Thanks for the replies.
Does the tub have a pressure switch or is it m7?
Well, the heater tube does have a pressure switch. Set to about 2 PSI if I'm reading it correctly.
Does the board have open contacts or the black sealed ones?
Sealed ones. The old board had open relays and I did actually find a sticking one, which is why I replaced the board.
reset the tub once it is running and heating test at the heater should be 220. Then turn the temp down below the water temp so the heater shuts off then test and make sure the contacts released
Good idea; I'll give that a try.
Also run the pump on high for 3 or 4 cycles and make sure it doesn't shut off
I'll do a specific test of this, but for what it's worth the tub never shuts off while in use (with pump on high).
How old is the pump? Do the jets have plenty of pressure?
As far as I know, the pump itself is original (so, 12 years old or so). I *think* I may have had the motor replaced several years ago, but it was not a DIY job so I don't have specific records. Maybe both parts are original.
The jets
seem to be working normally. On low speed the "shoulder massage" jets on one seat are not just dribbling out - they seem to be squirting water the way I always remember. On high speed when we're in the tub it all seems pretty normal. But I've had this tub for over a decade and I've had the OH problem for at least 3 years so maybe it's degraded and I just haven't noticed.
So if I'm understanding both of you correctly, the modes of failure we're considering are:
1. Flow is low (due maybe to aging motor and/or pump) and hi-limit is being triggered during heating
2. Motor is overheating and stopping, causing zero flow and tripping the hi-limit
3. Stuck relay is causing heater to remain energized after heating cycle is over (perhaps during filter cycles)
Since the tub water is, apparently, actually getting over 98F I'm leaning towards #3, or some other board failure. If the tub is actually at 102, and the panel reads 102, the heater and pump clearly operated longer than the required time. If it was just the hi-limit, I would expect to have normal (or low) water temp in the tub. I'll try the tests and post the results.
Thanks again.