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Editorial Article

A Cold Plunge Is a Public Spa, and the Code Assumes a Thousand Gallons

Updated October 18, 2026

The inspector settled it in a guidance document, not a market study

On 20 March 2025 the Indiana Environmental Health Association published a cold plunge guidance document written by one of its sanitarians. It is eleven pages long, it was circulated to local health departments, and it does more to explain the economics of the cold plunge category than any market study published this decade.

Its finding is unambiguous. Both the Indiana Swimming Pool Code (675 IAC 20) and the Indiana Public and Semi-Public Swimming Pools Rule (410 IAC 6-2.1) apply to prefabricated and built-in-place cold plunge units that are not drained and refilled between users. The Indiana Department of Homeland Security determined that such units, when placed for public or semi-public use, are to be regulated as public spas under 675 IAC 20-1.1-18(h) — the definition of a public spa being a spa that is neither for the sole residential use of two or fewer owner families and their guests, nor operated for medical treatment or physical therapy under medical supervision. And the federal Virginia Graeme Baker Pool and Spa Safety Act applies to any unit with a submerged suction outlet.

So the vessel is a spa in law. It is not a wellness device, not a fitness appliance, not a bathtub with a chiller bolted on. And the code it now has to satisfy was written for something else entirely.

What that does to the market is the subject of Global Cold Plunge & Recovery Equipment Market Size, Share & Forecast 2026–2033: three revenue pools sized separately, a commercial ticket 30.67 times the residential one, and a forecast that is almost entirely a unit story because the compliant product is the cheap product.

Two numbers in the code that no plunging product can meet

The document lists what Indiana’s spa rules require, and then lists the requirements the IEHA’s own cold plunge committee recommended not be enforced, describing them as “challenging if not impossible for these units to meet.”

The two that matter are both dimensional.

The volume floor. 675 IAC 20-3-2(c): public spas shall not be less than one thousand gallons in volume. One thousand US gallons is about 3,785 litres. A commercially sold plunge tub holds 200 to 400 litres. The code’s minimum is therefore between nine and nineteen times the size of the product. A public spa in Indiana cannot legally be as small as a cold plunge, which means the category does not sit inside the rule — it sits underneath an exception to it.

The turnover rate. 410 IAC 6-2.1-32(a) requires spa water to turn over once every half hour, and 675 IAC 20-3-7(a) requires circulation capable of turning the entire capacity in not less than thirty minutes. On a 200-litre tub that is 1,514 litres an hour, or roughly 25 litres a minute — trivially achieved. On a 1,000-gallon basin it is 7,570 litres an hour, which needs a pump and filter of an entirely different class. The turnover rule is only hard because the volume floor exists. The two requirements were drafted as a pair for a vessel with eight bathers in it.

The rest of the list reads the same way: a handrail or ladder for every fifty feet of perimeter (675 IAC 20-3-3(a)); a deck not less than four feet wide around at least half the perimeter (675 IAC 20-3-4(a)); a Design Release before construction under 675 IAC 12-6; an emergency shutdown that cuts power to the pump (675 IAC 20-3-5(c)); and — the one that shows how little the code anticipated this product — both a surface skimming system and dual drain outlets at the lowest point (675 IAC 20-3-6(c)). The IEHA’s own note concedes that these small units “will usually have a surface skimming system OR a pair of main drains separated by three feet, but not both.”

So the product is designed around the exemption, not around the water

Here is the part of the document that is really a product specification. The IEHA describes the units it is talking about as “low-volume, prefabricated, single-user cold plunge or cold immersion therapy units that are not drained and refilled before each use and plug into a 110V wall outlet,” and notes that Indiana’s Department of Homeland Security has taken the position that these specific units, designed for one user at a time, of relatively low water volume (less than 200 gallons), which plug into an existing 110V wall outlet, do not need the Design Release every other spa must obtain.

That sentence is the category’s engineering brief, and it is not written in the language of thermodynamics. It is written in the language of permits. Four clauses — prefabricated, single-user, under 200 gallons, 110 volts — and every one of them is a constraint on performance rather than an optimisation of it.

