Adding an additional stop valve to existing supply stop valve

Question

I need to replace some parts on my dishwasher that will require me to turn off its hot water supply for at least a few hours, maybe a day or two. There's an existing supply stop valve (see picture below) but it doesn't fully shut off the water, even when cranked really tight. It also turns off the kitchen sink's hot water which is annoying. I can turn off hot water at the water heater, but I can't leave it off for a long time or my family will kill me.

I'd replace the supply stop valve, but given the rust around its connection to the water pipe, I'm nervous that I'd break the supply pipe while removing the old supply stop valve.

Over the years I've had to make multiple dishwasher-related repairs, and the difficulty shutting off the water makes it a pain every time. And I suspect this won't be the last repair that I have to do here. So I'd like to figure out a longer-term solution to the incomplete-stop valve.

So as part of this repair, I was thinking that it'd make sense to add a new supply stop valve downstream from the current one, and attach it where the current flexible hose is attached. (See red arrow below.)

Questions:

• Is this a good idea, or are there gotchas with having two supply stop valves in a row?
• Should I connect the new valve directly to the old one, or should I attach a short supply line to the current valve, attach the other end of that short line to one end of the new valve, and attach the old hose to the other end?
• What part(s), both the stop valve and any other parts like washers, will I need for this work? It's 3/8" compression (see label in picture below) on the end of the supply line heading to the dishwasher.

Don't blame me for the shoddy drywall work here... it was like that when we bought the house!

Answer 1

Add-on Stop Valve:

The downstream add-on stop valve you are looking for is called an inline "repair stop valve". Its upstream end has a captured nut and seat to exactly fit on the "compression" outlet of your existing valve - just remove the outgoing hose, install the repair stop on the outlet, and put the hose on the outlet of the repair stop.

Here's an example "3/8″ O.D Comp X 3/8″ O.D Female Comp 1/4 Turn Straight Repair Stop Valve"

Another Replacement To Do

The rubber/plastic hose inside your braided dishwasher supply hose has also been subject to degradation from hot water over time. This is a great time to replace it with a decent-quality new supply hose.

Fixing Leak in Current Valve:

Your current valve is almost certainly leaking because the rubber gasket inside has degraded over years of exposure (this happens much faster with hot water than cold).

It seems to be a multi-turn shut-off valve, which should be able to have its gasket replaced without removing the valve from the pipe. Rather, after shutting off the water, one removes the "packing nut" (surrounding the white (plastic?) shaft) and carefully pulls out the shaft and gasket assembly, keeping track of the order of any washers - take multiple photos.

Make sure any broken pieces of rubber gasket are removed from the insides (I usually remove the attached outgoing hoses to make sure nothing goes into them). Then, then, reassemble using replacement parts from something like this multi turn valve stem repair kit:

The larger rubber washer fits inside the packing nut and the metal washer fits over the shaft of the stem (packing nut tightens against it). Here is a sample view of old worn parts on left and new parts on right. The metal shaft shown is higher quality than the plastic and is available individually as a brass part, which I would recommend instead of the plastic replacement for durability.

Many good videos are available on YouTube showing this repair; I like the ones from This Old House