What size chiller for a cold plunge?

What size chiller for a cold plunge tub

A cold plunge chiller is a refrigeration unit that pulls heat out of the water as a pump circulates it through the tub. The size you need depends on how much water there is, how warm it starts, and how fast you want it cold. If you already know the size you need, go to our pick of the best cold plunge chillers.

Start with what the people selling them say.

Why horsepower is a poor guide

Look at the last column. Three chillers carry the same 1/3 HP label and draw 280, 600 and 780 watts. One of them uses nearly three times the power of another. They cannot all be doing the same amount of cooling.

Horsepower on a chiller describes the compressor loosely, and these listings show that sellers do not apply it the same way. Two figures tell you more.

Of the chillers we looked at, two makers publish a cooling capacity: TURBRO gives 9,553 BTU per hour for the FrostCore F85, and Active Aqua gives 4,020 for its 1/2 HP. If a maker gives no BTU figure, you are left with its stated tub size and cool-down time, and you should take both as claims.

Work out the size you need

Take 100 gallons at 70°F and a target of 50°F. The heat to remove is 100 × 8.34 × 20, which is 16,680 BTU. A chiller that removes 9,553 BTU an hour needs about an hour and three quarters, at best.

The maker’s own claim fits the sum. TURBRO says the F85 cools water to 37°F “in just 4 hours”. Four hours at its maximum is about 38,000 BTU, enough to take 132 gallons down by roughly 35 degrees, from the low 70s to 37°F.

The first cool-down is not the job

Sizing by cool-down time makes chillers look undersized, and mostly they aren’t. You cool the tub from tap temperature once. After that the chiller only replaces the heat that leaks in, and a small unit left running can hold a temperature it would take all night to reach.

That changes the question. A small chiller suits you if you set it and leave it. A large one suits you if any of these is true:

• The tub is over 100 gallons. There is more water to hold, and more surface gaining heat.
• It lives somewhere hot. A garage in summer or a sunny patio pushes heat in all day.
• You want it under 45°F. The colder the water, the faster it gains heat and the harder the chiller works.
• Several people use it back to back. Each body warms the water, and the chiller has to recover between sessions.
• You refill often. Every refill is another first cool-down.

Insulation works on the same sum from the other side. A lid and a shaded spot cut the heat coming in, which lets a smaller chiller do the job. The running costs for each size are worked through in what a cold plunge costs.

What temperature can a chiller reach?

Every chiller in the table claims a minimum between 36 and 42°F, which is colder than you need. The range medical sources recommend is 50 to 59°F, and our guide to cold plunge temperature explains why colder brings no measured benefit.

A stated minimum is a best case. The Pod Company gives 41°F for its Pod Chiller on one page and 39°F on another. AS ColdPlunge’s listing says 42°F in its description and 39°F in its spec table. Read the minimum as “the low 40s, in a cool room”.

Is a 1/3 HP, 1/2 HP or 1 HP chiller right for you?

Our picks at each size are in the best cold plunge chillers. The one 1/2 HP spec sheet we have read is the Active Aqua’s: 4,020 BTU an hour at 510 W, for 90 to 172 gallons. By the sum above it takes 100 gallons from 70°F to 50°F in a little over four hours at best, against an hour and three quarters for the TURBRO.

One more check before you buy a large unit. The TURBRO F85 can draw up to 14 amps, which is close to the limit of a standard 15-amp household circuit. It wants a socket that nothing else shares.

Common questions