A stock tank is the best-value cold plunge in this hobby and the worst thermal envelope in it. Galvanised steel is a radiator: it conducts heat in from the air, from the sun and from whatever surface it is sitting on, with no foam anywhere in the path. That matters here because every gallon ceiling published in this category is derived from an insulated plunge tub, and nobody publishes a figure for bare steel.
So the number on the listing is not the number for your build. Our read — stated as a read, not as a manufacturer rating — is to treat a seller's gallon ceiling as an insulated-tub figure and plan for a tank no larger than about two thirds of it. The tables below carry both numbers side by side: the published ceiling, and that ceiling derated. On that basis a 100-gallon stock tank wants a unit rated for 150 gallons, not 100.
Two other things change when the tub is a farm tank, and both are in the ranking. The chiller sits outside, and only one listing in the whole category publishes a weather rating. And the tank has no ports, so the loop runs over the rim, which makes a self-priming pump a requirement rather than a feature. We own none of these units and have measured none of them; what we do instead is read the published specs and reason from them out loud.
How we ranked them
- Published gallon ceiling, derated. The first filter, because sizing is the decision. 6 of the 8 chillers we track publish a ceiling; the two that do not cannot be sized from their own listing, which costs them.
- Published cooling capacity in BTU/h. The spec that actually predicts pulldown, and only 3 listings in this category state one. Horsepower is a proxy for it; BTU is the thing itself.
- Does it say anything about living outdoors? One chiller publishes an ingress rating. That is worth a rank on a page about a tub that cannot come inside.
- What else is in writing? Minimum temperature, filtration, voltage, warranty. Amperage and rated watts are blank on nearly every unit in this category, which is a real limitation when you are planning an outdoor circuit.
No scores and no pulldown times. Only one listing here publishes a cooling rate and it names no ambient temperature, so it is not checkable; inventing a time-to-temperature figure would be exactly the kind of testing theater this site exists to avoid.
Why the derate, in plain terms
A chiller does not cool water once. It removes the heat that keeps arriving. In an insulated pod with a lid, that arriving heat is small, so a modest compressor holds temperature while cycling on and off. In a bare steel tank in the open, heat arrives continuously through the walls, the base and the open surface — and the same compressor runs near-continuously to stand still.
That is why the fix is not always a bigger unit. The three biggest heat paths in a stock tank build, in rough order, are the open water surface, the uninsulated walls, and conduction through the base into whatever the tank sits on. A lid addresses the first for well under a hundred dollars; rigid foam or a reflective wrap addresses the second and third. Do those and a 1/2 HP unit is a genuine answer on 100 gallons. Skip them and a 1 HP unit is working hard. The detail is in the stock tank build guide and the sizing arithmetic in what size chiller do I need.
The plumbing problem a stock tank creates
Purpose-built plunge tubs increasingly ship with bulkhead fittings you can plumb a chiller into. A stock tank has nothing of the kind, so you have two options and they are not equivalent.
Hoses over the rimis the reversible one and what most builds use. The requirement it puts on the chiller is a pump that can lift water over the tank wall and re-prime itself after a drain — two of the picks here state a self-priming pump on the listing. Keep the runs short: every extra foot of hose outside the water is a foot of hose warming up in the sun.
Drilling a bulkhead gives a tidier loop and commits the tank permanently. It also introduces the one failure mode a plunge setup genuinely cannot tolerate, which is a leak below the waterline. If you go this way, use proper bulkhead hardware rather than a hose barb and sealant, and do it before the tank is in position rather than after.
Outdoors means GFCI, not maybe
A stock tank build puts a 120V appliance, a pump and several hundred gallons of water in a yard. Outdoor receptacles require GFCI protection under the code — this is not a preference and not a nice-to-have — and the chiller's own listing is almost never going to tell you about it. Nor will most of these listings tell you the amperage, so you cannot confirm from the spec sheet whether the chiller and anything else can share the circuit.
The practical sequence is: site the tank, then site the receptacle, then buy the chiller — not the other way around. Our cold plunge electrical requirements page covers what to ask for, and do you need a GFCI for a cold plunge answers the specific question directly.
Who should skip this page
If your tub is an insulated pod or a foam-walled barrel rather than a farm tank, the derate on this page is penalising you for a problem you do not have. Go to best cold plunge chillers, which ranks the same units on horsepower and published cooling capacity without the uninsulated-vessel assumption, or best cold plunge chiller for the money if price leads.
And if you have not bought the tank yet, it is worth pricing a complete insulated tub-and-chiller system against tank-plus-chiller-plus-lid. The bundled systems arrive matched, insulated and ported, and the price gap is smaller than the stock-tank route's reputation suggests once you have bought the lid and the foam.
