I have an in-ground pool and spa that use the same equipment. I closed the pool in September and have left the spa up and running. I currently have it set to run every day from 7pm-8pm (after dark when the temp is starting to drop outside) and have the heater set at around 85 (higher if we're going to use it that day). With our hot tub cover sitting over it, it keeps it mostly warm throughout the day, so by 7pm when it kicks on it still reads about 75 degrees. The hour I give it to run is mostly to circulate the water as it only takes about 10 minutes to warm it back up to 85 and the heater shuts off.
So anyways, now that much colder weather is here, just starting to get into freezing temperatures, I started worrying. Just because the water in the hot tub is still 75 degrees doesn't mean the water in the pipes/filter/pump/etc is that warm. So I'm worried that the water in the lines could actually be sitting near ambient temperature, so if it drops below freezing, the water in the lines will still freeze, regardless of how warm the water in the spa is.
So the obvious solution is to run the system 24/7 if the temp drops outside, but I'd rather not if I can help it. I could set a second timer to run a few hours during the coldest part of the night to make sure nothing freezes if that would help. But what happens to the water in the lines when the system is shut off? Will the water in the pipes expand back into the hot tub, no damage caused (as long as the water there is still nice and warm)? Or is it stuck in the pipes and freezes there, causing pipes to burst or damage to my heater/filter/pump? And I assume the thickness of the pvc adds some minor insulation, so can the water in the pipes actually drop from 85 degrees to <32 overnight?
Am I worrying over nothing, as long as I'm running the warm water through the system daily, the water in the lines shouldn't be able to drop to below freezing that fast?