Hello, my first post. I hope this is the right forum... I'm considering a heat exchanger to solve a problem. I've purchased a tankless water heater for home usage - look at it as a black box, cold water flows in, hot water flows out. Output temperature is quite constant due to a digital servo. It just happens the model I've purchased minimum output temperature is a few deg C above the temperature I feel comfortable with during summer. I cannot mix cold water with the hot water. If I use a heat exchanger consisting of two copper tubes soldered together side by side, input cold water would flow trough one, output hot water trough the other one, how many deg C of cooling per meter could I expect from this setup? (The digital servo will compensate for the increase in cold water temp so output temp will be the same). Copper tube diameter 20mm, wall 1mm, water flux 10 liters/min. Input water temp 20 deg C, output water temp 37 deg C. Thanks for your help!