Why does the water from my shower get hotter when I use a shut-off valve?

Question

The water temperature in my shower is hard to control, so I bought a shower shut-off valve hoping to be able to conserve water by setting the temperature at the beginning of my shower and to temporarily turn it off while I lather myself with soap. I installed the valve between the handheld showerhead and the shower hose.

I've noticed the following:

1. I set the shower faucet to my desired temperature and wait for the water to reach that temperature.
2. I close the shut-off valve.
3. I wait a few minutes.
4. I re-open the shut-off valve.
5. First there is a small amount of cold water. (I assume that this is water in the shower hose that cooled off.)
6. Then there are a few seconds of very hot water.
7. Finally the water is at my original desired temperature.

Why are there a few seconds of very hot water?

I've considered that maybe this was a psychological effect: perhaps I got cold from being soaking wet while the water was off, and the water only seemed hotter? I haven't formally measured the temperature, but I have tried reopening the shut-off valve while dry, and the water is still initially very hot, so I don't think it's an illusion.

(I don't think we have a water circulation pump since we usually need to let water from the hot tap flow for a while before hot water actually comes out.)

Answer 1

Let's flesh this into an answer.

This happens with my kitchen faucet and one of those "quick-flip - not quite total shutoff" valves that screw into the aerator. My guess is that some hot leaks back into the cold pipes, (the control valve is open, the shutoff beyond the control valve is closed) so the mix is not the same temperature as it was until that's flushed out...

Responding to comment that

I don't think water will 'leak' in a pipe unless there is an opening so the water can move through the pipe.

Indeed. But if any cold water is used anywhere in the house (an opening), the opening between the hot line and the cold line at the control valve means that some of the cold water drawn elsewhere can be sourced from the hot line at the shower, resulting in hot water entering the cold water pipe for some distance. The draw might be only some fraction of what is used (with the rest coming from the cold supply as per usual) but it ends up with a slug of hot water in the cold supply pipe to the fixture.

When returning to normal flow, instead of a mixture of hot and cold, you have a mixture of hot and hot, until that water is flushed back out of the cold supply pipe.

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If you wanted to prevent this, a check (one-way) valve on the cold water feed to the shower should do that.

Answer 2

If you have a modern shower valve with an "anti-scald" feature and are shutting off the water with the shower shut off valve, then turning the water back on is setting up transient swings until the system settles down again. I know these shutoff valves can save water, but I don't think modern shower valves work with them properly.

Note that when you shut off the water at the shower head, you are fully pressurizing the mixing chamber of the valve and the lines from the mixing chamber to the shutoff valve. This could be causing leaks in seals and at connections inside the wall. I doubt that these shower valves are designed to be pressurized like this.

Finally, you didn't state what kind of water heater you have. If it's a tankless, then you are putting the moving parts through more actions than necessary.

If you are in an extremely water stressed environment, you could just shut the shower valve to stop the flow while you lather up.

EDIT

To shower in the least amount of water with a given shower head take a small bucket (or large bowl) into the shower with you. Direct the shower head into the bucket and turn on the valve to highest flow and highest temp. Run water into the bucket until the water in the bucket is the right temp and then shut the valve off.

Pour or ladle water over yourself and apply soap. A stainless steel "Sierra Club cup" makes a good ladle. Use any remaining water in the bucket to start rinsing, then turn on the shower valve and adjust the temp. Transient swings of temp should be minimal because the valve is at operating temp and the hot supply is loaded with hot water.