Heater Thermal Cut-off

I have a Tiger River Spa Sumatran (model N) It has been working fine for the last 6 months. I have not used it for 2 weeks and it is due for water change out. Water was cold and power light blinking. Troubleshooting section says it is either 1)GFCI and high limit thermostat 2) No-fault 6000 heater and heater thermal cut-off or 3) Silent flo 5000 circulation pump and circulation pump thermal cut-off. Solutions are the following: 1) clogged filter 2)blockage withing the plumbing 3)non-functioning heater circulation pump 4)power was not disconnected before draining 5) air lock in the plumbing lines. I drained the spa and followed refill procedure and ran spa and jets to bleed off any air. Once the heater gets to hot than same thing. Booklet only says it is one of the 5 problems. Once fixed it should work. 1)new filter 2)don't know 3)don't know 4)power was cut off for draining 5)I filled according to instructions and ran jets to instructions.

Once the heater cools everything is fine but will trip in about 15 minutes? How do I check 2 and 3 and is there anything I am missing that you can suggest? I even put a plunger over the inlet were the heated water comes into the spa and I can move water and can not see any blockages or hear any air pockets. HELP

Your getting a low flow of water through the heater.

Chances are you have a circulation pump thats not pumping enough water. The inlet you spoke of -should actually be the heater return line as the intake goes through the filters. So.. if your not getting a steady flow of water from that return line -- your circ pump is most likely dead or at least lazy. It should be moving one gallon every 30 seconds,andy less and the high limit could trip.