wierd situation

[Ned Flanders]

i figured thats what you ment just making sure.

[PipeDown]

So the water never gets cold. You've compared this to the water temp coming out of the city supply so you know it could be cold right. And this happens under static (no flow) conditions. Shouldn't have any gravity fed convection unless there is a cross connection and you ruled that out already.

Maybe the cold pipe is being heated by a source other than injection of heated water. Does the pipe run through a hot space (like an attic) or a heat sink (concrete slab or wall exposed to direct sunlight) or is it directly in contact with the hot pipe or another warm surface?

I bet on an unintentional solar heat sink.

Just my thoughts. (If it don't look like a duck, maybe it really isn't a duck)

[PipeDown]

More to add....

When did the owner do the add on plumbing? Was it after last winter? If I am right about an unintentional solar collector, the pipe will be susceptable to freezing this winter. If the problem is seasonal, this also supports my idea. Add the check valve (since it is eaier than the alternatives), if it doesn't improve significantly you may be back in December.

[PipeDown]

nevermind, I just reread the OP more carefully. The part about the cold inlet getting hot proves my theory wrong

[PipeDown]

Allow me to try again (Me; a hack)

Does the cold supply from the city flow past the hot WH at a Tee close to the inlet. If there is a restriction, you could be seeing a venturi effect which generates negative pressure at the cold inlet when cold water runs across it. Is the pipe copper or galv steel. I have a restriction in my hot pipe near the kitchen sink that causes a loud cavitation in the pipe at the tee when the water is run at full blast (so much negative pressure a vacuum bubble forms then collapses sending a pressure wave down the pipe, or at least that's my theory for now). That pipe is galv steel and the restriction is certainly rust, I'm not sure how a copper pipe would develop such a restriction or even if it could.

Restriction inside T creates a venturi that causes a negative pressure that sucks hot water out of WH and mixes with cold.

City cold > -------T------- to house ->

| | WH- inlet

Plausible, especially if the Tee is positioned very near or below the top of the tank (like in some tight spaces). If the distance from the Tee to the WH is long or has a substantial vertical climb, this is less likely but still possible.

In any case the check valves should do the trick but replacing the tee would improve flow. Adding check valves is like changing the design. It is much more satisfying if you can fix the problem without changing the design, then you know you were right and you know you understood the problem.

Running with this idea, have you made any repairs that have improved flow rate downstream of this hypothetical Tee? Increased output flow rate but the same input pressure would make it easier for a venturi to develop enough negative pressure to cause a reversal of flow. Did the problem develop gradually (slow buildup) or suddenly (as a result of an unrelated repair or dislodged rust chunk moving to the tee like a stroke)

I bet if you run the cold at one tap and hot at another tap, you can get it to flow in the right direction at the WH. If the restriction is a loose particle, try turning off the city water and opening an outside hose bib, then run water backward through the system and try to flush (don't know where you get the water though since I had you turn it off, maybe a friendly neighbor or a recirculation pump). In my house it would require fitting an adapter to a sink and running a hose to the neighbor then opening the front hose bib then running water first with the hot open then with the cold open. This way I can flush all 3 ports on the Tee in the reverse direction. While this works great on sprinkler systems, I have not actually tried this on a house, its just an idea.

I way overexplained this if you are already familiar with the concept.

[Mike Grooms]

"Ned Flanders" <

I just wonder, really, how much actual water will push back through the cold from the WH. I can't imagine it being more than just a little.

The new suspect is that the apartment shares a wall in common with another apartment. I'll do a little tearing apart and let everyone know.

[Mike Grooms]

"Mike Grooms"

And here's the weird part...I was there to see this actually happen, and the lady who lives in the apartment verified that it's been happening for a while, but for the past week, it hasn't happened again. I promise that when I was there, you'd run the cold, and warm water would come out, and I could tell from putting my hands on the WH pipes, that it was backing up through the cold inlet.

Now, for the past week (basically from the day after I was there) things have been just fine. I am mystified, except that I figured out that her bathroom shares a wall with another apartment. It just doesn't make sense.

My question is this. Since I was there, and the problem stopped the next day, how much do I charge?

[Ned Flanders]

you have also consulted her on what it could be and what it will take to be sure. how long were you there? mabye like 150-200 bucks.

btw that sounds more like a tempering valve

[PipeDown]

Agh, Ahh, ek------ arg &%# Oop.................. nevermind (so the wrong flame but it could have gone with my false start just above)

[PipeDown]

You're diagnostic procedures seem to have knocked whatever it was loose. It will probably come back later and so will you. Charge her now for a little labor but let her know that a perminant fix is likely to include the check valves which will prevent the symptom regardless of the real cause.

Still wondering, copper or galv steel ?

[Mike Grooms]

Gosh Guys!

It was just a joke. I wouldn't charge in a case like this. Geez...

[Snowbound]

I had this situation once. The previous owners of our house converted the heat from electric forced air to oil. They also converted the electric HWT to feed the water through the new boiler. This made the hot water extremely hot and we have young kids so I put in one of those thermostatic mixing valves which feeds a little cold water into the hot. After a few years the back flow preventer stuck open. It was noticable wit the bathroom sink that's directly above the connection.