For those curious, I posted this on ReefCentral a while back in the Wine Country Reefers forum. ================= In the cold winter months RODI water filters do not work that well due to the cold water being denser and significantly lowering your filtering ability and increasing time to filter water. Normally, you may get a 1:4 ratio of filtered water vs waste water. With the denser cold water, your ratio may go to 1:6 to 1:8 or more. That means (a) you are wasting more water down the drain. For a 20 gal water change that means maybe 120-160 gal of waste water instead of 80 gal. Also, it takes longer time to make the same water. A 75 GPD (filtered water device may decrease to a 40GPD filter, etc. The culprit is water temperature/density. RODI filters are meant to be run optimally at 70-80 deg F. Cold water, especially during winter is like 45-50 deg. That's 20-30 deg difference. My solution, use an anti-sweat valve. Anti-sweat valves are used in areas MUCH colder than California where very cold water enters warm houses (in particular toilets). This causes the cold water to cool the toilet reservoir so much that water condenses on outside of the toilet reservoir and causes a lot of "sweating" onto your bathroom floor. Anyways, anti-sweat valves work by combining cold water with an adjustable hot water. You could do this with a T-fitting, but you would probably end up with higher cold water back pressure pushing cold water into your hot water pipes. Not a good idea if your RODI will be running for 4-10 hours to make water and you try to take a shower. You'll get luke-warm water. Anti-sweat valves have one-way valves built into each inlet. This means no cold water can enter hot water inlet line and no hot can enter the cold inlet line. Anti-sweat valves allow you to adjust HOW much hot water you want to mix/combine. So you basically adjust it to around 70-80 deg F outlet temperature. NOTE: remember, when it is summer, your cold water line will get warmer, so either you need to adjust it again as weather warms up, or you need to lower the mixed outlet water temperature now a few degrees (during winter) so water can rise a bit and not exceed 90-100 deg F. Too hot water temperature (above 100-120 deg F) will ruin your RO membrane. ================== So how do you make it and how much does it cost... Parts: Anti-sweat valve ($15 on Amazon) I bought a used one on Amazon - $8 (free shipping) This uses 1/2 FIP for inlet & outlets. Dual outlet shutoff valve ($18 each, need two, one for hot, one for cold water) (I used dual outlet with teo knobs so I can independently shutoff the water lines to RODI). My house copper pipes are 5/8, so I got a 5/8" compression inlet with two 3/8" compression outlets. 3/8" compression to 1/2" FIP braided faucet connector (12" long in my case). ($6 each, need two, one of hot, one for cold). 1/2" FIP to 1/2" FIP braided pipe ($8, new one) Connects outlet of anti-sweat valve to my RODI filter (in this case, it goes to a ball valve & then RODI Johnny Quest 1/4 plastic piping. Total cost: $15+$18+$18+$6+$6+$8 $71 total cost. Technically, you should already have some of the parts already if you hooked up your cold water faucet line to RODI. For me, it cost me about $50 for the modification. And no I can make water faster (in case I forget to make it ahead of time). And less water goes down the drain (volumetrically). Yes, some people will argue I am dumping lukewarm water down the drain and therefore also wasting natural gas that is used to heat my water heater. But for me, I just want to cut my RODI water production time in half (during winter months).