Pool and Spa
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Do you know how old the hot tub is? I am thinking that the spa may be in economy mode. Most spas have a Standard mode which is what you want everything would work fine then you have an Economy mode and the spa will only heat when in it's normal filter cycle which only happens twice a day for two hrs so it can appear to not heat. Now without a meter to test voltage I do not know what you will be able to diagnose on your own but you need to test voltage at the control box coming in then there are several safety switches that control power to heater...the Thermostat, High Limit and pressure/flow switch all of these must be working correctly for the heater to get power from the circuit board. You can buy a multi meter rather cheaply at Home Depot or Lowes. You need to be sure that the pump that supplies the water through heater is running good and has good water flow, next check the topside display to see if there are any error codes on the readout like Fl or OHH or Dry. From there set the temp to 100 degrees and see if you get a light for heater to come on or an icon showing that the heater should be powered. Once that is good you need to test voltage across the two posts of the heater itself and you need to have 220 volts across both terminals.
No the heater light would be an icon on the readout of the display or a small red light just off of the display screen. In most Balboa control boxes to change from Standard to Economy and back you would press the temp button followed by the light button within 3 seconds of each other. Your spa would have to be 20 years old or more to have a reset button on high limit in the newer circuit board type controls there are no reset buttons. If NO error codes are in the display area then it has to be a mode setting or a bad heater but you need to test voltage at heater to note if there is power and no heat. Now when you say there are bubbles I need to to determine if you have water PRESSURE coming out of all jets. How many pumps do you have? If you have a smaller pump then that may be a circulating pump which would be the one that pushes water through the heater housing and that would be the one to look at. If you only have one pump then it is a 2 speed pump low and high speed and low speed would control heating and filtering and high speed would be for therapy.
OK well let me know what you find with the modes.
OK so now I see you have a Spa Builders control pack and that helps a little. Here is a link to the Spa Builders service manual now look at page 42 this goes over no heat in spa. This will show you how to test voltage at the heater. NOW KEEP IN MIND that you will need to have the power from breaker ON and the control box open to expose the circuit board and heater posts, so take EXTRA CARE when touching inside this box as you do have live power at this point (220) volts and it can zap you if not careful, with that said you can do this just go slow and pay attention. Here is link.
If the heater indicator light is not on or is not flashing follow the test for using a separate thermometer to test if temp reading is within 2 degrees from readout number. It may be a bad circuit board or temp probe.