I went through this recently on my system (recently had Aqualink automation installed). This occurs if I enable Freeze Protection on the pool and spa, because it causes the system to cycle between pool and spa every so many minutes and (depending on your return valve settings) some water is lost from the spa on each switch.
I considered two separate ways of dealing with this.
First method is to 'cam' the return valve actuator so that when the system is in pool mode, it does return some small amount of water to the spa (like 90% pool, 10% spa). You lose some spa water on each cycling of the valves, but it gets replenished by the 10% spa flow when the system cycles back into pool mode. If you're already doing this and are still draining the spa on each cycle, then you either need more spa return flow (80/20, etc) or you have another issue.
I don't like this approach for a couple of reasons:
* It means I have go out and partially disassemble the valve actuator to adjust/cam every time a hard freeze comes and goes (because during normal weather, I do not want to return water to spa in pool mode).
* I personally don't like cycling the spillway during a hard freeze, I don't want it to ice up between the cycles. My stonework is already in less than perfect condition and I don't want to take any chances.
So here's what I did instead. Not saying this is best, just what I did. I only engage freeze protection on the pool (not the spa). This ensures the pump comes on at low temps, but does not cycle between pool and spa mode. Then, I used the manual rocker switch on the bottom of the return valve actuator to manually set it to 90% pool / 10% spa and then leave the rocker switch in the middle/neutral position. With the rocker switch in this position, the valve actuator ignores any automation calls from the system. So I have a gentle spillover 24/7 and no risk of the spa draining, barring other unforeseen issues. This keeps water flowing in all pipes except for the spa drain (spa suction into pump). I wrapped that one pipe with a towel, covered equipment with a tarp, and put a 40 watt incandescent bulb under the tarp using a hanging work light fixture. So far so good, knock on wood. With this approach I do have to visit the pad to setup the towel/tarp/light, but I don't have to touch my automation schedule, and I don't have to disassemble any Jandy valve actuators to cam them. When the hard freeze is over, all I need to do is remove the tarp and flick the rocker switch on the valve actuator so it goes back to its normal scheduled operation.
I wouldn't do all this for a quick overnight light freeze, but this is what I'm doing for our current multi-day hard freeze.
