How to close a pool for winter

When to close

Wait until the water stays below about 60 to 65°F. Algae grows slowly in cold water, so closing then gives it the least chance to bloom under the cover. Close in a warm spell and you'll often open to a green pool in spring.

In places that rarely freeze, many owners don't close at all: they keep the pump running on a shorter schedule and keep testing. If you do close in a mild climate, the steps below still apply, and the chlorine step matters even more.

A week before: test and balance

Do a full test, including CYA and calcium, and adjust anything that's out. Water left alone for months needs to be balanced to start with.

Lean slightly toward scaling, not corrosive. Cold water is more aggressive: the same water that's balanced at 80°F is corrosive at 40°F. For plaster pools, aim for a water balance score (LSI) of 0 to +0.3 at your winter water temperature, nudging calcium or pH up if needed. The chlorine calculator covers chlorine; Pool Predictor Pro shows LSI at any temperature.

If the water is at all cloudy or you've had algae, SLAM it and pass an overnight chlorine loss test before you close. Closing a pool that isn't truly clean is the main reason pools open green.

Closing day

1. Clean the pool. Skim, brush walls and floor, and vacuum. Leaves and debris left in the water feed algae all winter.
2. Raise chlorine to your SLAM level (40% of your CYA) with liquid chlorine, and let it circulate for a few hours. It falls slowly in cold water and keeps the pool sanitized well into winter.
3. Optional: add a winter algaecide. If you use one, choose a polyquat 60 product. Avoid copper-based algaecides, which can stain.
4. Remove floaters and tablets. A tablet left sitting in still water bleaches the liner and eats away at whatever it touches.
5. Clean the filter. Backwash a sand or DE filter, or hose off cartridges.
6. Lower the water if your cover needs it (see the sections below).
7. Winterize the plumbing and equipment, then put the cover on.

In-ground pools: blowing out the lines

Water left in underground pipes can freeze and crack them, which is an expensive dig to fix. Blowing out the lines pushes the water out and seals the pipes with air.

1. Lower the water level to just below the return jets if you'll plug them with the water out of the way. With a mesh safety cover, most manufacturers call for 12 to 18 inches below the tile; with a solid cover, 4 to 6 inches below the skimmer. Follow your cover's instructions, and never drain a vinyl pool further than you need: the liner can shrink and pull out of its track.
2. Turn off the power to the pump, heater and any timer at the breaker.
3. Use a pool line blower or a shop vac set to blow, connected at the pump or filter. Low pressure and high volume is what you want. An air compressor at high pressure can damage pipes and fittings.
4. Blow the returns one at a time. Run air until you see a steady stream of bubbles at each return jet, then screw in a winterizing plug while the air is still blowing, so water can't flow back in.
5. Blow the skimmer lines the same way until air comes out of the skimmer, then plug the skimmer line and put a Gizzmo (a collapsible plug that absorbs ice expansion) in the skimmer.
6. Main drain: blow air until bubbles come up from the drain, then close its valve while the air is still flowing. That traps an air lock in the pipe, which keeps water out. Don't try to plug a main drain underwater.
7. Optional: pool antifreeze. In very cold areas, some owners add pool antifreeze to the lines after blowing them out. Use only pool antifreeze (propylene glycol), never automotive antifreeze, which is toxic.

Winterizing the equipment

• Pump: remove the drain plugs and let it empty. Store the plugs in the pump basket so they don't get lost.
• Filter: open the drain, remove the pressure gauge, and set a multiport valve to Winterize or Closed. Cartridges can be cleaned and stored indoors.
• Heater: open the drain plugs and blow out the heat exchanger following the manufacturer's instructions.
• Salt cell: remove it, rinse or clean it, and store it indoors. Put the union back with an empty spool or plug if your system needs one.
• Chlorinator or feeder: empty out any tablets and drain it.

Above-ground pools

Above-ground pools are simpler because the plumbing is above ground and easy to drain.

1. Don't drain the pool. The water holds the walls and liner in shape. At most, lower it just below the skimmer and return.
2. Plug the openings instead: install a skimmer cover plate and a winterizing plug in the return fitting. That way you can leave the water at its normal level, which supports the walls.
3. Disconnect the hoses from the pump and filter and let them drain.
4. Drain the pump and filter, open their drain plugs, and store them indoors if you can. Freezing water inside a pump or filter housing cracks it.
5. Put an air pillow in the middle, inflated about three-quarters full and tied in place. It takes the pressure of ice off the walls and helps rain and snow run off the cover.
6. Fit the cover and tighten its cable and winch, adding cover clips on a windy site.

Through the winter

• Pump rain and snowmelt off a solid cover so the weight doesn't stretch it or pull it into the pool.
• Keep leaves off the cover.
• In milder areas, peek under the cover once or twice. If chlorine has run out, add some before algae gets a start.

When spring comes, see how to open a pool.