I've always resisted trying to give instructions on clearing lines, since there's a LOT of variation in what needs to be done, depending on locale and the individual plumbing layout at YOUR pool. I will offer some comments:
1. Using a compressor to blow out lines will not reliably clear the lines, but *may* break them. The full explanation is complicated, but the bottom line is the FLOW from a compressor is too low, and the PRESSURE is too high. If you feed high pressure low flow air into the system, you may succeed at clearing the lines (due to the expansion of the high pressure air) but you may also succeed in breaking or shattering one of the lines or fittings.
2. To plug return lines, you have to remove the in-pool eyeball and replace it with either a rubber plug, or a PVC threaded plug.
3. Use of antifreeze is optional, but if you do use it make sure it's food-grade and not auto-grade antifreeze. Here's a link to Amazon:
Propylene Glycol - Food Grade USP - 1 Gallon
Most RV supply stores also carry it.
4. Most valves need to be left in a half-open state; otherwise water trapped in the valve internals can cause freeze damage. Multiport valves MUST be left in the lid-up, between positions state. Gauges must be stored in doors.
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