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I am installing a R/O filter under my sink. The drain for the waste water attached to my dish-washer drain hose provides a 3/8" connection (Watts Premier 164020 DLA-D Drain Line Plumbing Adapter). But, the drain with my unit is a 1/4" hard plastic tube. I am not sure what to do, but maybe I can connect the 1/4" tube to the 3/8" tube somehow.

Any ideas?

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up vote 1 down vote accepted

Check the plumbing section of your local hardware or big box store for a double barbed adapter. It may be a direct 1/4 by 3/8 or a multi size barb that increases size from 1/4 to 3/8.

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Might need a short length of flexible Tygon tubing on the RO unit's hard plastic for the double barbed adapter to work. – Wayfaring Stranger Jun 1 '13 at 13:41
Home Depot had a connector with 1/4" on one side and 3/8" on the other. Thanks for the reply. – ADH Jun 1 '13 at 19:59

Watts connector described has a push-in connector for poly tubing. What size is the push-in connector for? You cut off the poly with a sharp knife and push it into the fitting if it's the proper size.

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Other options, Universal Dishwasher drain coupler. Usually, they come with a new disposer. They look something like this. If you search online, all sorts of sizes are available.

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Also, given that 1/4" tubing is a standard size, you can adapt it up in size to 3/8" by using an appropriate brass compression fitting. I'm assuming that it is hard poly and barb fittings won't work as it isn't flexible enough. The compression fitting ring will have bite into its surface enough to seal and hold. For Vinyl and Poly type tubing, they make a brass insert so the compressing fitting has better bite. Shown is a sample of a union coupler with inserts, what you need will have to be selected for your situation

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