I started down this path planning to buy a 50' roll of copper and build a chiller and pre-chiller. I asked questions about the length of each (25/25, 30/20, etc.) and all answers were to steer clear of a pre-chiller as being inefficient. I started research and found quite a bit of the same sentiment on this forum and others. I found a site (lost it now) that measured a relatively minor temp drop in water temp using a pre-chiller in an icebath. Most recommendations were similar. Running 70 degree water through a pre-chiller sitting at 32 degrees might get you down in the fifties or sixties. You then have a differential of cooling water in the fifties and wort at 100. Why not just pump the 32 degree water through your wort chiller for the greater temp differential which will give you a faster heat transfer rate? My plan is to use a $18 garden pump from Harbor Freight. Sit it in a bucket and connect to my not-yet-built dual chiller. Use a garden hose to keep the bucket full and start the pump. When the wort temp drop slows down significantly, start dumping ice into the bucket. When the chiller discharge gets down to garden hose temp, divert the discharge into the bucket and turn off the garden hose. Continue dumping ice until wort temp is where I want it.