Size from delivered cooling capacity under expected conditions, not horsepower alone. The key inputs are water mass, required temperature drop, desired pull-down time, ambient heat gain, insulation, cover quality, pump heat, and how often warm users enter.
Core heat-load calculation
For US units, an initial pull-down estimate is gallons × 8.34 × temperature drop in °F ÷ hours. Add a margin for ambient heat, sunlight, pump heat, plumbing, opening the cover, and real-world performance. Use the calculator for a starting range.
Example
A 100-gallon tub cooling from 75°F to 50°F in 12 hours requires about 1,738 BTU/h for the ideal water pull-down alone. The installed requirement is higher because the environment continues adding heat. Hot outdoor conditions and weak insulation can dominate the load.
Performance data to request
- Delivered BTU/h or watts at relevant ambient and water temperatures.
- Minimum and maximum water flow.
- Maximum pump head or pressure limits.
- Electrical input, circuit requirement, and starting current.
- Outdoor rating, operating ambient range, and freeze protection.
- Heat-exchanger material and approved water chemistry.
Oversizing and undersizing
An undersized unit can run continuously and still miss target. Oversizing may cost more, require more flow and power, and short-cycle if controls or water volume are unsuitable. Correct sizing includes the whole loop rather than choosing the largest label.
Related guides and tools
- Chiller troubleshooting hub
- No-water-flow diagnostic
- Pump troubleshooting
- Chiller size calculator
- Water-volume calculator
Step 1: calculate usable water
External tub dimensions overstate water volume because walls, benches, freeboard, plumbing, and the user displace water. Use internal dimensions and fill depth with the water-volume calculator. Dosing and heat-load errors both begin with a bad volume.
Step 2: define the job
Separate initial pull-down from maintenance. Cooling 100 gallons from 75°F to 50°F by tomorrow is a different requirement from holding already-cold water at 50°F through a hot afternoon. State the starting temperature, target, allowed hours, ambient range, sun exposure, cover, insulation, and session frequency.
Step 3: calculate ideal pull-down
Water weighs about 8.34 pounds per US gallon. Ideal BTU/h equals gallons × 8.34 × temperature drop ÷ hours. This covers only the water. Add pump heat, plumbing, tub walls, air, ground, sun, cover opening, and occupants. The calculator provides a planning margin rather than false precision.
Step 4: use performance data
Hydrofarm lists its 1/4 HP example at 3,010 BTU/h with a defined flow range and notes that starting water and ambient temperatures affect duration. Those details matter more than “1/4 HP.” Compare capacity at the intended conditions, not a favorable test point.
Step 5: verify the loop
- Pump operating flow falls within the chiller range after all system resistance.
- Filter and hose sizes do not starve the exchanger.
- Maximum pressure is not exceeded.
- Heat-exchanger material supports the treatment chemistry.
- The circuit, plug protection, and installation comply with the manual and local rules.
- Indoor/outdoor rating matches the location.
Sizing examples
| Scenario | Implication |
|---|---|
| Small indoor insulated tub, covered | Lower ongoing heat gain; pull-down time may drive size. |
| Large outdoor tub in sun | Ambient and solar load can dominate; shade and insulation may be cheaper than more chiller. |
| Frequent warm refills | High repeated pull-down load; water-management changes may reduce energy. |
| Very low target | Capacity can decline as conditions become more demanding; use performance curves. |
Do not size by shortcut alone
Rules such as “one horsepower per X gallons” ignore climate, target, insulation, time, and actual product performance. They can be a rough screen, not the final selection method.
Reference
Hydrofarm Active Aqua product data.
Questions to ask a seller
- What BTU/h is delivered at my expected ambient and target water temperature?
- What water volume and pull-down time does the performance chart assume?
- What operating GPH and pressure are required?
- Is the unit rated for my outdoor, rain, or freezing exposure?
- What pump and filter arrangement is supported?
- What is the warranty if used for cold-water immersion?
Plan for the worst normal day
Size against a realistic hot day, not a perfect showroom condition, but avoid extreme once-a-year assumptions that make the system unnecessarily large. Shade, insulation, a cover, and shorter plumbing often reduce the required capacity more economically than moving up another equipment size.
How much sizing margin is reasonable?
A margin should cover normal hot weather, routine opening, pump heat, and performance variation—not mask an uninsulated tub in full sun. Model the realistic load, then compare adjacent sizes using delivered capacity, flow, circuit, sound, cost, and cycling behavior.
What if performance data is unavailable?
Treat that as uncertainty. Ask the seller for capacity at your conditions and a supported volume/pull-down example. A return policy and written suitability confirmation are valuable. Avoid converting horsepower into an invented BTU number.
