial pointer. (The lower temperature settings are to the right.) Replace the access panel(s) and insulation and then turn on the water heater. Do not remove the protective cover on the thermostat.

Wait for water to heat and then check the temperature at various faucets. Make sure to adjust the anti-scald temperature limiter at any faucet that may be accessed by a small child or adult who might be injured by high water temperatures. (Just put a thermometer in the flowing water. Check the faucet installation information. The high temperature limiter will be under/behind the handle for single handle fixtures.)

I hope this helps.

Cindy Wells

Raising temperature above 120 degrees requires service call.
Or change dip switch settings and risk voiding warranty and causing service call expense.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html#temperature

Earliest versions of Rheem tankless 2006-8 let you set temperature up to 140 degrees F using remote.
However the hotter setting caused error codes because the burner cannot keep up with both high temperature and large volume of hot water rushing through pipe. Especially in winter with cooler incoming water that must be heated more to reach temperature set point.
The burner simply cannot keep up.
So Rheem changed maximum temperature at remote control to tepid 120 degrees, which is not hot enough for folks wanting hot shower, or folks filling whirlpool tub because water in tub cools before tub fills.

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

According to the 2012 manual, 'the LED display only shows F.'
Bottom page 65. Yet the F temperatures are also shown as C in the manual.
http://waterheatertimer.org/pdf/How-to-set-Rheem-tankless-temperature.pdf
You probably have different model, or older model?.?
Older manual (page 15) shows same temperature range with both F and C, but typical of Rheem, the manual is totally vague about settings and say that qualified tech needed to adjust temps beyond factory default range (with no mention of display showing C) ... which is safeguard since the damg product cannot always heat water fast enough to reach higher set points above 120F.
http://waterheatertimer.org/pdf/TanklessOutdoor_95_84_64X_UC_AP15318.pdf

Call Rheem 800-432-8373 and give your product serial number and model number located on side of unit, and ask them.

Don't forget to keep the product maintained to avoid common error codes.
http://waterheatertimer.org/pdf/Rheem-CondensingTankless-maintenance-2012.pdf

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

You might have crossover problem caused by bad faucet cartridge.
http://waterheatertimer.org/Crossover.html

The green standby light should come on when 240Volt power is applied. Page 6 manual.
Both red lights should come on when tankless is activated. Page 6 manual

RTE 13 requires 60 amp breaker and 6 gauge wire
http://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater.html
http://waterheatertimer.org/pdf/Rheem-TanklessElectricUC_AP15447.pdf

Factory temperature setting is usually default 120 degrees.
Adjust temps page 11 manual
Thermostat calibration is always approximate.
120 degrees is not hot enough to burn the skin.
Exceptionally cold incoming water temperature can cause tankless not to heat enough hot water: install tempering tank to preheat water to room temperature.
Tankless cannot be exposed to more than 11 grains hardness in water. Delime tankless yearly, clean water filter monthly, do not install low-flow restrictions or conserve water since the flow rate must remain high for tankless to activate.
Rheem does not post service manuals for their tankless water heaters.
Call Rheem 800 432-8373

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

Read the manual for your specific product.
http://www.rheem.com/products/tankless_water_heaters
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Typical manual: dip switches show on diagram page 21
http://waterheatertimer.org/pdf/DirectVentManualRTG-74PVN-1.pdf

If you are adjusting temperature upward, following resources are useful:
http://waterheatertimer.org/images/Maximum-hot-water-flow-rate-200.jpg
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html#flow

1) The best place to find this information in your manual.
Typical Rheem manual in my files shows temperature is adjusted by pressing UP and DOWN temperature buttons on remote.

2) Manual says: Temperatures of up to 140°F
(60°C) for Light duty models and up to 180°F
(82°C) for Heavy Duty models can be
achieved with the MAIN (UMC-117) remote
control and a dip switch adjustment. Only
qualified service personnel should perform
this adjustment. Only factory authorized
remote control(s) should be used.

3) Typical manual shows information on page 22-23. Search pdf for 'temp'
http://waterheatertimer.org/pdf/Rheem-RTG74IndoorOutdoor_owners.pdf

4) This information squares with what I heard before that trained service technician is needed to adjust water temperature.
I think Rheem does this since tankless will not achieve that rise in temperature if incoming water is cold.
BTUs put out by tankless burner has limit.
If incoming water is colder than certain temperature, then tankless cannot raise temperature quick enough as water passes through burner.

5) You might also need a tempering tank, to modify cold water in winter:
http://waterheatertimer.org/Tempering-tank.html

6) Suggest you also read the yearly maintenance required for tankless.
For example tankless are prone to lime build up and need to be de-limed each year with vinegar.
Lime build up will certainly affect water temperature.

7) Following 2 links might be helpful for tankless owner:
http://waterheatertimer.org/pdf/Delime-tankless-water-heater.pdf
http://waterheatertimer.org/pdf/Paloma-Tankless-Error-Codes.pdf
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Add a comment for more free help.
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Fixya is always less expensive than a service call.

Hi,
You set the temperature by turning the round knob on the silver box near the bottom front...CW should be hotter and CCW should be cooler...but different ones may vary...

heatman101

Hi,

Here is a tip that I wrote to help people understand how to adjust the temperature on their electric hot water heater.

Electric Hot Water Heater Temperature Adjustment
heatman101

if it is electric remove the metal plate that is about 1/3 from the top usually colored blue or whitethere is pull dome of the insulation out fof the way you will see red and blue wires going into the element and a plastic cover underneath the cover is a setting with an indicator it is usually adjustable with a thin flat head crew driver the indicator has high medium and low temp setting

Just turn indicator to temp desired


good luck