Freezing of Hot and Cold Water Pipes

Why do hot water pipes sometimes freeze up and burst when adjacent cold water pipes do not? That is an intriguing question recently asked by Anna Kvistad of Clear, Alaska and Tom Busch of Nome.

My first thought on the matter was that the statement of the question itself was in error because I could not understand how it could be possible for a hot pipe to freeze and burst without a cold pipe doing the same, and probably quicker. But then I asked around and found that several people had observed the same seemingly illogical phenomenon described by Ms. Kvistad and Mr. Busch. In fact, hot water pipes exposed to freezing temperatures can freeze and burst while identical cold water pipes beside them remain intact.

The reason for this curious difference is thought to be related to the existence of foreign solid particles in well or stream water, that is, in any except distilled water. Compared to the size of water molecules, the foreign particles are quite large, though perhaps still small enough to pass through even a highly effective filter. However, prolonged heating breaks up the foreign particles so that they become smaller, and they remain small even after the water is cooled down. Consequently, normal tap water that has been heated for a few hours is different from cold water. This difference affects the temperature at which the first ice crystals appear in water.

To see what happens consider two identical pipes, one with hot water inside and one with cold, and both exposed to outside temperature far below freezing. Now it so happens that the larger foreign particles within the cold water pipe actually do cause the water in that pipe to begin freezing before the water in the hot water pipe, even if both pipes are brought down to the temperature at which ice made from distilled water melts, 32°F or 0°C. Note that this temperature is the temperature at which ice melts but is not necessarily the one at which the first ice crystals appear as the water freezes, even though we commonly refer to 0°C or 32°F as the freezing point.

According to the leading idea on the subject of bursting pipes, ice starts to form inside the cold water pipe soon after the temperature there drops slightly below 0°C. Heat energy given off as the ice forms increases the temperature of the ice-water mixture to 0°C. As time goes on, more ice freezes in the pipe, but the growth is slow. The slowness of the growth allows water to flow down the center and be forced out the ends of the freezing section of the pipe. The result is that the cold water pipe eventually freezes solid but does not burst.

The situation in the hot water pipe is quite different. Here, previous heating of the water broke up the foreign particles to a much smaller size that does not promote freezing of the water, as in the other pipe. Instead the water in this pipe cools well below 0°C without turning to ice. Such water is called supercooled because it is still liquid well below 0°C. Supercooling always occurs even if the water is vigorously stirred, but still water with very small or no foreign particles contained in it typically will cool several degrees below 0°C before freezing. But when the freezing does take place in the hot water pipe, it occurs very rapidly throughout the entire pipe. The temperature of the ice and water mixture rises to 0°C as in the other pipe, but in this case a far more extensive network of ice forms in the pipe.

So even though the water in the hot water pipe freezes later than in the other pipe, it freezes so rapidly that the pressure created by the volume increase cannot be released by flow of water out the ends of the pipe. Result: the pipe bursts.

Thus we see that cold water pipes really do freeze before hot water pipes, but the freezing process is different enough to sometimes burst the hot water pipe without harming the cold water pipe. Next week I will discuss the differences in freezing rates of open pans of hot and cold water.