A friend asked me a question. If you’re taking a shower and someone runs the faucet water in the kitchen what would be a cause for the shower water to go cold?
Hot water is directed towards that tap, momentarily disrupting the flow.
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old style faucets…
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Lack of hot water. 
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A possible cause is the WH is too small and or has a very slow recovery rate.
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Undersized Supply lines, (restricted) Galvanized piping, DIY plumbing repairs/flipping activities, etc…
I run into this issue in newly “flipped/remodeled” homes also.
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The shower valve is not an anti-scald valve.
The kitchen is upstream of the shower. “Path of least resistance”. The capacity of the two valves is greater than the supply line capacity from/to the water.
Insufficient water pressure to overcome the static pressure of that plumbing design.
i.e. too many 90 degree elbows between the two valves or too much pipe.
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I suspect this is more common than we generally realize. I don’t know of any inspector trying to actually test for this, because it is very time consuming in a house with 3 or more baths. My wife used to amuse herself by flushing the toilet to give me a hot surge when I was in the shower.
As stated the basic reason is not enough hot water. Why falls into three overlapping categories: 1) your supply is low (small tank, slow recovery, already used most) and 2) hot water diverted by plumbing routing. 3) water temperature is too low at the tank (bathing is about 100° if this is the hot water setting you are not mixing cold water to it and consequently draining the tank faster).
We really should not diagnose the problem…we should however describe what we observed followed by the consequences of such a defect in their home…Such as… “Shrinkage may be Significant”.
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Agreed. It is way beyond the SOP, time consuming and I don’t get paid for it. That said, my post was simply to inform the OP, who may not be a home inspector (yet) and get them to think about the system and how hot water is delivered.
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Cause, changes in water pressure.
Compensate; install a pressure balancing shower valve-faucet.
I have an auto temperature balancing/adjusting shower faucet. Mine is not as effective as advertised.
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