How do you set the maax crescendo tub water temp?? my tub will only get to 91 degrees... help

the thermostat has an upper and lower setting at which point to turn the unit off and back on
most have a 10 degree variation between off and on so that a steady temp is maintained
check to see if the lower setting can be raised closer to the 80 degrees
if not consider a new thermostat with closer settings

Water heater can be added to any incoming water line, either on hot or cold.
http://waterheatertimer.org/How-to-install-under-counter-water-heater.html
Under-counter water heater located near tub will supply hot water just like ordinary household water heater. But it will not reheat water that circulates inside the spa tub.
Otherwise, a spa heater is made with electric element that can work with 120V or 240V. Water from tub circulates through tank that holds element. Element must be fully immersed in water.
To retrofit spa tub with this heater would require cutting water line near pump, running both ends of water line into tank with element.
Then of course you would need to retrofit adjustable thermostat to control when element is on. And integrate element operation into air control switch at tub that can override thermostat so element is not heating when tub is not in use.
Unless there is a pre-packaged kit to retrofit heater, I think adding heater would be difficult.

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

Check for crossover to eliminate plumbing problems:
http://waterheatertimer.org/Crossover.html

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

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

Id set on A then in a day or so run water from tap & check w/ thermometer even one used in oven ..

Yes you should be able to adjust the eyeball. Simply grab it with your fingers and move it where you want it to be. Some jet trim are adjustable from the outside face of the jet. take a hold of the outside of the trim and turn it both directions until you feel more water pressure come out. this means the jet is open.

How big is your bathtub? Is your water heater set on the hottest setting? What is the temperature of the hot water at the tap?

Keep in mind that hot water can cause severe burns. Children and those with diminished mental capacity can be seriously harmed by hot water.

The "40 Gallon" rating on the water heater is usually the amount of hot water the unit can produce in one hour. So the tank, and the amount of immediately available hot water, might be smaller than 40 gallons.

My bathtub is 2 feet wide by 1 foot deep and 4 feet long. This is 8 cubic feet or almost 60 gallons. So a 40 gallons of water at 120 degrees, mixed with 20 gallons of cold water (60 degrees), fills the tub with 100 degree water. My January cold water is 42 degrees, so it will take 40 gallons at 129 degrees to fill the tub. If the tub is cold to start it will take even hotter water.

So the water heater might be just fine.

Possible solutions:
1. turn the temperature up on your water heater- or the temperature control could be malfunctioning and not getting the water hot enough.
2. insulate your hot water pipes so that the water loses less heat on the way to the tub.
3. bigger water heater.
4. smaller bathtub, or less full bathtub.

We will just assume that "a" is the lowest temperature
to correspond to 120 degrees, while "c" for 160 degrees.
So try setting it at "a" and feel the hot water that flow.
Or it could be the other way. Just feel the hot water that
flow. It will be done in a few minutes. I hope this will help
you.

I think some one mis informed you, the normal temp on any water heater from the factory is 120, it should not go above that for reasons of sevear burns, as far as it't smart part it's just a name, you will lose a couple of degrees just comming torm the tank to the faucets, the water heater will allways keep the temp in the tank at what ever it is set at, it doesn't have the brains or computer to know what time of day it is but when the cold water comes in to the tank it comes thru a dip tube that goes to aprox 1 inch from the bottom of the tank, if you take a long shower and do not have a energy efficant shower head you can use up to 5 gals, per min of hot water where an energy efficant one only uses 2.5 or less and still has the same pressure at the shower head, the temp in your water heater tries to stay at the setting it is at depending on how much is used at one time meaning washing machine, filling tub or dishwasher at the same time, it lets more cold in as the hot goes out and as you know it can only heat up so much at a time and the cold water is just going across the bottom of the tank, so the name is just that no diffrent then any other water heater unless you went to a tankless water heater.

the pump just pumps, do you have a heater it would be about 2inch in diameter adn in line with a cord comming off of it that plugs in seperate from the pump that is the heater,