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

[Mary]

If the circulation pump (http://www.hottubworks.com/SpaParts/PumpsCirculation.php) is running you should feel water coming out of the return line in the spa. If you don't you need to verify that the pump is getting power from the circuit board. If you are but it is not pushing water but the motor is on you may have a clogged or broken impeller. To test the spa heater (http://www.hottubworks.com/SpaParts/HeatersLanding.php) you need to verify that it is getting power when the thermostat is turned up. If you have an clamp on amp meter you can clamp around one of the wires going to heater and see how many amps it is drawing while in a heat call.

Thanks for the reply Mary. I ended up looking at the Circulation pump because I could not see water coming out of the return line. After unwiring the spa, moving the heater, clamping hoses I got to the pump. The last owner had a lot of plant debris blocking the impeller but there was a massive calcium buildup. Pump is clean and calcium free now and the spa is running smooth. Later this month I am going to run spa purge to clean out the system so I can do the last fresh fill for the winter. Thank you again for the advice.