I am hooking up a used faucet (see photos below) for use as an outdoor sink, and I only want to supply it with cold water.
If I attach the cold water supply pipe to the cold water fitting and simply do nothing with the hot water fitting (i.e. leave it open), what will happen? Will the faucet function as usual so long as I turn the handle to the cold side, or will all the water flow out the bottom through the open hot water fitting?
Top view:
Underside:
No you cannot. A single handle faucet like yours has a mixing valve inside it that connects the cold and hot side with each other. In other words, there is a point where the hot and cold water mix together.
The cold water will travel back through this point and out the bottom of the disconnected hot side. Big mess. Even if you keep the faucet all the way to the cold side, some mixing could occur. Also you’re now relying on every possible user to not turn the faucet even slightly away from the maximum cold setting. Simple human error will cause water to come gushing out under the sink.
Just cap off the hot side. If you leave the hot side open, you’re asking for trouble. As Ecnerwal said in the above comment, one good solution is to add a tee fitting to the cold supply line and bring cold water to both the hot and cold side of the faucet. Doing so will ensure that there’s always water flowing no matter which way the faucet is turned.
You'll need to somehow cap off the hot to prevent backflow from the cold being that's a single handle faucet.