The most binding is the last. A standard North American 110-volt branch circuit on a 15-amp breaker delivers at most about 1,650 watts, and in practice a plug-in appliance is designed to draw well under that so it does not nuisance-trip the circuit. That caps compressor size, which caps cooling capacity, which caps how fast the tub recovers temperature after a body gets into it, and how deep the setpoint can go in a warm room. It is why the category’s chillers cluster at half a horsepower and one horsepower — not because 1 HP is the right answer, but because it is the largest answer that fits a wall socket. A product manager who wanted a faster plunge would have to specify a 220-volt circuit, and would then have built a constructed public spa.

Cold water breaks the disinfection model the residual was written for

The chemical requirements in 410 IAC 6-2.1 are specific, and they are specific about warm water. Continuous automatic disinfection with an EPA-registered chemical imparting a measurable free residual (410 IAC 6-2.1-30(a)); a free residual for spa pools of 2.0 to 7.0 ppm chlorine or 4.0 to 10.0 ppm bromine (410 IAC 6-2.1-30(b)); pH between 7.2 and 7.8 (410 IAC 6-2.1-30(k)); no chlorinated isocyanurates indoors; superchlorination to breakpoint when combined chlorine reaches 0.5 ppm; testing for pH and disinfectant daily before opening and at least twice more during hours of use; total alkalinity weekly; combined chlorine twice a week; and all results recorded, kept for a minimum of one year, and produced on request (410 IAC 6-2.1-38(a)). Plus a bacteriological sample taken weekly from each unit, with a satisfactory result obtained before that unit opens for use (410 IAC 6-2.1-31).

The guidance document states the problem in its own overview, and it is a problem the code has no answer for: the concerns raised in the literature include “the reduced effectiveness of disinfection in colder temperatures.” Every one of those residual figures was set for a spa held at 38 to 40 degrees Celsius, where hypochlorous acid reacts quickly and bather load is the dominant chlorine demand. Hold the same water at 5 degrees and the chemistry that made 2.0 ppm sufficient no longer behaves the same way, while the number the inspector reads off the test kit is unchanged.

The commercial consequence is not a chemistry problem, it is a labour problem. A residual range that must be verified before opening and twice during use, with a weekly outside laboratory sample and a year of retained records, is a daily task performed by a named person at a specific site. It cannot be shipped inside a tub. Which means the operating cost that decides whether a plunge installation is profitable is not the chiller’s electricity — it is the fifteen minutes someone spends with a photometer and a log book before the first client arrives, and the possibility that nobody does.

The chiller is a heat pump, and the duty cycle is the bill

The physics is small enough to do in full. Raising the temperature of 200 litres of water by one kelvin takes 837 kilojoules. Dropping 200 litres from 23 degrees to 5 degrees means removing about 15,070 kilojoules of heat, which is 4.19 kilowatt-hours of thermal energy that has to be moved.

What it costs to move it depends on a coefficient of performance, and there the only operating figures on the record come from the manufacturers themselves — a chiller supplier’s own engineering note for a 1.0 HP unit on a 200-litre tub gives roughly 0.8 kW of input during the active phase, about 3.5 hours to pull the water from 23 to 3 degrees for around 2.8 kWh, then a hold state of about 40 watts for the circulation pump and ozone, with the compressor cycling on briefly as heat enters, consuming roughly 2.0 kWh over the remaining 20.5 hours. Total, about 4.8 kWh a day. At the US average residential rate of about 15 cents a kilowatt-hour, that is 72 cents a day — and 1,752 kWh and roughly 263 dollars a year.

Two things about that number are more interesting than the number.

First, the split matters, not the total. Only 2.8 of the 4.8 kWh is the product doing its job. The other 2.0 kWh is heat leaking back in, and heat ingress scales with the temperature difference between the water and the room, the insulation of the shell, and whether there is a lid on it. A plunge in a 26-degree gym in July is not the same appliance as the same plunge in a 19-degree garage in November, and the difference lands entirely in the hold state. The figure a buyer is quoted — a day’s electricity for less than a dollar — is a temperate-climate, well-insulated, lidded number.

Second, the machine rejects its heat into the room it is in. A chiller moves heat; it does not destroy it. That 4.19 kWh per cooldown, plus everything the compressor removes in the hold state, is discharged through a condenser into the surrounding air. Put a plunge and a sauna in the same small room — which is precisely what a recovery studio does — and the chiller is a heat pump running against a heater, in a space where the second law is not on anybody’s side.

