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?

  • I just bought this house so I have no idea if this is a new or old issue.
    – DAB
    Commented Jun 22, 2021 at 19:11
  • Not familiar with Mexican construction. How is the plumbing built into the concrete? In some countries, new concrete houses have conduits built in to the walls and plastic pipes (Pex) run through the conduits, with all fittings exposed. I don't imagine you're so lucky or you would have said. What do you have?
    – jay613
    Commented Jun 22, 2021 at 19:17
  • You have a main cold water valve this is normal if the main is not fully closing there will still be water why do you think they are backwards? Water back flowing through a valve or recirculating system is also not unusual. What are you trying to solve?
    – Ed Beal
    Commented Jun 22, 2021 at 19:44
  • 1
    Can you post a picture of the lower floor shower valve. My guess is that valve is allowing the cold to flow through it to the hot side. By chance is there another valve in that shower to control the flow to the shower head. Commented Jun 22, 2021 at 20:08
  • 1
    Based on your last comment about the quick response to opening/plugging the cold side of the shower valve and that it is sharing a wall with the heater, it would be reasonable to 1) confirm your "timing" experiment by repeating it at another fixture that is further away. See if that takes longer to respond. 2) If it does take longer to respond, the cross is probably in that wall. I think you've made it clear though, you either need to live with this or start breaking walls.
    – jay613
    Commented Jun 24, 2021 at 13:05

2 Answers 2


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!

  • Thanks for coming back to let us know what you did!
    – FreeMan
    Commented Aug 31, 2021 at 12:28
  • Yes, thanks for the follow up.
    – P2000
    Commented Sep 3, 2021 at 18:39

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.

enter image description here

  • The sizes seem to be consistent throughout the house with the cold water about 1" and the hot water about 1/2". I tried opening up the shower again. There was already a slow trickle from the hot without touching the cold. When i blocked cold with my finger it took about 5 seconds to get some bubbles from the hot then 3 more seconds until it was at it's peak. Meanwhile with the kitchen sink open, there was no water on either side. If i plugged both sides of teh shower it took only 4 seconds until there was water to either the hot or cold. Kitchen is approx 25 to 30ft away from the shower.
    – DAB
    Commented Jun 25, 2021 at 14:55
  • i tried shutting the shower completely to see if the kitchen would do the same thing but since it's a single mixer it seemed to have constantly the same pressure in both hot and cold
    – DAB
    Commented Jun 25, 2021 at 14:57
  • 1
    @DAB I edited my answer to include the calculations. Looks like the cross is not far. I hope this helps. Are you saying it took 4s for flow to start in the kitchen? That could be because of residual water in the pipes.
    – P2000
    Commented Jun 25, 2021 at 21:26
  • Yeah that's what I was thinking, probably all the pipes below the night of the shower valve were full? Because the water would be level across all pipes.
    – DAB
    Commented Jun 26, 2021 at 23:57
  • I think at least gives me confidence in opening the wall behind the shower. Fingers crossed!
    – DAB
    Commented Jun 26, 2021 at 23:58

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