# Why do hot water lines freeze before cold ones?

Discussion in 'The Okie Corral' started by rtl, Jan 11, 2010.

1. ### rtlRobby The GuyMillennium Member

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This is so logic defying. My BIL is a plumber and his explanation that "the heat actually attracts the cold" sounds like total jive. He even says that many plumbers will use the hot line to feed ice makers because the hot water freezes faster. Someone please provide a jive free explanation!

2. ### HerrGlockScouts OutCLM

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http://www.scientificamerican.com/article.cfm?id=is-it-true-that-hot-water
"To the first part of the question--'Does hot water freeze faster than cold water?'--the answer is 'Not usually, but possibly under certain conditions.' It takes 540 calories to vaporize one gram of water, whereas it takes 100 calories to bring one gram of liquid water from 0 degrees Celsius to 100 degrees C. When water is hotter than 80 degrees C, the rate of cooling by rapid vaporization is very high because each evaporating gram draws at least 540 calories from the water left behind. This is a very large amount of heat compared with the one calorie per Celsius degree that is drawn from each gram of water that cools by regular thermal conduction.

"It all depends on how fast the cooling occurs, and it turns out that hot water will not freeze before cold water but will freeze before lukewarm water. Water at 100 degrees C, for example, will freeze before water warmer than 60 degrees C but not before water cooler than 60 degrees C. This phenomenon is particularly evident when the surface area that cools by rapid evaporation is large compared with the amount of water involved, such as when you wash a car with hot water on a cold winter day. [For reference, look at Conceptual Physics, by Paul G. Hewitt (HarperCollins, 1993).]

"Another situation in which hot water may freeze faster is when a pan of cold water and a pan of hot water of equal mass are placed in a freezer compartment. There is the effect of evaporation mentioned above, and also the thermal contact with the freezer shelf will cool the bottom part of the body of water. If water is cold enough, close to four degrees C (the temperature at which water is densest), then near-freezing water at the bottom will rise to the top. Convection currents will continue until the entire body of water is 0 degrees C, at which point all the water finally freezes. If the water is initially hot, cooled water at the bottom is denser than the hot water at the top, so no convection will occur and the bottom part will start freezing while the top is still warm. This effect, combined with the evaporation effect, may make hot water freeze faster than cold water in some cases. In this case, of course, the freezer will have worked harder during the given amount of time, extracting more heat from hot water."

Robert Ehrlich of George Mason University, in Fairfax, Va., adds to some of the points made by Takahashi:

"There are two ways in which hot water could freeze faster than cold water. One way [described in Jearl Walker's book The Flying Circus of Physics (Wiley, 1975)] depends on the fact that hot water evaporates faster, so that if you started with equal masses of hot and cold water, there would soon be less of the hot water to freeze, and hence it would overtake the cold water and freeze first, because the lesser the mass, the shorter the freezing time. The other way it could happen (in the case of a flat-bottomed dish of water placed in a freezer) is if the hot water melts the ice under the bottom of the dish, leading to a better thermal contact when it refreezes."

Still feeling skeptical? Fred W. Decker, a meteorologist at Oregon State University in Corvallis, encourages readers to settle the question for themselves:

"You can readily set up an experiment to learn which freezes earlier: water that is initially hot, or water that is initially cold. Use a given setting on an electric hot plate and clock the time between start and boiling for a given pot containing, say, one quart of water; first start with the water as cold as the tap will provide and then repeat it with the hottest water available from that tap. I'd wager the quart of water initially hot will come to a boil in much less time than the quart of water initially cold.

"The freezing experiment is harder to perform, because it ideally requires a walk-in cold storage chamber that is set to a temperature below freezing. Take into the chamber two quart-volume milk bottles filled with water, one from a hot tap and the other from a cold tap outside the chamber. Time them to freezing, and I would wager again that the initially colder water will freeze sooner than the initially hot water."

[We would add that, if you don't want to suffer in a walk-in freezer, you can conduct a reasonably good version of the above experiment in the freezer compartment of your refrigerator; just don't check the water too often-in which case it will never freeze-or too infrequently, in which case you may miss the moment when one container is frozen but not the other.]

Decker concludes that "much folklore results from trying to answer such a question under conditions that do not make 'all other things equal,' which the foregoing experiments do.

