I have a 1/2" od PVC outside my home with a steady drip of water. It is a few inches above the line that runs to the TPR valve on the water heater (confirmed by opening the valve). It's not the AC condenser line either. There are two more for them and I checked both units in the attic to confirm there was no water coming from the lines.

Water heater

Looking at the top of my water heater, I'm baffled on what the set up is. I'm guessing one or both of the mystery 1/2" lines running off the hot and cold lines are the source of the drip in the 1/2" line outside since it's next to the TPR line and right on the other side of the wall from it.

As a guess, I figure they are there to relieve pressure instead of having an expansion tank? The pressure coming out of the tank drain is @ 50 PSI. When I cut off both the gate valves on the cold water line, the drip is still there so that leaves me thinking it's the hot water line.

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I believe the culprit is this valve coming off of the hot water line. I don't exactly what it is or why it is there. It's sprayed over with paint but it is a metal valve of some sort. It reminds me of a needle value for a hydraulic line which might make sense if it's some other mechanism to relive pressure but I think it's more likely a ball or gate valve I'm not familiar with.

Anyone know what this valve is and what it's purpose is? I'd love a larger explanation of why the 1/2" line on the cold water exists as well but my main priority is to determine what the one on the hot water line is and replace it to see if that stops the drip.

2 Answers 2


It's not exactly like mine, but it looks like it's probably an overpressure (no temperature) type valve (aka pressure relief valve), and it should not be dripping unless you have overpressure - so a constant drip means it's likely in need of replacement. (Or possibly just cleaning, if you get lucky; don't expect to get lucky.)

adjustable pressure relief valve

Indeed, it looks very much like this adjustable one.

The line on the cold water line could be intended to feed a hose bibb.

Given that it's probably intended to relieve pressure events before the P/T on the tank so that the P/T on the tank does not become leaky through frequent activation, you probably could replace it with a pressure tank.

  • Thanks. I stumbled across the same valve on THD website about the time you posted and agree. The manual for this valve - s3.supplyhouse.com/product_files/… - make it seem like it's typically used to keep the TPR from dumping. Now I just have to debate the easy fix of replacing it or putting an actual expansion tank in :)
    – DSS
    Commented Mar 26, 2016 at 2:23

It looks like a check valve to me. It is hard to tell without knowing a couple of things. First, is the line going in the metal pipe or is it punched through the wall?

It could be used for allowing the return flow of your hot water supply from its furthest point. Normally the how water returns to the cold water supply at the tank.

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