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Thread: New reverse osmosis system. Flow rate seems low. Please advise

  1. #1
    DIY Junior Member ribs1's Avatar
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    Default New reverse osmosis system. Flow rate seems low. Please advise

    Hey guys,
    I just finished having a reverse osmosis system installed as part of my new kitchen project.
    I bought a black and white unit from pure water products in texas with a permeate pump.

    Everything is working now but I think the flow rate is too low. I have never had one of these system though so I guess I'm not sure what the flow is supposed to be.

    To test, I filled a 1 quart container in 35 seconds. is this normal? What can I do to increase flow rate from my RO faucet?
    Here's some more details
    1. Unit is mounted on a shelf in the basement just under the kitchen cabinet.
    2. All the tubing is 1/4 inch
    3. The tank is mounted just under the RO system.
    4. I think the membrane is a 24 gallon per day.

    Any other tips would be appreciated.
    Thanks

  2. #2
    In the Trades Tom Sawyer's Avatar
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    what is the pressure? And...how much water do you expect to get through 1/4" tube
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    DIYer, not in the trades LLigetfa's Avatar
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    RO systems are often placed directly under the sink. You can lose about 4 PSI by having it in the basement. The long run of 1/4" tubing doesn't help. A different faucet might make a small difference.

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    DIY Junior Member ribs1's Avatar
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    Quote Originally Posted by Tom Sawyer View Post
    what is the pressure? And...how much water do you expect to get through 1/4" tube
    I don't know how much water should come through a 1/4 inch tube. I don't really know how to measure my pressure.
    I guess I could look at the pressure gauge by my well pressure tank?
    Should I change this whole thing to 3/8" tube and fittings?

  5. #5
    DIY Junior Member ribs1's Avatar
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    Quote Originally Posted by LLigetfa View Post
    RO systems are often placed directly under the sink. You can lose about 4 PSI by having it in the basement. The long run of 1/4" tubing doesn't help. A different faucet might make a small difference.
    The unit is on a shelf high on the wall in my basement pretty close to the ceiling directly under the sink cabinet. I didn't really have much room in the sink cabinet because I have a large apron front sink. I could have gotten it in there though.

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    Aspiring Old Fart, EE, computer & networking geek Mikey's Avatar
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    Quote Originally Posted by ribs1 View Post
    I don't know how much water should come through a 1/4 inch tube.
    One source (http://www.engineeringtoolbox.com/wa...se-d_1524.html) shows a little under 1gpm through 100' of tubing at 40 psi. If you've only got a few feet of tubing I'll bet the limiting factor is the RO unit itself.

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    In the Trades Tom Sawyer's Avatar
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    Somewhere in the paperwork that came with the unit there should be a chart that shows the volume / pressure chart etc for the unit.
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    That's all folks! Gary Slusser's Avatar
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    Doesn't seem to be anything wrong except customer volume expectations are not met.

    The size of the tubing or the basement install is not a problem.

    There is no way you can lose 4 psi in the distance from the basement to the faucet on the kitchen sink counter, well a kink in the tubing would do it but not by the length or dia/size of the tubing.

    Most under sink ROs I am aware of use 3/8" tubing from teh tank to the faucet but..... You can not use larger dia tubing and get more water because the faucet is 1/4" and the captive air pressure in the RO storage tank provides the pressure to the faucet which should be about 7-10 psi.

    The RO has to work against that pressure so you don't go any higher without severely decreasing the output volume of RO water and the time it takes to produce it.

    To get more product water in a shorter period of time you'd need a larger than 24 gal/day membrane. Or to add a pump.

    In the manual there should be instructions on how to check that air pressure. You do it with a regular tire pressure gauge and with no water in the tank.

    The well pump pressure switch setting has nothing to do with the pressure coming out of the RO's storage tank.
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  9. #9
    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by Gary Slusser View Post
    There is no way you can lose 4 psi in the distance from the basement to the faucet on the kitchen sink counter...
    In case you forgot, the formula is .43 PSI per foot just for the static column alone and the OP did not say how high in the basement the system was installed. There is also losses per foot on a long 1/4" tubing run, also not mentioned in the OP.

    So, there is a way you can lose 4 PSI.

  10. #10
    That's all folks! Gary Slusser's Avatar
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    Well the RO won't be sitting on the basement floor because that would not allow servicing it, and usually the tank is up on the wall above the RO. Main line water pressure moves the water into the storage tank.

    So how many feet of height do you think there is from the storage tank to the faucet? To lose 4 psi there would have to be over 8' of height to the faucet. A basement is usually 8' and kitchen counters are usually 33"-36" but...

    As I said, the captive air pressure in the RO's storage tank provides the water pressure to deliver the product water to the faucet.
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  11. #11
    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by Gary Slusser View Post
    So how many feet of height do you think there is from the storage tank to the faucet? To lose 4 psi there would have to be over 8' of height to the faucet. A basement is usually 8' and kitchen counters are usually 33"-36" but...
    Looks like you answered your own question but you forgot to factor another foot for the floor joists.

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    DIY Junior Member ribs1's Avatar
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    Hi Fellas,
    The RO unit is close to the basement ceiling. The tank is mounted just beneath the unit. Both are high up on the wall. It is not far to the sink cabinet and up to the faucet.
    It is likely that the problem is just that I am not used to these things. I have never owned one before.

    Also, after letting the system sit idle over night the performance is better today. I just filled that same quart container in 25 seconds.
    I guess I just didn't let the tank get full before testing.
    I do have a permeate pump

    Anyway, just curious, what is a typical flow rate out of these things? Am I worried about nothing?

    Thanks

  13. #13
    DIY Senior Member mialynette2003's Avatar
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    I think you are worried for nothing. You are dealing with a 1/4" line that goes uphill and the more water that is drawn from the unit, the less flow rate you will have. A qt in 25 sec would be normal to me. Enjoy the great taste, less filling (pun intended) RO water. LOL

  14. #14
    DIY Junior Member ribs1's Avatar
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    We are definitely enjoying the water and I am getting used to the flow rate.
    Really I had no problem drinking our water out of the softener but my wife hated it.
    I am very happy to not have to buy bottled water anymore.
    Ice cubes look nice too. Really clear.

    Coffee was good this morning too.

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    Water systems designer, R&D ditttohead's Avatar
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    Ro tank pressure shoud be set to 6 psi empty. The only way to test the tank pressure is when it is empty of water. If anyone tells you otherwise, they are wrong.

    Your RO systems pressure is directly affected by line pressure. Adding air to the tank will not increase pressure to the faucet. At least not for the first half gallon. It will only lessen the tanks volume. The permeate pump shoud be installed without an ASOV, or at minimum, a 90% ASOV.

    The tube length is always of great concern to flow rate avaiability. It has to do with frictional loss and capillary restriction calculations. I would recommend running any RO line between the tank and the faucet in 3/8" if it is more than 10', 1/2" if it is over 20'.

    Hope this helps.

    FYI, the flow rate should be approximately .5 GPM if the system is operating properly. Most faucets will flow at approximately 1 GPM. The loss of flow from the posy filter, tubing length, etc. can drop you to the 1/2 GPM range. This is fine since contact time is key for the post filter to provide the best tasting water.

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