Threaded plug or expansion plug?
Look at the hole. If the fitting has good threads, a threaded plug is the better answer: it screws in, the o-ring does the sealing, and it comes out in April with a quarter turn. If there is no thread, or the thread is chewed up from ten seasons of being cranked on, use a rubber expansion plug instead. That one does not care about threads at all, because it is a tapered rubber plug between two washers that swells outward against whatever it is sitting in when you turn the wingnut.
What do the numbers on an expansion plug mean?
They measure the plug, not the pipe, which is why the numbering looks arbitrary until somebody says so. Higher number, fatter plug. The trap: inch and a half pipe and an inch and a half fitting are two different holes, the fitting being wider, so the same spoken size wants two different plugs. Measure the hole you are actually plugging rather than the pipe it belongs to, and if you are between sizes take the larger one, because a plug that is slightly too big still seals and one that is slightly too small never will.
What is a duck plug for?
It solves the awkward part of blowing out return lines. Inside is a soft rubber bill that opens under pressure from behind and falls shut on its own: everything the blower pushes goes out, and the moment the blower stops the bill closes and nothing runs back down the pipe. Two things follow. You can clear all the returns in one go instead of working round them one at a time, and you do not have to drop the water below the returns first to keep the pool out of the lines.
Do the returns and the skimmer take the same thing?
No, and they are not even solving the same problem. A plug in a return line is keeping water out of a pipe. The skimmer is a box set into the wall or the deck with water sitting in it, and what threatens it is that water freezing and expanding against the walls of the box. That is a different failure and it takes the part built for it. Plugging the skimmer's outlet and calling it done leaves the box itself unprotected.
How tight should a winter plug go?
Snug, and then stop. On a threaded plug the o-ring is doing the sealing, not the thread, so turning it past the point where it seats is only stressing the fitting. On an expansion plug, hand pressure on the wingnut is what it was designed around, which is why the nut is oversized and there is no tool for it. Nobody publishes a torque figure for either kind. The test is whether it stops seeping, not how hard you can pull.
Can I use last year's plugs again?
Usually, and the rubber is what tells you. A threaded plug is good until the o-ring goes flat, cracks or goes missing, and the o-ring is replaceable on most of them. An expansion plug lasts until the rubber hardens or splits, and rubber that has spent a winter compressed in a pipe and a summer in a hot shed does both. Squeeze one before you trust it: if it does not spring back, it will not seal, and finding that out in November is cheaper than finding out in April.
