Inspired by a question from a colleague, I sought out to answer the question: how cold is my chilling water? Empirically, it’s painfully cold in the winter and tolerably cold in the summer, but I had no numbers to ascribe to those observations. As you’re not going to be able to cool your wort below the incoming temperature of your chill water—that is, without some additional pre-chiller or other funny business—knowing the incoming temperature of your chill water (in both your winter & summer months) seems like a valuable thing to know!
For this ad-hoc experiment, I ran the faucet for a full minute to ensure any slightly-warmer water that had been sitting in the pipes was fully flushed, before filling a small cup and testing with my Thermapen. As of the date of this post, for the northern suburb of Minneapolis, MN where I live, water straight out of the tap (i.e., from inside my house) was approximately 62 F in August. The average shallow groundwater temperature for the Minneapolis-St. Paul region should be somewhere in the 42 F to 47 F range, though it’s uncertain how that translates to cold water temperature out the tap.
Just for the sake of curiosity, I did the same little experiment for the hose spigot on the exterior of my house. The temperature of that water was more than a full, two degrees cooler, at approximately 59.5 F. This part of the test was performed after sunset, with NOAA reporting an ambient air temperature of 61 F.
More observation seems warranted.