The refrigerant inside is on its own schedule

There is a second regulatory clock in the product, and it is not about water. Annex IV to Regulation (EU) 2024/573, the fluorinated greenhouse gas regulation, lists the placing-on-market prohibitions. Point (4) prohibits, from 1 January 2025, any self-contained refrigeration equipment — except chillers — containing fluorinated greenhouse gases with a GWP of 150 or more, except where required to meet safety requirements at the site of operation. Point (3)(c) prohibits, from the same date, commercial refrigerators and freezers containing other fluorinated greenhouse gases with a GWP of 150 or more. And point (5)(c) reaches chillers themselves from 1 January 2030.

Which puts the plug-in plunge chiller in an awkward position that has nothing to do with how it performs. On the widely used GWP values, R-410A sits at about 2,088 and R-32 at about 675. Both are far above 150. Whether a given unit is caught in 2025 as self-contained equipment or in 2030 as a chiller is a legal question about how the appliance is classified; either way the destination is a refrigerant with a GWP under 150, and the practical answer is propane — R-290, GWP of 3 — which is flammable, and which therefore drags the design back to the electrical and charge limits that the 110-volt, single-user, low-volume envelope was already built around.

That is the structural point worth carrying out of all of this. The pool code, the electrical envelope and the refrigerant phase-down are three unrelated regimes, and they push the same product toward the same place: small, sealed, single-user, low charge, natural refrigerant, sold as an appliance rather than built as a facility. The category’s form factor is not a design preference. It is the intersection of three compliance perimeters.

The evidence supports a warmer bath than the one being sold

Then there is the uncomfortable part. A 2025 network meta-analysis in Frontiers in Physiology pooled 55 randomised controlled trials of cold water immersion after exercise-induced muscle damage — 42 reporting delayed onset muscle soreness, 36 reporting jump performance, 30 reporting creatine kinase — and ranked the doses.

The dose that best reduced soreness was 10 to 15 minutes at 11 to 15 degrees Celsius, with a standardised mean difference of −1.45. The dose that best improved jump performance and best reduced creatine kinase was 10 to 15 minutes at 5 to 10 degrees — SMD 0.48 and −0.90 respectively. The authors’ own recommendation is to use both, for different purposes.

Read those two sentences together and a marketing problem appears. There is no single setpoint that is optimal for everything, the optimum for the thing customers complain about is the warmer of the two, and the industry sells the coldest number it can print — 3 degrees, 1 degree, ice. The parameter the category competes on is the one the evidence ranks second for the outcome buyers actually care about, and it is the parameter least likely to be verifiable in a showroom.

And the customer with the strongest reason to buy is the one with the strongest reason not to. A 2024 systematic review and meta-analysis in the European Journal of Sport Science, titled with the conclusion it reached — “Throwing cold water on muscle growth” — found that post-exercise cold water immersion attenuates resistance-training-induced hypertrophy. Someone lifting to add muscle is trading adaptation for soreness. That is a legitimate trade and a great many people will make it. It is not a trade you put on the front of the box, which is why the category’s vocabulary drifted from “recovery” toward “wellness,” “resilience” and “mental clarity,” where the claim is subjective and the evidence stretches further.

The number I will not give you, and what can be counted instead

I am not going to publish a cold plunge market size. The perimeters do not reconcile: a tub, a chiller, a filtration package and a service contract are sold by different suppliers at different margins; a meaningful share of “units” are rotomoulded shells with a third-party chiller bought in; and the commercial segment’s revenue is a per-session fee at a studio, which is a different business from a five-thousand-dollar appliance sale. Any single figure would average four purchases and two business models.

What can be counted is the compliance perimeter, and it is unusually legible. A public installation has a jurisdiction, a code section, a residual range, a turnover rate, a weekly bacteriological sample and a retained log. Those are the line items that determine whether the operator can open, and they are public.

Which points at where the money in this category probably sits — and it is not in the tub. A rotomoulded shell is a commodity. A 1 HP chiller is a bought-in component. The recurring, defensible, high-margin work is the water chemistry programme, the daily test and log, the weekly sample, the quarterly service and the sanitiser supply — a service business with route density, exactly like commercial pool care, attached to an installed base that legally cannot be left untested.

