I assume there's no hot water return system, right?
If not, and by turning off the the cold supply to the water heater you only
have cold water to the cold side of all the fixtures your problem is almost
certainly pressure that is being created by the heating of the water.
Your maost practical recourse is to put in a check valve on the cold supply
of the heater and an expansion tank on the hot side.
Keep us posted how this turns out.
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 ->
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
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.
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?
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 ?
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.
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