The Watkins No-Fault 6000 heater on my spa appears to be causing a ground fault, so I began searching for a new heater. What I found is the No-Fault heaters typically cost around $300, whereas common flow-thru heaters typically cost around $90-$130. Due to my No-Fault heater being sealed, there also appears to be no way to replace the element, whereas flow-through elements are readily available for $39.

If I reduced the 2" bulkhead fittings (on the flow-thru heater) down to 3/4" (matching the No-Fault hose fittings on my HotSpring spa) and adapted the 2 temperature sensors to fit the flow-thru heater, would a flow-thru heater work as a NoFault (low flow) replacement? Heaters I am considering are all 220 volt, 5500 watt, such as 50096 or C-2550-0119, so I expect roughly the same heat output would be generated. Due to the lower than normal flow rate through the flow-thru heater (in my application), the exit temperature would be higher, but should be no higher than what was generated by the original No-Fault heater. Is this a concern?

The newest generation of no-fault heaters are incredibly reliable and in my opinion are worth the extra money VS spending all the time trying to retrofit a modification that may or may not work for you.

Not sure how you would modify the 2 thermistors to fit the new set-up either. If you decide to try it I would like to hear how it turns out and see some pics.