Safety regulations prevent hot tubs from exceeding 104°F (40°C). If your hot tub is not reaching 104°F, follow the steps below:
1) If your hot tub has a timer, make sure it is disengaged while troubleshooting.
2) Ensure the thermostat is turned all the way up. Confirm the spa water temperature with a separate thermometer in the water.
3) Does the heat indicator light stay on when the temperature is turned up? (The heat indicator light shows there is voltage at the heater.) If the indicator light stays on, proceed to step 4. If the light fluctuates on and off, proceed to step 5.
4) Check for excessive heat loss and increase insulation if needed. Make sure the element is the correct size for your hot tub and replace if it is too small.
5) If the heat light fluctuates on and off, check the pressure switch. Adjust the sensitivity or replace if necessary. If the pressure switch is fine, replace the thermostat. (Some thermostats can be calibrated. If your thermostat has a small Allen screw or flat screw on the back side, this is for calibrating. Start by turning a 1/4 turn clockwise to calibrate the temperature up. Wait and see how hot your spa gets and calibrate more if necessary.
It is also possible that your hot tub heater element is not working. If your pump is on for an extended period, you will generate heat from the pump. The motor will give off heat and heat is also created from the friction of the water going through the plumbing. Your spa may get higher than 70°F with your heater not working. If you think this is the case, you should read our article on troubleshooting a spa that is not heating.