If your spa heater is slowing down and things are getting chilly, there are a few steps you can take to determine what the problem might be. Before you replace your entire unit, make sure the heater's had enough time to do its job, check that the switches are working and the power supply is connected correctly, and if necessary, switch out the heating element.
Step 1 - Warmup Considerations
It takes many hours for a spa heater to make much difference in the water temperature. Most heaters will only warm the water by two or three degrees per hour, which could mean that heating the spa could take as much as a full day to complete. If the water seems to be warming up slightly, leave it alone for a few hours and check it again. Hopefully, you'll find that the process is slow, but there are no other problems.
Step 2 - Check the Switches
Check the switches for the hot tub heater and the motor or pump. The heater can't function properly unless the pump is running and water is circulating in the spa. Keep in mind that most supposed faults turn out to be some simple problem such as a switch that is caught partway between on and off settings. Turn off all of the switches and turn them back on again, making sure that they're fully engaged in the in ON position.
Step 3 - Check the Voltage
Another possible reason that the heater's not working properly is that it's not receiving the necessary voltage. Check whether the heater requires 110 or 220 voltage, then make sure it's connected to an appropriate circuit breaker. Generally, a 220 volt circuit breaker takes up a double slot in the panel box or resembles 2 breakers that have been slaved together.
Step 4 - Replace the Element
If all else fails, you may have to replace the heater element. On most units, the element can be accessed by removing two to six screws, setting the cover aside and pulling out the element. This will vary from model to model, but the new element will come with specific instructions. Make sure you replace the heater element with the same make and model to avoid the potential for severe electrical problems.
Step 5 - Replace the Heater
If you know the element is in good working condition, and you've verified that there's proper power coming to the unit, your only recourse may be to replace the unit completely. It is rare for a spa heater to become unusable, but it can happen from time to time. Remove the old one and install a replacement. If this still doesn't solve the problem, you may have a wiring issue between the switch and the heater.