I have been and am dripping Hot water and cold water together, been doing it for about 4 months now and no issues, heaters run less on the tank but also it takes a while to dial in because you can increase the tank temp allot with very little hot water and you can also decrease the temp with very little pure cold water running into the system, and my system is about 670 ish gallons. You need to have a line of each because as I said just a little bit of hot can increase a tank temp a ton and it takes me a few days to dial everything to the right temp, then of course ill mess with it and takes another couple days to change again I have a couple heaters set so that if it gets down to about 76 they kick on and if it gets a touch less then that all the heaters will turn on to cover incase for some reason something happens, Summer will be intersting to see what happens with the water, the winter I think is a little harder as the water is so cold from the cold water line, during the warmer months my water can come out at a much warmer temp because of heating with water like this I really wish I had a monitoring system that I could check on the temp, for now I check the temp all the time when ever I am home and also ask about the temp when my wife is home. Electrical bill is less and hot water heater does not seem to run any more at all, gas price is the same to a touch higher, but electrical sayings is in half of what I was using(counting pump and heaters) I am running filterless in my area with no problems so strait from the water line to the tank. As you are all aware all areas are different so do as you feel, Here is my tank, Heavy stock in my opinion, about 35 fish and 8 stingrays.
[video=youtube;7VkxnjzZj9M]http://www.youtube.com/watch?feature=player_embedded&v=7VkxnjzZj9M[/video]