How to Keep a Cold Plunge From Freezing in Winter

A frozen plunge rarely ruins the tub. It ruins the things holding small amounts of water outside the tub: the chiller's heat exchanger, the pump, the filter housing, the hoses and the fittings. Water expands by about 9 percent as it freezes, and a fitting has nowhere to put that extra volume. So this guide goes in the order the damage happens: chiller, plumbing, tub.

Your chiller's manual decides first.

Chillers handle winter in two ways. Some must be drained and stored at freezing. A few are designed to keep running, using a heat mode or built-in freeze protection to keep the water moving and above 32°F. Here is what the manufacturers publish.

If your chiller isn't listed, look for three things in its manual: the minimum air temperature it can run in, whether it has a heat or freeze-protection mode, and what the warranty says about freeze damage. If the manual is silent, assume it has to be drained.

Insulation beats wattage.

The fastest way to see why is to run the numbers. Take a tub with about 40 square feet of outside surface (walls, floor and lid), water held at 35°F and air at 0°F. Using the standard building values for the thin layer of air on a surface (about R-0.68 in still air and R-0.17 in a 15 mph wind) and roughly R-5 per inch for extruded polystyrene foam board, steady heat loss works out like this:

That is a rough steady-state estimate, not a measurement of any tub. The pattern is the useful part: a bare tub in the wind loses heat faster than a livestock de-icer can replace it, and two inches of foam cuts the loss by more than 90 percent. Insulate first, then decide whether you need a heater at all.

Start with the lid.

Open water loses heat to the air and to evaporation, and it is where ice forms first. A hard, foam-core lid that seals at the rim is the first upgrade to make. For a 100-gallon oval stock tank, the Polar Protector hard insulated cover is an EPS foam lid with a skirt and a six-point strap system; the listing gives its size as 32 by 52.5 inches, so measure your tank before you order.

Under the lid, a floating bubble cover cuts evaporation and adds a second air layer. The Sun2Solar 8 ft round solar cover is pool cover material (12 mil) that you trim to your tub's shape with scissors. It is not a lid on its own and does nothing against snow load or wind.

Walls: foam board, then a skin.

Rigid foam board is the simplest real insulation for a stock tank or DIY tub. Extruded polystyrene runs about R-5 per inch and polyiso about R-6 to R-7 per inch, according to the Department of Energy's guidance. Build a box around the tub, or cut panels to the curve, and close the gaps. Loctite Tite Foam Gaps and Cracks is a can of expanding polyurethane foam for gaps up to 1 inch, which is the right job for it: sealing seams between panels and around the drain, not insulating the whole tub.

Reflective bubble foil is easy to wrap around a tub, but read the label carefully. The US Energy Products double bubble foil is a quarter-inch polyethylene roll, 24 inches by 25 feet, labeled R8. Federal Trade Commission staff have written that reflective insulation needs an air space next to the foil to perform; wrapped tight against a tub wall, it will do much less than the number on the roll. Use it as an outer skin over foam board, with a small gap, rather than as the only layer.

Wind barriers matter most on a bare tub.

In the table above, wind quadruples heat loss from a bare tub because it strips away the still air on the surface. Once the tub has two inches of foam, the same wind barely changes the result. So a wind barrier is the fix for a tub you can't insulate, such as a metal stock tank you also use in summer. A fence panel, a stacked-firewood wall or an outdoor privacy screen on the windward side helps. Keep it far enough back that snow drifting behind it doesn't bury the chiller's air intake or the tub's drain.

Circulation protects the water, not the plumbing.

Moving water is slower to skin over than still water, which is why some systems are designed to run all winter. But a pump only helps while it is running, and it adds nothing to the hoses, valves and filter housing outside the tub once the power stops. If your chiller is drained for the season, a separate pump in the tub is just another electrical device in the water. A thermostatic de-icer does the same job with no plumbing to freeze.

De-icers: the right heater for this job.

A stock tank de-icer is a sealed heating element with a built-in thermostat, made to keep livestock water open in winter. It is the right tool for a tub without a running chiller. It is not a pool heater: K&H says its de-icers keep water just above freezing and cannot be adjusted higher, and Allied Precision (API) says its floating models don't switch on until the water drops to around 40°F. Your plunge stays cold; it just doesn't freeze solid.

