How to troubleshoot a hot tub that is not heating - This article is for a mechanical spa pack (not one that has an electronic circuit board)
Step 1) Is there voltage at the element? (Is the heat indicator light on?) If there is no voltage, proceed to step 2. If there is voltage, proceed to step 3.
Step 2) Check to see if the contactor is open. If it is closed, test the switches between the relay and the element on the primary side. i.e. the high limit switch
If the contactor was open, then manually close it and test for voltage at the element. If there is voltage at the element, then replace the contactor.
If there is still no voltage at the element, a control switch or circuit on the coil side is open. Test for voltage on the load side of each switch to determine an open circuit. Or, jump from line to coil to determine the side of fault. i.e. check the thermostat, pressure or flow switch, pump switches, blower switch and the top mounted spa side control.
Step 3) If you have voltage at the element, test the amperage draw at the element. If there is no amperage draw, replace the heater element.
If there is amperage draw, calculate the proper amperage draw using Ohms law:
Amps = Watts ÷ Volts
For example, if you have a 5500 Watt heater (5.5 kW) and your heater is wired to 220 Volts, then the formula should look like this:
Amps = 5500 ÷ 230 (your answer calculates to 24 Amps)
If the amperage draw from your test is not close to the proper amperage draw, then replace the heater element.
If the amperage draw from your test is the proper amount, look for loose wires or intermittent switches which may cause your element to cycle on or off. i.e. the pressure switch.