I installed my RO system this weekend and I'm pretty excited. The water pressure seems low though. Not unbearably slow but much slower than before the system was put in. The system is 6 stage and has a dedicated faucet, a 3 gallon tank, and is all 1/4 plex tubing.

I have a conditioned crawlspace that stays within 5-10 degrees of my house. I put the unit in the crawlspace so i would still have room under my sink. (It's about 1.5-2 feet below the floor)

I installed in by cutting into a 1/2 pex (or cvpc?) pipe and using an adapter to change to the 1/4 inch pex. At the far end of the RO system I used a 2nd adapter to change back to the 1/2 inch to travel to the sink (it's about 4.5 feet). After it goes through the floor i used a 3rd adapter to change it one more time to 1/4 pex to match the faucet pipe.

The reason I kept the bigger pipe was in hopes that it could be considered as an additional "storage tank" in that that long run would hold more water than using a 1/4 pipe.

The question I have is do you think this transition back and forth is causing a lower water pressure? enter image description here

  • Your pipe won't function as an extra storage tank as even when the tank is empty, the pipe will be filled with water. The tank has a stretching membrane between the water and compressed air that provides the pressure and forces the water out of the tank until it is empty.
    – Jon
    Dec 23, 2020 at 5:47

1 Answer 1


I would time how long it takes to fill a 1 gallon jug and see if the system is operating within specifications. With a 1/4” line I would not expect a large flow. But no the changing sizes won’t affect the flow as the 1/4” would be the limiter. The size change won’t really help or hurt in this limited length.

  • Are you sure the low pressure isn't from lack of pressure in the tank itself?
    – Phaelax z
    Dec 21, 2020 at 21:18
  • I'm not. That seems the test that @ed-beal considering too?
    – rudtek
    Dec 21, 2020 at 21:38
  • 2
    RO units are fairly slow the last one I installed under sink fairly high end was 10-12 gph so once the tank is exhausted it may take ~30 minutes to go to full pressure and after the tank is depleted 10 minutes to fill a gallon jug, just a guess based on the one I installed last. They sound like they produce bathtubs full a day but in reality the daily use averages at the gph , a tank at breakfast, lunch and dinner would put you at 9 as a daily average with a 3 gallon tank from what I remember but I may be off a little that was a couple of years ago. Once the tank is empty time a gallon.
    – Ed Beal
    Dec 21, 2020 at 23:59

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