• Sinking or floating. A sinking de-icer sits on the floor of the tub, under a lid and out of your way. A floating one rides at the surface, where ice starts, but it can bump a tight lid. Convertible models let you remove the float.
• Plastic and rubber tubs. Use a model the maker rates for non-metal tanks, or fit the guard it requires. API's instructions make its 88R guard mandatory in non-metallic tanks so the element can't touch the wall; the 521G version includes it.
• Wattage. API rates its 1,000 W sinking de-icer for 70 to 100 gallons. A lid and foam cut what the heater has to do, so a well-insulated plunge needs far less than an open livestock tank of the same size.
• Running cost. Watts times hours: 1,000 W for 24 hours is 24 kWh. The thermostat cycles the element, so the real figure depends on how much heat the tub loses. The insulation table above is the best predictor of your bill.
• Don't pair it with a running chiller. A de-icer that switches on near 40°F and a chiller set to 39°F will fight each other. Use a de-icer once the chiller is drained and stored.

GFCI: put it in the outlet.

A GFCI cuts power when the current going out and the current coming back differ by as little as 0.006 amps, according to the Consumer Product Safety Commission. Outdoor receptacles have required GFCI protection under the National Electrical Code since 1973, but older homes and add-on circuits may not have it. Test the outlet's button before the season. If it isn't protected, have an electrician install a weather-resistant GFCI receptacle, such as this Amazon Basics weather-resistant GFCI receptacle, under a while-in-use cover that stays closed with the cord plugged in. API also wants the outlet right next to the tub so no extension cord is needed.

Draining and winterizing a tub you won't use.

If you're taking the season off, drain everything, not just the tub. Coldture's takedown steps are a good model for most systems:

1. Turn the chiller off and close the tub valves.
2. Drain the tub through a hose to a spot where the water won't refreeze into a slick patch on a path or patio.
3. Disconnect the hoses from the tub and the chiller and let them drain.
4. Empty the filter housing and the inlet strainer, then tip the chiller slightly to drain what's inside. Penguin's manual says to hold its chiller upside down with the ports facing the ground. Aquavoss uses a shop vac on blow to push water out of every line.
5. Dry the filter and store it, and the chiller, somewhere above freezing.
6. Dry the tub, cap the fittings and cover it. Don't stack heavy things on the cover.

Plumbing and fittings break first.

Hoses, quick-connects, ball valves and filter housings hold a few ounces of water each, sit in the air and have no heat source. They freeze long before the tub does. A cracked filter housing or a split hose barb often isn't obvious until spring, when the system leaks on its first run.

• If the system runs through winter, keep hose runs short and wrap them in closed-cell pipe insulation.
• After any hard freeze, inspect fittings and the filter housing before you power up.
• Low points in a hose run hold water after draining. Lift and empty them.

Salt won't save you.

Epsom salt does lower the freezing point, just not by an amount that matters. Using the standard freezing point depression math (water's constant is 1.86°C per mole per kilogram), one pound of Epsom salt in 100 gallons lowers the freezing point by about three hundredths of a degree Fahrenheit. Buying one full degree takes roughly 31 pounds per 100 gallons, and that's the best case. Table salt is about 7 pounds per degree per 100 gallons. Either way, check your chiller's manual before adding anything. Dynamic Cold Therapy's warranty, for one, excludes damage caused by improper water chemistry.

For keeping the water clean through a long cold season, see how to keep ice bath water clean. To size a chiller for next summer, start with the chiller sizing guide. If you're shopping for a tub that is built for this, Sauna Marketplace's cold plunge selection lists which tubs heat as well as cool.

FAQ

How do I keep my cold plunge from freezing in winter?

Follow your chiller's cold-weather rule first, which usually means draining it at 32°F. Then insulate the tub, keep a tight insulated lid on it, and add a thermostatic stock tank de-icer if the water still freezes.

How do I keep an ice bath from freezing without a chiller?

Use a stock tank de-icer rated for your tank material, plugged into a GFCI-protected outlet, under an insulated lid. Unplug it and remove it before you get in.

Can I put a stock tank heater in my cold plunge?

Yes, if the tub has no running chiller and the de-icer is rated for that tank material. It keeps water just above freezing, so the plunge stays cold. Never get in while it is plugged in.

Can I leave my chiller running in winter?

Only if the manufacturer says so. Penguin, Dynamic Cold Therapy and Aquavoss say to drain and store at freezing; Dundalk LeisureCraft allows running at a hot setting; Renu's Cold Stoic 3.0 is designed to run all winter at 45°F.

Will Epsom salt keep water from freezing?

Not in useful amounts. It takes about 31 pounds of Epsom salt per 100 gallons to lower the freezing point by 1°F.

What's the best way to insulate a cold plunge?

Rigid foam board on the walls and floor, gaps sealed with expanding foam, and a hard foam lid. Reflective foil only helps with an air gap beside it.

Is it safe to cold plunge in winter?

Cold air adds to the cold water, and slippery ground adds a fall risk getting in and out. Read our cold plunge dangers guide and temperature guide before plunging outside in freezing weather.