Why won't my hot water heater's temperature change?

It is generally not recommended to set your water heater to the highest setting. Water heaters typically have a maximum temperature setting, usually between 120°F and 140°F, and setting the temperature higher than this can be dangerous.
Water that is too hot can cause scalding and skin burns, and can also damage pipes and fixtures. Additionally, high temperatures can cause increased wear and tear on the water heater, reducing its lifespan and potentially leading to costly repairs.
If you're running out of hot water, there are a few other things you can try before turning up the temperature. For example, you could insulate the hot water pipes to reduce heat loss, or upgrade to a larger water heater if your current one is too small for your needs. If you do decide to increase the temperature, be sure to do so gradually and monitor the water temperature to ensure it doesn't get too hot.

It sounds like the thermostat that operated gas valve is failing or the gas valve is bad. This is a repair left to professionals. Now I would price a new heater and install it myself. Repairing older heater is not a good idea, the rest of the heater is old and will fail.

1) Copy following link to adjust water heater temperature.:
http://waterheatertimer.org/How-to-adjust-temperature-on-water-heater.html

2) Add a comment if more help is needed. Jan 2013

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

1) Thermostat may be bad, or out of calibration. Set temperature higher, and/or troubleshoot by type of gas control.
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-adjust-temperature-on-water-heater.html


2) Dip tube broken
http://waterheatertimer.org/Water-heater-dip-tube.html

3) Water leak running hot water out of tank.
Turn off all taps and put ear against pipe listening for water leak.

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

I recommend safety changes to prevent ruptured water heater and assorted leaks.
135 degree hot water.
Maybe replace TP valve if it is dripping.
Drip pan under water heater with 3/4" drain line to outside.
Expansion tank on incoming cold water line to absorb pressure build-up, and prolong life of tank and plumbing.

Here's why:

1) Yes you can have scalding hot water.

2) I recommend 135 degrees since that will burn the evil right out of ya. And generally kill clothes washing bacteria.
http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html

3) 150 is dangerous and can seriously burn skin in 1.5 seconds. Not safe around children and elderly.

4) Temperature settings on electric water heater are approximate, since thermostats are calibrated approximately, and read temperature through tank wall.

5) Also if you want to increase amount of hot water for high-consumption household, then setting tank temperature upward is one solution since more cold water is mixed with hot, so amount of hot inside tank goes further.

6) High temperature can cause TP valve to release water.
TP valve is pressure-temperature safety valve. Do NOT cap off.
Rating should be 125 psi and 180 degrees ... but all things water heater are approximate, and TP valves need replacement now and then.
It is not recommended by water heater manufactures to use 150 psi TP valve when water heater came with 125 psi. Rating shows in small print on end of TP. New TP valves sold at local hardware. Let tank cool before installing new one.

7) Higher temperature and high pressure can rupture tank.
Pressure over 80 psi shortens life of water heaters and plumbing fixtures. Put BTG100 pressure gauge on water heater drain valve to check pressure.

8) You need a drip pan with 3/4" drain line to outdoors, or to drain vent line, to prevent flood caused by ruptured water heater.
Turn water heater off when going on vacation.
Inspect water heater each 6 months.
Drip pan available at local home center or hardware:
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html#drip

9) Heated water expands and puts pressure on tank and pipes.
Water does not compress... so when water gets hotter it expands against tank wall and against pipes.
Especially in closed system (check valve present) when system is without expansion tank.
Expansion tank is installed on incoming cold water line.
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html#expansion

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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

1. COLD MIX, HEATER DEACTIVATES
   If inlet water temperature is over 70°F due to geographic location, water maybe very hot out of the tap. This requires a lot of cold water to be added with it in order to attain a useable hot water temperature. The addition of too much cold water will overpower hot water demand from the tankless water heater. This slows the flow within the tankless water heater, decreasing it below activation point, which shuts off the heater. The end result is nothing but cold water coming out of the outlet. See: TWH E2-04 Overcoming fluctuating temperatures [PDF].
2. FLUCTUATING WATER PRESSURE
   If the water pressure in the home is erratic and the water flow is not consistent while a tap is opened, then the temperature of hot water will fluctuate. The minimum water pressure for the home should be 30psi or greater. For people on well systems the minimum pressure range is 30-50psi. The use of a pressure reducing/regulating valve is an effective way to maintain constant water pressure to the water heater. Watts brand 25AUB- ¾" or N35B-¾" pressure reducing/regulating valves or equivalent is suggested.
3. CHANGING FLOW RATE
   Major changes in flow rate can adversely affect the output water temperature of the heater. Increases from one major fixture running to multiple fixtures running at one time can cause the temperature to fluctuate. Stay within the heater's specifications. See: TWH E2-03 Outlet temperature vs. flow rate [PDF].

From the Bosch website.

Good Luck,
Gary

Sounds like there is not enough flow in order to keep the burner in the heater firing continuously

If steam or excessively hot water shoots out of faucets, or you hear boilding sounds inside the hot water heater, the appliance may not be shutting off at its set temperature. The job of the temperature- pressure (TP) relief valve is to release excessive pressure from steam buildup, but this valve may have been improperly installed, or it may be faulty. Either way, this condition can be dangerous, both because hot water and steam can scald people and because enough pressure could build up to rupture the water heater. Immediately turn down the water heater’s temperature setting and allow the water to cool.