The risk to that installed base is the same as its origin. The category exists because regulators decided that a small, single-user, 110-volt prefabricated unit is not a public spa for the purposes of a Design Release — a decision that the IEHA’s own committee wrote down as a recommendation, because the alternative was a code the product could not satisfy. That is a discretionary tolerance, not a settled rule. If a jurisdiction decides instead to enforce the volume floor, the deck requirement or the dual-drain requirement as written, the product does not need to be redesigned. It needs to be replaced.

Correction and sourcing note. The Indiana guidance document is advisory: the recommendations to waive specific code sections are the IEHA cold plunge committee’s, not the Department of Homeland Security’s, and a local health department may read its own code differently. The chiller consumption figures are a supplier’s own engineering estimates for one chiller on one 200-litre tub and should be read as such. All energy arithmetic in this article is derived from first principles where stated, and the GWP values quoted are the conventional assessment values rather than a reading of Annex I of Regulation (EU) 2024/573.

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Frequently Asked Questions

Is a cold plunge regulated as a swimming pool or a spa?

At a public or semi-public facility in the United States, generally yes. The Indiana Environmental Health Association's 20 March 2025 cold plunge guidance document states that both the Indiana Swimming Pool Code (675 IAC 20) and the Public and Semi-Public Swimming Pools Rule (410 IAC 6-2.1) apply to prefabricated and built-in-place cold plunge units that are not drained and refilled between users, and that the Indiana Department of Homeland Security treats them as public spas under 675 IAC 20-1.1-18(h). The federal Virginia Graeme Baker Pool and Spa Safety Act applies to any unit with a submerged suction outlet. The widely used exemptions — under 200 gallons, single-user, 110-volt plug-in — are a recommendation from the IEHA's cold plunge committee, not a statutory safe harbour, and a local health department may read its own code differently.

What water chemistry does a commercial cold plunge have to hold?

Under 410 IAC 6-2.1, a spa pool must be continuously and automatically disinfected with an EPA-registered chemical, held at 2.0 to 7.0 ppm free chlorine or 4.0 to 10.0 ppm bromine, at a pH of 7.2 to 7.8, with no chlorinated isocyanurates indoors, superchlorination to breakpoint when combined chlorine reaches 0.5 ppm, pH and disinfectant testing daily before opening and at least twice more during use, a weekly bacteriological sample with a satisfactory result before the unit opens, and all records retained for at least one year. The complication is temperature: those residual ranges were set for spa water at roughly 38 to 40 degrees Celsius, and the same guidance document flags reduced disinfection effectiveness in colder water, so the number on the test kit means something different at 5 degrees.

How much electricity does a cold plunge actually use?

The thermodynamics is small and fixed: raising 200 litres of water by one kelvin takes 837 kilojoules, so dropping 200 litres from 23 to 5 degrees means removing about 15,070 kilojoules, or 4.19 kilowatt-hours of heat. What it costs to move depends on the chiller. A chiller supplier's own engineering note for a 1.0 HP unit on a 200-litre tub gives about 0.8 kW of draw during a roughly 3.5-hour active phase for about 2.8 kWh, then a hold state of about 40 watts drawing roughly 2.0 kWh over the remaining 20.5 hours — 4.8 kWh a day, about 1,752 kWh and 263 dollars a year at 15 cents a kilowatt-hour. Note that only 2.8 kWh of that is the machine working; the rest is heat leaking back in, so the real figure depends on room temperature, insulation and whether the tub is lidded.

Why is cold water immersion disputed for strength athletes?

Two recent meta-analyses pull in opposite directions for the same buyer. A 2025 network meta-analysis in Frontiers in Physiology, pooling 55 randomised controlled trials, found the best reduction in delayed onset muscle soreness at 10 to 15 minutes in 11 to 15 degree water, but the best improvement in jump performance and the best reduction in creatine kinase at 10 to 15 minutes in 5 to 10 degree water — so no single setpoint is optimal for everything, and the optimum for soreness is the warmer of the two. Separately, a 2024 systematic review and meta-analysis in the European Journal of Sport Science, titled 'Throwing cold water on muscle growth', found that post-exercise cold water immersion attenuates resistance-training-induced hypertrophy. The customer with the strongest reason to buy is often the one with the strongest reason not to.