Copper pipe runs off top of water heater to exterior wall, exits to PVC drip pipe

[Confoozled]

Hello all. First, thanks for this resource. I've spent a lot of time reading threads and have learned a lot. My issue:

I noticed a ground-level PVC drip pipe recently began flowing, about a quart a minute. First time in 20 years. Not huge, but enough that the patio and ground around it are now soaked. I know where my primary and overflow drip tubes are for the AC condensate and it is not one of them (plus I checked to verify AC pan is dry, normal condensate line running per usual).

The water is quite hot. I pulled the basement drop ceiling above where the PVC pipe exits the wall and find a hot water pipe (copper) directly in line above it, which turns downward at that point as if it runs down the wall to the PVC pipe exiting the house (I haven't ripped out the wall board to directly prove this, but circumstantial evidence link the two).

This copper pipe runs off the water heater pipe shortly above the hot water exit, runs up to the joist/deadspace level, and then straight across to that basement exterior wall and then down that wall as mentioned. There are no sinks or water-using fixtures on that wall.

Does anyone have a guess what this mystery pipe is for? It's not connected to the heater's TPV, which is side-mounted and its pipe just runs to the floor. I keep a bucket under it just in case and few minutes ago tripped it for 10 seconds to ensure it ran/stopped as expected.

Also, there must be some sort of valve (perhaps another pressure valve?) inside the wall, just upstream of where the hot copper pipe meets the PVC drip pipe, or surely this thing would just be shooting water out all the time like an opened tap. There's no shutoff on this mystery pipe, but I wouldn't shut it off at this point anyway since I don't know what it is and it might be some kind of safety feature. House was originally plumbed in 1999 if that helps, for example if there was some short-lived piping fad from around that time.

Many thanks in advance if you've ever heard of anything like this and know why it might have been installed.

 

[Reach4]

How about turning of the supply water to the WH for a while. See how quickly the drip stops or slows.

 

[Sponsor]

 

[Confoozled]

Thanks Reach4. It does stop it (edit: although it took a while). But unfortunately (didn't want to go into all this in the initial post) the thing is set up backwards and I have to turn off the whole-house supply to stop it.

What I mean is, the original plumbing placed the shut-off on the hot water pipes and failed to put shut-off on incoming feed, so there's no shutoff between the water heater and the incoming house supply.

I found this out 10 years ago when I installed the current WH, but I only had 4 hours to buy and install it from when I noticed the prior one was leaking (was heading out on a business trip and didn't want the wife to have to deal with it), so I didn't have time to install a shutoff fixture too. After that it became a "round tuit" that I never got around to.

But at that time I didn't trace all the hot water piping so I failed to notice my "mystery pipe".

 

[Fitter30]

With just the hot water heading off is there a dish washer and a overflow pan? Maybe a 5 gallon water heater down stream leaking? Maybe its in the attic?

 

[Confoozled]

Thanks fitter30, none of those. Just this pipe runs straight from the water heater to the exterior wall, then down the wall, then the PVC drip tube below it leaking hot water.

 

[Confoozled]

I found this thread mentioning a direct-installed TPR valve (i.e., on the water heater itself) and then another TPR valve installed along a pipe leading away from the hot water outlet, with TPR about about 20 feet away from the hot water heater, and terminating in a drip pipe outdoors.

(quote is, "It... appeared to be a standard TPR valve connected to the hot water line and situated about 20 feet from the water heater (which had its own properly installed and working TPR valve.")

Here's the thread, if I am able to post links https://terrylove.com/forums/index.php?threads/inline-tpr-valve.173/

I'm thinking in my situation there's another device like a TPR (maybe a pressure-only valve) behind the wallboard on that exterior wall. Maybe the original water heater (ca. 1996 make, if I'm remembering correctly) had a temp-only valve direct installed and this was an additional safety measure.

In any event, the basement was not originally finished - the first owner did that a few years later - but if I am correct then I'm left wondering why they would have installed it (edit: "it" here means the second pressure valve) at a point on the pipe after the pipe turned down between exterior wall studs? Surely finishing the basement was foreseeable. Why not install it somewhere accessible, along the way?

 

[Jadnashua]

Excessive pressure will occur if you have a closed system (check valve or a pressure-reduction-valve installed) when you use water, then the WH reheats it, expanding the water. Since the basic water supply system is rigid, any expansion will immediately raise the pressure. That pressure rise is normally enough to cause the safety valve on the water heater to release some water. THat valve is not designed to be used regularly, and if it is, it often gunks up and needs to be replaced since it no longer can close properly. That valve is designed to open at 150psi, or 210-degrees F.

To avoid having the T&P valve open on the WH in a closed system, some people put in a pressure relief valve. Some of them are adjustable, but 120-psi seems to be the default setting. Note that plumbing code calls for homes never to exceed 80-psi. So, when one of those valves does open, it represents an undesirable situation.

So, if you have a pressure relief valve that is opening, you either have a bad valve, or excessive water pressure in the home. If you do have a PRV, it may have failed, allowing the pressure to get too high, or the utility's supply pressure for some reason is higher than it was.

Get yourself a water pressure gauge, preferably with a second, peak reading tattle tale hand, and leave it connected for 24-hours to catch a peak reading as well as you reading it for the current value periodically. YOu can get one with a hose connection, and attach to say an outdoor hose bib, a utility sink, or most anywhere else with some adapters. You could put it on the drain valve of the WH, but keep in mind, when it comes time to remove, it WILL be hot!