A shop vac sucks or blows air depending on the setting used on the machine. Shop vacs help close a pool by blowing air into the swimming pool's plumbing lines, removing 99 percent of the water in the pipes. This prevents the risk of water in the pipes freezing and causing damage. Hooking up a shop vac to the pool for use in closing involves setting the device to blow air and attaching the vac's hose to the appropriate pool plumbing line.
Position the shop vac near the pool's filter system. Ensure the vac has power and is set on the "blow" setting.
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Hook up the shop vac's hose end to the return lines of the pool near the filter system. Alternatively, remove the pump basket and hook the vac's hose end up to the pump's drain line. Turning the shop vac on while in these positions pushes water out of the return lines. Run the shop vac until a constant stream of air bubbles rises from the pool return lines. Plug the lines and shut off the vac.
Hook the shop vac hose up to any secondary lines such as the skimmer, main drain or slide lines. Turning the vac on in these positions clears the skimmer, main drain and slide line of water, prepping them for closing.