When I turn on cold water it runs hot for a while then cold. This happens at all faucets and also in toilet bowls which leads me to believe it is right at the water heater. The cold pipe going into the water heater is warm and the hot water pipe 10 feet or so downstream from the heater is hot, not warm, like the hot water is running, though no water is on. A while ago (over a year) I ran a cold and hot line off of the cold and hot water lines, after the water heater, to supply an outdoor shower. Could this cause mixing at the water heater? I have briefly felt warm water from faucets in the past but didn't think anything of it. Now, It seems to be getting worse.

Thanks.

Post a photo of the outdoor shower, if at all possible.

It depends on what you used for the outdoor shower faucet, but that would a logical place to start looking for the crossover .

I assume you have a basement, but if not, then you are describing a broken hot water line under the floor.

Here are some photos of the plumbing.
The first photo shows the watere heater. We have plenty of hot water but the cold inlet pipe is warm.
The second photo shows where I came off the main lines for the shower. The shower lines are cold to the touch but the main cold line is warm before and after the shower line connection. The main hot line is very warm.
The third photo shows the shower plumbing. One thing I didn't think of before is that I do not ususally shut the mixing valve, it is left on hot as shown. The flow is shut of using the ball valve on the shower head line. Even so, both hot and cold pipes are cold to the touch indicating there is no water flow and so I don't think that is the problem. The shower head valve is there in case I want to use the sillcock - washing feet, garden hose etc.

you have created a perfect example of a cross connection.
shower valves that are designed to do what you are doing have integral check valves to avoid cross connection.

Ok, but what is going on? What is happening to cause hot water to enter the cold side? Is this a simple matter of shutting off the mixing valve like you would in a regular house shower or are there plumbing changes that I need to make? Thanks for the help and explanation.
George

Quote

Is this a simple matter of shutting off the mixing valve like you would in a regular house shower.....

YES

the HW is mixing with the CW through the active shower control

turn it off when not in use (the control, not the shower head valve)

ps. it appears you may also have tempered (shower) water to the hose connection

this is OK providing you remember to turn the shower control OFF when hose connection not in use

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

I think there has to be something else going on. Earlier today I shut off both of the shower lines. But the inlet to the water heater was still warm hours later and so was the cold line after the shower shutoff. Turning off the shower lines should have taken the mixing valve out of the equation.

The cold water inlet piping to the water heater will normally get warm from natural convection.

So here is where this problem stands. The shower valve is shut and the ball valves on the shower lines are shut off. The cold water inlet to the water heater feels cold and the shower llines feel cold. But, there is still warm water coming out of the faucets and in the toilet tanks, and the main cold water line away from the water heater feels warm.
I have the following thing behind a wall panel. Is this a mixing valve? I think that the toilets may be plumbed to have warm water so they tanks don't sweat. This is a very humid area.
If this is a mixing valve, is there a way to tell if it is not working? Is this repairable or would it require full replacement? Could there be one on each toilet or could this service all of them? I hope the former because if there are others they are buried in the walls. The water coming from the faucets and in the toilet tanks seems to be hotter the closer they are to this valve.
Lots of questions, thanks for the answers.

IT is a tempering valve, but the only way it would affect the other toilets or faucets would be if they were all connected to the outlet pipe, which would not a common way to do it.

If this valve were to fail or if a diferent temper valve were to fail (if there are any) would it cause the crossover problem in the house? I am at the point of calling in a pro but want to get as much information as I can.

yes, it could

continue to investigate for any other 'equipment'

show the 'stuff' to the pro

then

stand aside and let them work

PAY ONLY FOR RESULTS (including all their time, but DO NOT PAY if they fail to fix)

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638