I also have a 1999 Cal Spa, and I had drained for the winter, and refilled recently. Everything was working when I drained it. I refilled the tub this spring, and pumps, etc work, but it does not demand heat even when the tub is at 77 degrees. I replaced the heater (not the first time I had to do this), so I thought I was set. Same issue with a brand new heater installed. I saw in another post to check the pressure switch on the heater. That is also fine (checked with an OHM meter). It appears that the relay is not being tripped to turn on the heater.

Any other ideas? My local Cal Spa dealer is worthless.

I found what the issue was with my Cal-Spa. The pressure sensor checked out OK with an OHM meter, but I still had no power to the heater. On a suggestion from a local hot tub repair service, they suggested that I jump the pressure sensor connection on the circiut board. When I did that, the relay energized and he heater came on. It turned out that the connector cable was not making a clean contact on the circiut board. After cleaning the contacts on the board and the cable connector, and replacing the connectors that attach to the pressure sensor, the relay energized and the heater turned on. And I was 2 clicks away from ordering a reconditioned circiut board... so all is now well and the hot tub is almost up to full temperature.