Hot water freezes faster than cold water

Cecil's Columns/Staff Reports
1

I happen to be passing through and read about hot water freezing faster than cold water. I happen to have some information that may be helpful.

I am an HVAC technician for 20 years and have worked with many plumbers during this time. I have personally repaired over a dozen split and leaking pipes due to freezing due to inclimate weather. In all cases, if there were hot and cold water pipes side by side the hot water pipe ALWAYS froze before the cold water pipe. I have asked many of my plumber friends and they have all agreed.

Now the temperatures of the water may not be drastic. If both pipes havent been used for some time the hot water pipe will only be perhaps 10 degrees warmer than the cold water. I really don’t know the scientific answer to this but I have a couple of theories:

a) Due expansion of the hot water, when the pipe cools down, the water contracts, which may provide a lower pressure or vapor in the pipe that frrezes faster.

b) The cold water and the pipe are aclimated to the cold so the process slows down, the hot water and pipe cools more rapidly when exposed to the colder temperatures because the molecules move at a faster rate.

Either way I don’t know the true reason but ask your local plumber and I’m sure he will agree.

Jim129

2

I worked in North Texas one winter and rented a travel trailer for a couple of months. The water lines for the bathroom ran through a small compartment on the back of the trailer and invariably, when the outside temperature was low enough, my hot water line would freeze but the cold water lines were clear. Luckily they never broke but in order to take a hot shower I had to thaw the frozen hot water line with a hair dryer. I eventually set up a low wattage light in the compartment to prevent this.

I don’t really know why this happens but I have an idea that it has to do with the change in the salt content of the water being lower after it is heated. This would definately lower the freezing point of the water. If you’ve ever seen the build up in a well used water heater you’ll know what I’m speaking of. They get full of stones made up of calcium and iron salts.

3

I thought I’d throw this in:
When I drove a Zamboni we always used hot water to replace the surface of the ice being scraped off. It not only seemed to freeze faster but made a much smoother surface for the skaters.

4

Link to column in question.

5

It seems obvious that, everything else being equal, cold water will freeze more quickly than hot water.

That leaves us to explain what it is that is not equal between cold water pipes and hot water pips in a typical house. This is assuming that hot water pipes actually do invariably freeze before cold water pipes. I live in a frost-free area so I cannot contribute direct observations.

One thing that I can think of is that perhaps the hot water pipe is intentionally less coupled to its environment. It is connected to a small local reservoir of water (the hot water tank) rather than a huge remote one (the town water supply). Perhaps the fittings that physically connect it to the environment are designed to limit the flow of heat? I am not a plumber but this does sound reasonable. Normally this would prevent the loss of heat from the water to the environment, but it would also work in the opposite direction. That is, if the water starts becoming cold enough to freeze, it would not be able to draw heat from the environment quickly enough to prevent this.

Well how about it - does this at least seem worthy of further investigation?

6

I think this debate the freezing rates of hot and cold water needs to define things a little better.

First, there is the question of pipes. I too have noticed that when two pipes, hot and cold, are side by side in a space that gets cold enough to freeze during very cold weather, the hot tends to freeze first. But let’s get our terms right here. It is not hot water that is freezing. If it were still hot water, it would not freeze. It is water in the hot water pipe that has probably sat there in the cold air space for several hours overnight until it became cold enough to freeze. But why does it freeze before the water in the cold pipe that is right next to it?

Here is one possible reason:

The hot water system gets used less than the cold, especially at night, when the pipe-freezing temperatures are most likely to occur. If there are several people in the house, they may get up and flush the toilet a few times, or run a glass of water. So the cold water keeps running, and even in winter it is maybe 5 or 10 degrees warmer than freezing.

Meanwhile, the water in the hot water pipe may have been sitting there since somone took a bath or a shower. And some people wash in the morning, not in the evening. So by the time the outside temperature is coldest, around 6 a.m., the water in the “hot” water pipe has been sitting there in a freezing space between the walls for maybe seven hours or more, just getting colder and colder and finally forming an ice plug in the pipe.

Now, there may be other factors such as the fact that the pressure in the hot water system is lower. But generally speaking, the time/use secnario would explain it.

Second, there is the “fact” that a pot of boiling water set in a freezing temperature will freeze faster than the same amount of cold water. This is a different matter from pipes freezing. And this phenomenon has been proven. However, all that is really happening is that the hot water evaporates faster than the cold, so that by the time it has cooled down to just above freezing, there is less of it to freeze.

Therefore, do not believe people who tell you to fill your ice cube trays with hot water because it will make them freeze faster. All you will be doing is making them faster because you are making smaller ice cubes. And you are making your freezer work harder for nothing.

7

The phenomenon where hot water freezes faster than cold water is called the mpemba effect. and it is only certain circumstances where it is so. If you make two trays of ice cubes, one with hot, one with cold, the cold tray will freeze first.

Here is a link about the Mpemba effect:
http://www.school-for-champions.com/science/mpemba.htm

But the gyst of it is, if your cooling source is above the water, it causes rotating currents that mix the water and cause it to freeze faster because of a more even distribution of cold.

Thanks.

Donuts.

8

Zambonis use hot water only because it provides a smoother surface. The hot water melts the rough surface of the ice to be smoothed. It has nothing to do with freezing faster.

And if you always used hot water, how can you say it seemed to freeze faster? Faster than what?

9

Coincidentally, this month’s issue of Physics World has an article about the Mpemba Effect by Philip Ball. He doesn’t come to any definite conclusions about what, if anything, is going on, but it’s a nice overview.

He links to this recent review (a pdf) of the subject. (This doesn’t appear to have been published in a journal as yet, but it was only submitted to arxiv a few months back, so that’s not too surprising.) That even cites Cecil’s column, even if it isn’t terribly impressed by his reasoning.