Most of us cool our wort using a chiller that relies on water as the working fluid. Tap water goes in one end, heat is transferred through a metal wall from the wort to the water, and hot water comes out the other side. Whether you use a coiled immersion chiller, a plate chiller, or a counterflow chiller, the heat transfer principles are the same.
These devices work beautifully in the winter, when tap water can run as low as 40–50°F (4–10°C) in some locations. But in summer, tap water is often warmer than your desired pitching temperature. How can you cool your wort to 65°F (18°C) with tap water that may be considerably warmer?
Enter the humble pond pump. Available at garden centers and home improvement stores for less than $20, a garden pump might become your best friend in the summer. Submerged in a cooler of ice water, a pond pump can deliver much colder cooling water than your household spigots.