My water lines are mixing but i'm not sure how or where

Question

UPDATE: here's a video of the situation: https://youtu.be/NoTIYk_x8EU In this video the hot water heater valves are closed. There is no recirculation. All sinks and toilets are closed at the valves. All the shower cartridges are removed like the ones shown.

If I disconnect the water heater and turn on the main, cold water flows through BOTH pipes that go to the water heater. The cold line (correctly) and the hot line (this is backwards).

I have turned off the value on both lines, turned off all the valves on all the sinks and appliances. The only thing left is the shower valves. I thought it might be a valve, so i removed the cartridges from all of them exposing hot and cold holes in the valves.

I turned on the cold water (the water heater is completely shut off with valves on both pipes) and of the 3 showers, only the shower on the bottom floor had water come directly out of the valve. If i put my finger in to plug the cold water hole, cold water also comes out of the other. If i remove my finger, the hole that's supposed to be hot loses pressure but still flows quite a bit.

The lines are definitely crossed but i can't see where. The house is made of concrete and cinder block (this is typical in this area of mexico) so it's not easy to break open the walls. Is there any way I can detect where this crossover is? Is there anything else it can be?

Answer 1

I was able to solve this problem. It involved opening a few walls and experimenting with cutting some pipes. Unfortunately listening devices were of no use. I was able to track down some people who worked on this house and others nearby that had the same problem, although the location of the problem was a little different.

In both the downstairs and upstairs bathrooms, the shower spout and knobs are on the west side of the shower. They were originally designed to be on the east side. They had already put pipes going to the east side of the shower and instead of capping those off, they decided to connect them together. Why? I have no idea. Maybe they didn't have caps, thought that nobody would know, or forgot to fix it later. In any event, the solution was to cut these connections and cap them off.

The first was slightly easier to find. The pipes for the second floor can be seen in the ceiling through the holes for the lights. There were hot/cold pipes going to the wrong side of the shower. We figured that was the issue, opened the ceiling and cut it. For the first floor, the pipes ran under the shower, maybe in the foundation, so we weren't sure how to open it. But one of the guys had the idea to open the wall on the outside. With some measurements about where the shower would be, they were able to cut a single hole about 3 inches wide right where the pipe was. Cut it and capped it and we're all good.

Thanks for all the suggestions and comments!

Answer 2

Your cross connect could be in any mixing valve, inside or outside.

Sometimes an outside shower or hot supply can have a deliberate cross valve near its shutoff to allow blowing for winterization.

Another purpose of the cross valve is to allow a pressure balanced outside shower to work even if its hot supply is shut off. (A pressure balanced valve will block flow if either cold or hot has no pressure, e.g. when either one is shut off)

Also, if you know the diameter of the tubing/piping, you can estimate the location of the cross flow by measuring the time to build pressure, as you have observed, on the hot side.

You need the flow rate with the main valve squeezed as you have in the video, and the diameter of the pipes. If you need help with the math, let us know.

If the diameter varies and you don't know where, then regrettably this won't work directly, but you could place bounds on the distance: calculate the distance once based on assuming one diameter, and then again based on another possible diameter. Perhaps these bounds are close enough to be useful in narrowing the search.

For the math: determine the flow by measuring the time it takes to fill a container of known volume, e.g. 1Ga milk jug. Now you have the flow rate .

If it takes for instance 80s to fill a 1Ga jug, your 8s timing at the shower means 0.1Ga in the pipes. The fill time of course depends on the setting of your main shut off.

Type your numbers in the calculate at https://www.omnicalculator.com/construction/pipe-volume

Using the 80s example with 0.5 in pipes, that comes to almost 10ft distance.

The accuracy also depends on the residual fill of the pipes. So for a reliable measurement it would be good if you could blow the pipes (I realize this is not easy). That could explain the comparatively short time to get flow at the kitchen.