How to Size a Chiller for a Cold Plunge
Size a cold plunge chiller in three steps: water volume, heat load, then cooling capacity with margin for ambient heat and daily sessions.
Size a chiller in three steps. Measure the water volume in litres, calculate the heat you must remove for your temperature drop, then pick a cooling capacity that can do it in the time you are willing to wait, with margin for ambient gain and repeat use.
Most sizing mistakes start with the tub. A "120 gallon plunge tub" may hold less once you fill it to a sensible depth, and a tub sold by outer dimensions hides its real water volume. Everything downstream depends on that first number.
Step 1: get the water volume right
Measure the water you actually use, not the tub's outside footprint.
- Measure the internal length, width and water depth in metres.
- Multiply them to get cubic metres.
- Multiply by 1,000 to get litres.
- Or fill the tub with a flow meter or a known-volume container and count.
| Common tub sizes in litres | ||
|---|---|---|
| Tub described as | Approximate water volume | Notes |
| 80 US gallons | 300 L | Small home tubs |
| 100 US gallons | 380 L | Most common home size |
| 120 US gallons | 450 L | Deeper or wider tubs |
| 150 US gallons | 570 L | Large home or light commercial |
| Two 100 gallon tubs | 760 L | Manifolded, sized as one body of water |
Round up. A tub that holds 380 litres when full is a 400 litre job once you account for hoses and a filter housing.
Step 2: calculate the heat you must remove
The formula is the same one used for any body of water:
Energy (kJ) = litres × 4.186 × temperature drop in kelvin
Divide by 3,600 to convert to kilowatt-hours. That tells you how much heat has to leave the water, not how fast a given chiller will do it.
| Worked examples for a 400 litre tub | |||
|---|---|---|---|
| Start temperature | Target | Drop | Energy to remove |
| 24°C | 15°C | 9 K | 15,070 kJ (4.2 kWh) |
| 24°C | 10°C | 14 K | 23,440 kJ (6.5 kWh) |
| 30°C | 10°C | 20 K | 33,490 kJ (9.3 kWh) |
| 14°C | 10°C | 4 K | 6,700 kJ (1.9 kWh) |
These numbers ignore heat leaking in while you cool. Treat them as a floor.
Step 3: turn capacity into waiting time
Now divide the energy by the chiller's rated cooling capacity to estimate a pull-down time.
| Pull-down time for 400 litres, 24°C to 10°C (6.5 kWh) | ||
|---|---|---|
| Rated cooling capacity | Approximate pull-down | Best fit |
| 1.0 kW | About 6.5 hours | Small tub, patient owner |
| 1.4 kW | About 4.6 hours | Home tub, overnight cooldown |
| 1.7 kW | About 3.8 hours | Home tub, faster recovery |
| 3.3 kW | About 2.0 hours | Gym, studio, repeat sessions |
Divide your own energy figure by your own capacity. The table is only there to show the shape of the relationship.
Add margin for the real world
Every tub loses cold to its surroundings while the chiller works, and every user adds heat back. Add margin on top of the calculated capacity:
- +10–20% for a covered, insulated tub in a temperature-controlled room.
- +25–40% for an outdoor tub, a hot room, or an uninsulated tub.
- +50% or more when several people use the tub in a day, or when sessions run back to back.
That margin is not padding for sales. It is the difference between a chiller that holds a set point comfortably and one that runs flat out and never quite gets there.
A worked comparison: home tub vs gym
Home tub, 380 litres, two users a week. Target 12°C, indoor room at 22°C, insulated and covered. Energy for a 12 K drop is about 19,100 kJ, or 5.3 kWh. A unit in the 1.4–1.7 kW band pulls that down in three to four hours and holds it easily.
Gym, two 450 litre tubs, 40 sessions a day. Here the pull-down is irrelevant; the holding load dominates. Each session adds body heat and mixing, and the tub has to recover between sessions. Size for the combined volume with a large margin, or treat each tub separately.
The same formula, a different dominant term. That is why asking "what size for a gym" without numbers gets a useless answer.
The five mistakes that keep showing up
- Sizing from the tub's marketing name instead of litres.
- Using the compressor's horsepower as if it were cooling capacity.
- Ignoring ambient temperature and insulation.
- Forgetting that recovery time matters more than initial cooldown.
- Buying one oversized unit for two tubs without designing the manifold.
How to sanity-check your answer before you buy
Once you have a candidate, compare it against your own arithmetic rather than the seller's summary.
- Ask for the pull-down time for your volume and temperature drop, in writing.
- Recalculate it from the rated cooling capacity and see whether the two agree.
- Ask what ambient temperature the capacity figure assumes.
- Ask what flow the unit needs, and whether your pump and hoses deliver it.
- Ask what happens to recovery time with four users an hour.
- Ask which model they would choose for your numbers, and why.
If the supplier's answer is "the bigger one is better", you have learned something about the supplier. A good answer names the volume, the drop, the capacity, the margin and the limiting condition.
Frequently asked questions
How do I convert gallons to litres for chiller sizing?
Multiply US gallons by 3.785. A 100 gallon tub is about 380 litres; a 120 gallon tub is about 450 litres. Work in litres for the heat calculation.
What cooling capacity do I need for a 400 litre tub?
It depends on your target temperature and how fast you want to get there. For a 14 K drop, the water needs about 6.5 kWh removed. A 1.4 kW unit does that in roughly four and a half hours; a 3.3 kW unit in about two.
Is it bad to oversize a chiller?
Not for performance, within reason. Oversizing costs more up front and can short-cycle if the controller is poor, but a unit that is too small will fail to hold a set point under load. Size to the job plus margin rather than to a round number.
Does insulation change the chiller size I need?
Yes, substantially. An insulated, covered tub in a cool room may hold temperature with a modest unit. The same tub uncovered outdoors may need a size or two larger.
How long should a chiller take to cool a tub?
There is no standard. Decide how long you are willing to wait, then choose capacity to meet it. Overnight cooling and thirty-minute recovery are two different products.