I took Randy's suggestion one step further, although there is some risk in what I did. I run tap water through a loop in my sump, then into my RO/DI, then out to water the lawn. Warming the water coming into the water purification system allows it to work more effectively. The danger is the loop is under a fair bit of pressure, and if it split tap water would be flowing into my sump.
I just set this system up a few days ago. I purchased a valve from Home Depot for a sprinkler system (I think it was about $15-20?). I then had to purchase a controller for it to deliver 24 V to the valve. However, the controller was too complex, and then I realized I could just wire the controller's AC/DC converter direct to the valve. If you have a 110 V AC to 24 V DC converter hanging around you'd be all set. I plugged the valve into a temperature controller.
I only have one 50 ft' coil in my sump. It's in a spot of decent water motion, and clearly the water is warming coming out of the sump, but I think a longer length would more completely warm the water (still some condensation on the line coming out of the sump). For me though I need to think about the pressure drop as I need decent pressure for the RO system to work well. Perhaps I'll splice in another two lengths of the tubing so that I have three in parallel (no increase in pressure drop). The tubing is super cheap, 50' for like $3-4 from HD.
I also need to use a fan and chiller on really hot days. I also need to fire up my geothermal cooling loop. It works, but not that well because the ground the loop(s) are in just heats up too quickly, and the heat does not radiate out in the ground very well.
