I'm attempting to add an under-sink water filter to my existing wall-mounted kitchen faucet. There is no shut off valve in the sink cabinet for the wall-mounted faucet. The 1/2" copper cold water supply line for the wall-mounted faucet runs straight up the inside of the wall, turns 90 degrees, comes out of the wall and connects directly to the wall-mounted faucet (i.e., the faucet body itself connects directly to the copper supply line coming out of the wall; there is no flexible supply line between the faucet and the copper supply line). The water filter assumes you have a shut-off valve and a flexible supply line from the faucet that would connect to the 3/8" compression fitting on a typical shut-off valve.

Since I have neither the shutoff valve under the sink nor the flexible supply line from the faucet, here's my plan: I have no problem accessing the 1/2" copper supply line in the wall so I want to cut the supply line, attach a pipe at 90 degrees that comes into the kitchen sink cabinet, and then attach a standard under-sink shut off valve. Then I will connect the water filter to the 3/8" compression fitting on the shut-off valve.

The faucet itself will still be connected to the 1/2" copper supply line and so I would have to connect that supply line back to the 3/8" compression fitting on the water filter. If I do that, though, what I will end up with is a 1/2" water supply line TO a shut-off valve with 3/8" compression fitting TO water filter TO 3/8" compression fitting (built into the water filter) TO a continuation of 1/2" copper supply line TO the faucet.

My question: does going from the 1/2" supply line down to the 3/8" compression back to the 1/2" supply that attaches to the actual faucet going to cause a pressure or water flow problem? If so, any other ideas on how to tackle this?

  • 1
    The filter itself, the aerator on the faucet, and the built-in restrictors most modern faucets have will more than likely reduce maximum faucet flow far more than any of the pipe size alterations you mention.
    – kreemoweet
    Jun 13, 2023 at 19:54
  • I thought this may be the case, but I appreciate you confirming.
    – newbie
    Jun 13, 2023 at 20:52

2 Answers 2


Your faucet has 1/2 inch connections for practical reasons, because it's designed to be connected to a half inch supply in the wall. Not because it's supposed to provide more flow than all other faucets. That's not the point of it. So maybe the flow will be a bit less, probably not, and it will be the same as any other faucet fed by that filter.

The only improvement I can think of to your plan, is if the pipes behind the cabinet happen to come directly from an unfinished basement, you could do exactly the same rig-up in the basement, the result will be the same, and you can keep your cabinet whole.

You need to make sure that the pipes going up the wall to the faucet don't continue to some other fixture where you don't want filtered water.

  • Very good to know the practical reason for this set up. I will proceed as planned. And I don't have an unfinished basement, but do have a crawl space which I could do this in, but then every six months when I have to change the filter I would have to go back into the crawl space and change it. Worth considering, though, because would make life much easier than opening up the wall.
    – newbie
    Jun 13, 2023 at 20:54
  • Another possibility, cut in in the crawl space, run a pair of copper pipes there through small holes in the cabinet floor, so the filter and hoses are in the cabinet. Just don't run hoses through the floor.
    – jay613
    Jun 13, 2023 at 21:04

Yes, the reduction in pipe diameter, as well as the additional fittings and not least the new filter you're adding, will reduce your pressure somewhat. It is unlikely to be a big problem, as most faucets go through some 3/8 hoses and the filter is presumably designed for this type of use. However, the details depend on your specific home water pressure, faucet, and filter plus your personal preferences.

Alternative options to consider:

  • Use rigid fittings to make the required transitions, eliminating the hose. This is more for longevity but might help with pressure. The downside is that it'll be harder to replace the filter in the future.
  • Install a separate filtered water output. These are often installed on the side of a sink, but may require drilling your sink or countertop.
  • Install the filter somewhere else. You could put it on your fridge if it's just for drinking water and your fridge has a dispenser.
  • Switch to a whole house filter. That's a different project and may still affect your pressure, but is approximately the same amount of effort as what you're proposing doing and may offer other benefits.

Overall I'd say your proposed approach is reasonable and shouldn't cause major pressure problems, but you should at least think through some alternatives.

  • 1
    I wish I could accept both your and jay's answers, but wanted to comment here to say I appreciate your detailed explanation here and the alternative ideas. Things to think about...
    – newbie
    Jun 13, 2023 at 20:55

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