My hot tub (coleman model 137) will not warm past 47 degrees. The pumps all run fine, the light that indicates the heater is working is red. Its below zero outside so I know the heater is working some what. Any help would be appreciated.

check circulation pump if running check pressure switch circuit make sure unit isnt on maintain mode or economy

Well the first thing is that it is inflatable and doesn't have the same insulation as a regular hot tub and number 2 keeping in a 65 degree environment on a concrete floor isn't helping either. The heater I'm certain is working but your variables are taking the positives away.

Bath tub heaters will only maintain the water temperature in the bath. They will only heat the water approximately 1-2 degrees an hour.
Whatever water temperature you start with is what you will be able to maintain.
There is a red indicator light on the front of each heater indicating whether or not there is power to the unit when the tub is full of water and the water pump is turned on.
If there is a red light on then it has power to it and is working.
This heater will never feel warm to the touch.
There are three different styles of Bath Heaters available.

• Tee Heater - installs on the discharge fitting on the pump
• In-line Pressure - installs anywhere in the plumbing line before any jet.
• In-line Vacuum Heater - installs before the suction of the pump

If the heater you purchased is installed in the wrong place it will not operate.

Rod
US Parts Center

make sure the heating element isnt touching the casing inside the heating tube. if its an indoor tub, make sure the filtration cycles are not set to constant. constant filter cycles on an indoor tub will generate residual heat from the circ pump running 24/7, causing tub to heat past 104 degrees

check to make sure all the jets are open, this can cause a restriction, that makes the tub think its the filter

Most hot tubs have a freeze protection that will lock you out and run the pumps full blast with no heater if the temerature is under 50 degrees F.The circulating pumps create heat until the water temp is above 50 degrees, and then the heater will come on after that.
Hot tubs do this to protect themselves and it is normal operation.

This is normal with the dream maker spa. The dream maker spas are poorly insulated, and only run 120 volts with a very small heater.
When the unit is first filled, the hot tub will not heat full time either. Most new hot tubs will have freeze protection below 50 degrees F. They will recirculate automatically using the friction and the pumps heat to warm the water above 50 degrees, and then the heating unit will kick on. These hot tubs do not heat while the jets are on high either, so once you are in it using the jets on high, the temperature will start to fall.

Now that you are up to 80 degrees, it should heat up fairly quick however.

To keep the Pilot running... TYPICALLY the instructions say hold the pilot down for 30 seconds to allow the thermocouple to assume control... If extending that 30 seconds a wee bit does not work and your "pilot" keeps going out...

You have indeed found your "trouble"

The obvious defferences being length (first
(see generic parts below)

Follow the instructions on the replacement &
- DO NOT overtighten...!!

https://www.bing.com/search?q=thermocouple+rinnai+REU-95GS&qs=n&form=QBLH&sp=-1&pq=thermocouple+rinnai+reu-95gs&sc=0-28&sk=&cvid=AE29392FA0A049CDA62B934C932F13A7

Generic
Amazon com replacement thermocouple

Carnac the Magnificent

The tub experienced an overheat situation. This may be due to leaving the t stat turned all the way up and not monitoring or the hi-limit sensor is defective. The hi-limit sensor will shut down the heating cicuit if temp gets above 105 D optimally.the fact that the heater light is on is a good sign that things are working at this point.Whether or not a suitable temp(100-103 D) will be arrived at is to be seen. If spa heat kicks off before set temp is reached then replace hi-limit sensor.