Keeping Your Pool Open All Winter: How to Protect It When a Freeze Hits

Not everybody closes their pool for winter. If you live somewhere with mild winters, you probably keep the pool open all year, enjoy the view from your warm kitchen, and feel just a little superior to the folks up north wrestling with winter covers.

Then the forecast drops "hard freeze tonight," and suddenly that pool full of water and those pipes full of water feel a lot less relaxing. Good news: an open pool can ride out a cold snap just fine if you do a few things right. Here's how, based on guidance from the companies that build pool equipment.

Quick heads-up: Every pool setup is a little different. Use these tips together with your equipment manuals. When they disagree, your manual wins.

Why freezing is such a big deal

Water expands when it freezes. The water sitting in your pump, filter, heater and pipes doesn't care that those parts weren't built for it. It expands anyway, and plastic housings and pipes crack. You usually don't find out until the thaw, when water starts showing up in places it really shouldn't.

The pool itself is rarely the problem. A big body of water holds a lot of heat. It's the small stuff, the equipment and the plumbing, that freezes first.

Rule #1: Moving water doesn't freeze (easily)

The single most important thing you can do during freezing weather is keep the pump running. Water that's moving through the system is much harder to freeze than water that's sitting still in a pipe.

1. Run the pump continuously whenever temperatures are near or below freezing, day and night. Yes, all night. Your electric bill will survive. A cracked filter tank might not.
2. If you have a variable-speed pump, don't leave it on its lowest whisper-quiet setting. Run it fast enough that water actually moves through every line. Many pumps and controls have a built-in freeze protection speed, so check your manual.
3. Get water flowing in every line, not just the main one. Spa lines, waterfalls, fountains, cleaner lines and any plumbing that only gets used now and then are the most likely to freeze. Open those valves partway so every pipe gets some flow.
4. Keep the water level normal so the skimmer doesn't run dry and the pump doesn't lose its prime in the middle of the night.

Let freeze protection do the worrying

If you'd rather not get up at 3 AM to check on the pump (reasonable), let a control do it for you.

• Pool automation systems usually have a freeze protection setting that uses an air temperature sensor. When the air gets close to freezing, it turns on the pump and the circuits you choose. The exact trigger temperature depends on the system, so look it up and make sure the feature is actually turned on. Plenty of people find out it was off the hard way.
• Check the air sensor. It should be in the shade and reading correctly. If it's damaged or giving weird readings, replace it before the cold arrives, like this air/solar temperature sensor kit.
• No automation? No problem. A timer with built-in freeze protection can switch the pump on when it gets cold, even outside its normal schedule. For example, this 240V freeze protection control timer. For a basic setup with a standard 24-hour timer mechanism, just switch it to run all the time while the freeze lasts.

Freeze protection is a safety net, not a magic spell. If it's going to be a hard freeze for hours, run the pump continuously anyway and check on it.

What about the heater?

Here's where opinions get spicy, but the safe basics are simple. A heater that's running with good water flow isn't going to freeze. A heater that's sitting with water inside and no flow can crack, just like a pump.

• If you're using the heater, keep the pump running so water keeps moving through it.
• If you're not using it, make sure water still flows through it when the pump runs. Don't close it off in a way that leaves water trapped inside.
• Many heaters have their own freeze settings and drain plugs. Your heater manual is the boss here.

Keep up with the boring stuff

Cold weather makes the pool slower and lazier, which is nice, but it doesn't mean you get to ignore it until spring.

• Test the water about once a week. Chlorine lasts longer in cold water and algae grows slower, so you'll likely need less sanitizer. But "less" isn't "none." A good complete water test kit or quick chlorine test strips will keep you honest.
• Keep the water balanced. Cold water can be harder on plaster and equipment when pH, alkalinity or calcium drift out of range. Keep them where they belong.
• Keep the filter clean so flow stays strong. Weak flow means less freeze protection.
• Empty the skimmer and pump baskets. Winter leaves are sneaky, and clogged baskets choke flow right when you need it most.
• Check pump lid and filter o-rings. Cold can make old rubber stiff and leaky, and a leak can let the pump suck air and lose prime. A fresh lid o-ring is cheap insurance.
• Keep an eye on the water temperature with something simple like a floating thermometer. Also, the polar bear one is seasonally appropriate. We don't make the rules.

A solar cover also helps the water hold onto heat between sunny days, which makes cold nights a little less dramatic.

Plan B: when the power goes out

Freezing weather and power outages love to show up together. If the power is out and temperatures are going to stay below freezing, your pump can't protect anything. That's when you protect the equipment the old-fashioned way: by getting the water out of it.

1. Turn off the power to the equipment at the breaker, so nothing starts up dry when the power comes back.
2. Open the drain plugs on the pump, filter and heater and let them drain. Keep the plugs somewhere you'll actually find them later (a zip bag taped to the pump works great).
3. Open the filter air relief valve so it drains fully.
4. Wrap exposed above-ground pipes and equipment with towels, blankets or pipe insulation. It won't stop a long hard freeze, but it buys time.
5. When the power comes back, put the plugs back in, close the air relief, prime the pump, and check for leaks before you walk away.

Don't drain the pool itself. The water in the pool is helping, and an empty pool brings a whole new set of problems.

If you're in an area where long freezes happen now and then, keep a couple of winter freeze plugs around. They're handy for plugging lines if you ever have to blow out the plumbing.

After the freeze

• Walk around the equipment and look for drips, cracks or puddles.
• Make sure the pump is running and holding prime, and the filter pressure looks normal.
• Put valves back to their normal positions if you changed them.
• Test the water and get back to your normal (slow, lazy, winter) routine.

The short version

Keep the water moving, make sure freeze protection is on and working, keep flow strong with clean baskets and filters, and have a plan for when the power goes out. Do that, and your pool will come through the cold snap looking a lot better than your tomato plants.

Need a part before the next cold front? Browse our pool parts and supplies and get it before the weather gets serious.