What is this drainage pipe under kitchen sink that leads outside?

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rigg8280

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I've known about a 1/2" pvc pipe leading out of my house since I moved 18 months ago. I assumed it was drainage for AC condensation in the attic until it was still running even when AC isn't on. It caused a leak because it wasn't angled enough (imagine slowly pouring a glass of water so it runs under the underside and back to your hand) and the water dripped back into the wall. So I ripped out the drywall and traced it up to the main floor and just realized I have this offshoot of the hot water line under my kitchen sink (see picture). Same size pipe and directly above so I'm pretty confident this is the source of the pipe leading outside. I know to fix the angle of the pipe, but I'm curious what this is and if I should run it away from my house with more pvc (currently dripping under my deck and likely into the foundation) or if I can cap it.
 

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Maybe a remote temperature and pressure relief valve and drain line for the water heater?
 
Maybe a remote temperature and pressure relief valve and drain line for the water heater?
The water heater is one level down with plenty of access to the exterior wall (where the drainage goes now along with ac condensation). However, this under-sink setup was before expansion tanks were required so maybe related to pressure relief? My other concern is if this has a steady drip, I'm guessing this isn't helping my water bill.
 
Maybe you could convert the relief valve into a shutoff valve.

Then convert the pipe outside into a sillcock.

Just an idea.
 
You are right, I forgot it was hot water.

But still might be welcome, as you suggested.
 
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