Still feeling skeptical? Fred W. Decker, a meteorologist at Oregon State University in Corvallis, encourages readers to settle the question for themselves:

"You can readily set up an experiment to learn which freezes earlier: water that is initially hot, or water that is initially cold. Use a given setting on an electric hot plate and clock the time between start and boiling for a given pot containing, say, one quart of water; first start with the water as cold as the tap will provide and then repeat it with the hottest water available from that tap. I'd wager the quart of water initially hot will come to a boil in much less time than the quart of water initially cold.

"The freezing experiment is harder to perform, because it ideally requires a walk-in cold storage chamber that is set to a temperature below freezing. Take into the chamber two quart-volume milk bottles filled with water, one from a hot tap and the other from a cold tap outside the chamber. Time them to freezing, and I would wager again that the initially colder water will freeze sooner than the initially hot water."

[We would add that, if you don't want to suffer in a walk-in freezer, you can conduct a reasonably good version of the above experiment in the freezer compartment of your refrigerator; just don't check the water too often-in which case it will never freeze-or too infrequently, in which case you may miss the moment when one container is frozen but not the other.]

Decker concludes that "much folklore results from trying to answer such a question under conditions that do not make 'all other things equal,' which the foregoing experiments do.

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4. ### CarrysInquisitive

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The BIL's explanation makes it quite clear why plumbers aren't brain surgeons, rocket scientist, or anything else that requires serious thought and common sense.

But then again, many rocket scientist can't fix their own broken pipes either.

Guess everyone has their place.

Just goes to show that they should understand that.

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logging on

7. ### brboyer

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Your BIL is FOS.

8. ### trifecta

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My thoughts too. There was a thread on here the other day about it and everybody jumped on the "it's true" bandwagon. Everything says its "possible" under certain conditions that aren't completely understood. It's interesting it can happen at all, but to act like it is a given under all circumstances is not too bright.

9. ### chris in va

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FOS huh? My friend lives in a double trailer and she reported her hot water lines froze a couple days ago. Cold water lines are fine.

10. ### JJohnsonCall me Jeff

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Perhaps she had flushed her toilet got a drink, refilled her ice maker, etc., keeping the cold water lines moving thus less likely to freeze?

11. ### trifecta

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Too late for logical thought. It's been a couple of days since it happened so everybody she knows will have a personal story of hot water freezing-not just first, but instead of the cold water.

12. ### certifiedfundsCosmopolitan Bias

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The answer is because people use the cold water lines much more than the hot. The water in the "hot" water pipes sits more. Not used when flushing toilets, quick handwashing, etc. It doesn't remain hot for long sitting in the pipes. And it sits, so it freezes.

Add, most folks who leave a faucet dripping to prevent freezing, they don't think to leave a hot water trickle too.

And, water heaters have exposed pipes going too and from the unit.

And, they are usually located in an out of the way place....outside storage or attic.

Hot water does NOT freeze faster than cold.

I actually got into a nasty argument with my FIL over this. Finally I posed a bet: He and I would strip down to our boxers and go sit outside. I would hold a dish of cold water. His dish would hold hot water. We could only come inside when our respective dish froze.

13. ### JAY-TX

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Your cold water gets used more than you know. maybe a slow leak inside the toilet, and the ice maker come to mind. There is also considerably more pressure on your cold water line vs the hot. That has more to do with the myth than anything else.

14. ### Diesel_Bomber

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Because everyone leaves the cold water faucets dripping and figures the hot water side will be fine.

Edit: certifiedfunds beat me to it.

15. ### MooseJawNRA LiferCLM

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Hot water pipes wouldn't freeze so quickly if they were circulated..

16. ### bootsx

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That must be why licensed plumbers make more than doctors.

17. ### brboyer

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I never said hot water pipes do not freeze faster than cold. I simply said the OP's BIL is FOS, and he is.

BTW: You only need to know three things to be a plumber: Hot goes on the left, cold on the right and fecal matter flows downhill.

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Folklore...I think it got started with hot water lines to ice makers making clear ice cubes instead of whitish ones

19. ### TheLastDaze

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as a kid I'd take ice trays one with cold water the other hot water and pop them in the freezer to see which froze faster, iirc the hot water did freeze up a little faster..

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