Planning capacity with 35% headroom: 4,222 BTU/hr.
Water-only load: 0.92 kW thermal.
Do not select equipment from “horsepower” alone. Compare the manufacturer's delivered BTU/hr at your expected ambient and target temperatures.
The sizing formula
This first calculates the heat that must leave the water. Then add headroom for heat entering through the tub, open lid, plumbing, pump and sun. Hotter ambient conditions may also reduce the chiller's delivered capacity.
Water-only BTU/hr by cooldown target
Add at least 20–50% planning headroom, then check manufacturer performance at high ambient temperatures.
| Temperature drop | 4-hour cooldown | 8-hour cooldown | 12-hour cooldown |
|---|---|---|---|
| 10°F | 2,085 BTU/hr | 1,043 BTU/hr | 695 BTU/hr |
| 20°F | 4,170 BTU/hr | 2,085 BTU/hr | 1,390 BTU/hr |
| 30°F | 6,255 BTU/hr | 3,128 BTU/hr | 2,085 BTU/hr |
| 40°F | 8,340 BTU/hr | 4,170 BTU/hr | 2,780 BTU/hr |
Why horsepower is not enough
“1/2 HP” or “1 HP” can describe compressor motor class, not a guaranteed cooling output. The useful comparison is delivered BTU/hr at a stated ambient temperature, water temperature and flow rate. A unit that performs adequately in a garage may struggle outdoors in direct summer sun.
Hold load versus cooldown load
Cooldown sizing asks how quickly you can remove a one-time body of heat. Hold load asks how much heat continuously enters after the target is reached. Choose a chiller that can handle both, with room for filter loading and hot-weather performance loss.