I understand the concept of airlocking the main drain. You can't plug it, so blow air until it bubbles, then close the valve at the equipment pad. Its like putting your finger over the top of a straw. On the other hand, we blow out the returns and skimmers, then plug them, so water cannot go back into the line. The conventional wisdom says to leave these valves open all winter.
Would there be any harm in closing the skimmer and retun valves at the equipment pad? That way, if one of the plugs fails for some reason, you have your "finger on the straw", and water cannot back up into that line.
If the valves are best left open during the winter, why is it okay to close the main drain valve? If its okay to close the valves, why not close the skimmers and returns?
Thanks for your thoughts!!