1
Predict heat load
Estimate UA from tub geometry and insulation, then add bounded solar and pump heat instead of a flat safety multiplier.
Physics-first DIY planning
Build a cold plunge system that actually works together.
Size the chiller from BTU and heat load, then check pump flow, filter restriction, hose diameter, sun exposure and cooldown time as one system.
BTUnot horsepower alone
Flowafter real system losses
Rangeno fake 5.42-hour precision
What the calculator checks
Thermal + hydraulic, together
A chiller can be thermally large enough and still fail because the pump cannot maintain the manufacturer’s minimum flow through a restrictive filter and undersized hose.
Cooling loadWater mass, ambient, insulation, solar exposure, pump heat.
Operating flowPump curve vs. pipe, fittings, filter and chiller resistance.
Why this is different
Not another “buy a 1/2 HP chiller” guide.
Two nominally identical horsepower ratings can have different published BTU capacity. And the zero-head GPH printed on a pump is not the flow your chiller receives after the complete loop.
2
Solve the plumbing
Intersect a pump curve with system resistance. A 1/2-inch line can choke a pump that looks large enough on the box.
3
Only call it safe when it stays safe
Recommendations are ranked from physics and confidence. Affiliate commission is never part of the ranking logic.
Start with your actual tub and plumbing.
Enter volume, temperatures, insulation, exposure, hose size, filter and pump. The result shows a cooldown range, flow range, confidence level and compatible supported equipment.
Open the System DesignerBuilt for decisions, not just answers.
“Will it cool?”
Checks whether conservative chiller capacity remains above the modeled heat load at your target temperature.
“Will it flow?”
Checks actual modeled GPH against each chiller’s published minimum flow requirement.
“How long?”
Integrates cooling through the pull-down instead of assuming capacity and heat gain are constant at every temperature.
“What is uncertain?”
Shows ranges and confidence when sun exposure, insulation construction or pressure-drop data are not exact.