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Old 12-08-2014, 09:09 AM   #31
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Originally Posted by rockfordroo View Post
When you're showering, the hot and cold water mix together at the point where the shower hose starts. Shutting off the flow anywhere in the shower hose still allows the hot and cold water to be "connected." Since there is usually a larger pressure drop associated with either the hot or the cold (they'll never be exact), whichever one has the lower pressure drop (i.e., will have the highest pressure at the faucet) will "overcome" the other and push some of its water backwards into the other line at the faucet. Then when you turn the shower back on at the hose, you get a blast of whichever one pushed back into the other.

Since the hot water has to go thru the water heater, and there's at least two valves (winterizing) and some extra hose that creates additional pressure drops that the cold line doesn't have, I suspect that the lower pressure drop for most of us is the cold line, thus its higher pressure pushes some cold water into the hot line, and we get a blast of cold when we re-open the valve in the shower hose.
Thats what im talkin about. Fantastic explanation.
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Old 12-08-2014, 11:13 AM   #32
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Pressure drop is only an issue through a restriction during high flow. You can push water through a pin hole and get full static water pressure. When the shower head valve is off there is a minimal flow so the extra potential restrictions aren't the problem.

My theory has always been that as the warm water cools (while you soap up) in the long run back to the WH, it contracts and that pulls cold water backwards into the warm line at the shower. Then when you turn the shower head valve back on, you have cold water coming from both lines temporarily.
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Old 12-08-2014, 11:19 AM   #33
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Originally Posted by techntrek View Post
Pressure drop is only an issue through a restriction during high flow. You can push water through a pin hole and get full static water pressure. When the shower head valve is off there is a minimal flow so the extra potential restrictions aren't the problem.

My theory has always been that as the warm water cools (while you soap up) in the long run back to the WH, it contracts and that pulls cold water backwards into the warm line at the shower. Then when you turn the shower head valve back on, you have cold water coming from both lines temporarily.
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Old 12-08-2014, 05:51 PM   #34
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Pressure drop is only an issue through a restriction during high flow. You can push water through a pin hole and get full static water pressure. When the shower head valve is off there is a minimal flow so the extra potential restrictions aren't the problem.

My theory has always been that as the warm water cools (while you soap up) in the long run back to the WH, it contracts and that pulls cold water backwards into the warm line at the shower. Then when you turn the shower head valve back on, you have cold water coming from both lines temporarily.
I've designed several water systems in electric generating stations.

The amount of cooling that could occur during your soap-up and the amount of contraction associated with that cooling is MUCH smaller than the amount of water required to re-pressurize the line. (Not saying that won't contribute an ounce or two of water.)

However, there is a pressure drop associated with ANY component (valve, Tee, elbow, every foot of pipe, etc.) and while you are correct that it's more at higher flowrates, some pressure drop is always there and proportional to the flow, so it doesn't matter what the flowrate is, there is SOME pressure drop. The higher the flowrate, the higher the pressure drop. And most of us usually have the max flowrate we can get out of the shower head. For most of us, this means the cold is usually wide open and we warm it up as necessary with the hot, so the hot faucet isn't fully open, adding an additional pressure drop. So you will get SOME cold water flowing into the hot line until the cold line pressurizes the hot line. It doesn't take much cold water to go into the hot line to pressurize it (but more than you'll get from contraction due to cooling), but it's definitely enough to make you feel it when you turn it back on and you get hit with cold water from both sides of the faucet.
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Old 12-09-2014, 05:26 PM   #35
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techntrek (et al):

Well, you got me thinking, and I'm an engineer, so I couldn't help myself - I did some calculatin'.

Assuming 15 ft of 3/8" ID PEX on the hot water line, and assuming it starts at 140 degF, and then cools to 130 degF, the water volume in the line would "shrink" by about 0.436 cups. However, PEX is not a conductor, it's an insulator (not a great one, necessarily), so I think even this 10 degF drop would take WAY longer than it takes to soap up and turn the shower head back on. The cold water repressurizing the hot line only takes a few seconds, so I'm still convinced that it's not the water cooling off, but the cold line repressurizing the hot line.

For those who get hot water instead of cold, there may be something else going on with respect to greater pressure losses in their cold water lines for some reason. Perhaps larger diameter PEX in the hot line for some reason?
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