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The previous homeowner put in a GE SmartWater reverse osmosis filter under the sink, which is attached to the cold water. It was experiencing low pressure (the sink-top valve would spurt water for less than half a second and then drip). I purchased new filters and a new pressure tank, since the existing tank seemed to have a leak in its bag (diagnosed based on weight, and unwillingness to hold pressure from a bike pump).

I thought for sure replacing the tank would resolve the pressure issue. The new tank is very light, so it does seem to be working. But if anything, the pressure for the valve is even lower.

If I disconnect the pressure tank, the flow from the hose is pretty low.

I also tried turning up the water pressure from the source, which had no effect -- except on the pressure out the sink faucet.

There are no obvious kinks in the hoses. What's the best way to diagnose this?

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I'm assuming you replace / checked the membrane as well? –  UNECS May 21 '13 at 9:43
    
Yes. Replaced the membrane along with the filters. –  mohawkjohn May 22 '13 at 5:48
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2 Answers 2

What you call a pressure tank is actually a storage tank. RO filters always produce a very low flow which gets stored in the storage tank to supply the user.

You probably need to depressurize your storage tank below your water supply pressure so that the storage tank will accept the water from the filter.

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and wait for it to fill up to have usuable water 6 +hrs –  UNECS May 21 '13 at 21:26
    
OH! Wow. That explains a lot. Okay, I depressurized it down to below what my bike pump could measure (<10Lbs). Was still hissing, but only a little bit. It looks like there's already flow through the filter faucet. I'll try again in the morning. –  mohawkjohn May 22 '13 at 5:55
    
That's right, it takes several hours just to fill 10L. –  Matt May 23 '13 at 8:04
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If the water line to the filter unit is joined with the normal cold water line with one of those self piercing saddle valve units then I would suggest taking a good look at that to see if the valve is faulty, partially plugged or only partially pierced into the main water line.

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I hate those piercers, leaky, low flow. I usually recommend upgrading to 1/4 turn, full port valve. –  HerrBag May 21 '13 at 12:32
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