The hot tub heater keeps heating after it is reached temperature and turned off. How do you check the flow switch and adjust it possible solve this problem? Thank you in advance, Paul

It *may* not be a problem with the water heater itself necessarily. When you say "keep adjusting the water temperature when you're taking a shower" I presume you're talking about turning the shower hot/cold knob(s) and not the temp thermostat on the water heater itself.
If so, when it starts to run colder, turn the knob all the way to full hot. Is the water then very hot? If so, the water heater is working fine & doing its job, and the problem is likely to be in the shower faucet mixing valve itself. Sometimes a faulty one will expand the inner seals/cartridge as hot water flows thru it, and starts to (wrongly) restrict the hot water flow, thus causing the constant need to "turn the dial up a notch".

Try replacing heating element in hot water heater.

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

Bad thermostat is sticking.

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

Turn off all power to the hot tub as soon as you realize it is overheating. The spa should have an automatic shut off when the water temperature reaches about 112 degrees fahrenheit. Call your hot tub dealer or manufacturer for service options. There are a variety of conditions that could lead to the overheating of your hot tub, but may differ in various hot tub models and brands. Check the water level of the hot tub. Low water levels could be the culprit for the over heating, so you may just need to add more water to the hot tub.
Check the shut-off valves in the hot tub. If they are closed while the hot tub heater is on, it could cause the water to reach unsafe temperatures.
Clean the hot tub filter. A dirty filter could be blocking the flow of water, causing the water to sit in the hot tub and overheat.
Turn the power back on after finding the problem and talking to the hot tub dealer. The hot tub will probably not start back up until the water reaches a safe temperature

Information on the care of your product and troubleshooting is available here:

http://www.oasisleisure.ca/customer_care/SpaCare.html

Did you have a recent power outage? It is necessary to reset the temperature after a power outage.

Check the temperature setting on your hot tub; if it has defaulted do to a power outage reset to the desired temperature. If it is still set at your desired setting and is operating at 68 degrees then there is a problem that is preventing the tub from reaching the set temperature.

Some possible causes and actions are:

Pull filter (see if light comes on) / Replace filter
Knife valves not fully open / Open knife valves

Pressure switch cut of calibration / Adjust pressure switch
Faulty Pressure Switch / Replace pressure switch

Air leak, check pump suction line / Tighten pump union
Heating element corroded / Replace element

Faulty control panel / Replace panel

Find the Manual for you specifc model for more explansyion on Troubeshootig.

The problem with all that type of water heater is that once the hot tap is turned on sufficient flow must be maintained through the water heater to keep the main gas burner alight. If the heat adjuster is turned up to high, then the person using the hot water will turn it down to a low rate which means the main burner will turn off and result in cold water. Try setting the heat control to the lowest setting that still gives hot enough water. If you have a thermometer adjust control for a temperature of around 50 to 55 degrees Celcius. You may have to fiddle with fine adjustments to get the temperature and flow rates to maintain a constant output of hot water. You may still have intermittent problems. If the heater only supplies baths or showers you could set the temperatuer even lower (45 to 50) to ensure that the hot tap is used more than the cold which will maintain flow and therefore heat.

check to see that the flow switch is working properly, it may not have enough flow on low speed to allow the heater to turn on. Make sure your filter/s is/are clean. If you have a pressure switch instead of a flow switch, it may need to be adjusted or replaced.

'OH' means 'over-heat'.

Basically the high-limit temperature sensor has detected a temperature far in excess of what it should be.

Check the filter(s), water flow, pump etc....

Regards,
Ian.
http://hottub.synthasite.com