Step 1. Are there any errors ? Check the control pad of your spa to ensure it is set to whichever mode you prefer (usually STD, Standard, or ECN Economy), if a mode such as SLP is flashing on screen, the temperature will be kept at the lowest possible temperature before the spa freezes, therefore the heater will not appear to be working. Check for error messages such as LF or FLO, these are issues with the flow of spa water getting to the heater and this as a safety feature, most spas will cut off the heater, when insufficient water is flowing through it. To overcome flow issues, check your spa filters are clean and nothing is blocking the pipework within the filter housing. To overcome a flow issues, remove the spa filters, turn the hot tub off, then turn it back on again and run without filters, to diagnose the issue, if the spa runs fine without filters for 5 minutes, it is a problem with your filters, if it doesn't.....
Step 2. If all the above steps are OK and the spa is still not heating, it could now be an issues either with the control board not getting power to the heater element (rare) or the heater element itself damaged by scale and not operating (more likely). The best way to test a heater element is using an electrical tester. Turn off and remove all power to the spa, then remove the heater, you can then test hot tub heating elements for integrity, use an ohm meter on its lowest setting. With the meter's test leads, measure the resistance between the two terminals. The acceptable resistance range for heaters used in most hot tubs is between 9 - 12 ohms (a few may be as high as 25 ohms, depending upon kilowatt rating).A reading which is too low indicates a bad unit: short circuit. A very high (or infinite) reading indicates a bad unit: open circuit or limited conductivity. You will see either a burnt out element or scale damage which is limiting the heaters capability.
Step 3. If, after checking for any control errors, and inspecting the heater element, the error is still not visible, it is more than likely that it is a control board issue. By removing the faceplate to the control board you will see the layout on the back, you can then check the power source to the heater and see if any issues are present on the board itself.
These steps are more than likely to diagnose heater issues on a portable hot tub.
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Happy Hot Tubbin' !