I'm assuming that everyone who reads this blog has heard the urban legend that hot water pipes will freeze before cold water pipes. Some of you may have even heard some complex reasons why the hot water pipes will freeze before the cold water pipes. I've heard explanations ranging from the absurd to the excessively complex. Some of the explanations focus on the assumption that because the water in the hot water pipes is hotter and somehow that makes it so that it looses energy faster than the cold water and thus it will freeze faster. Though at some point it has to reach the same temperature as the cold water pipes and therefore it should then have the same heat loss rate as the cold water, thereby invalidating this theory, but I have even heard some complex explanations as to why the "hot" water, now actually colder than the "cold" water, still looses heat faster. There are other explanations involving the salinity of the water, the effect of super cooling, radiation, heat stress on the pipes and any number of random explanations.
So to counter all of these theories I propose a rather simple explanation that has more to do with money than thermodynamics. Yes that's right, money. My theory is is that hot water pipes freeze faster than cold water pipes because they represent an additional cost to the construction of a house. That's right, they freeze because they make the house more expensive.
To understand how this is possible we first have to understand how houses in the US (and other places) were constructed, and are currently constructed. Indoor plumbing did not become widespread in private residences until the 20th century and in many places it did not become common until after World War II. The problem is that at that time (1900-1950) the majority of the houses were built before indoor plumbing became common. This means that if a house were to be upgraded with indoor plumbing then that would represent a major retrofitting process. Sometimes this meant that water could only be piped to one place in the house (or maybe two if they could get the pipes there).
But the houses that were built during that time were being built to accommodate the new fangled indoor plumbing, and this meant putting the pipes throughout the house in such a way that every bathroom and the kitchen had running water. Thus with the new houses the water pipes were integrated with the house itself and in some cases ran through the foundation. But the only problem was that up until this time hot water was made by heating water on a stove (my grandmother had a large wood burning stove that took up half the kitchen, until it was replaced in the 1950's with a smaller electric one), or by having a gas or electric heater that hung on the wall over the sink to make the hot water. Those of you who have spent any time outside the US have probably seen these types of hot water heaters. In the US they are called Tankless Water Heaters. While these type of water heaters can service an entire house, typically the smaller (and cheaper!) ones are used to service a single point in a house. These water heaters were at one point common in the US. This means that if someone wanted hot water for a shower or for the kitchen then they would have one of these heaters mounted on the wall in their bathroom or kitchen.
The reason why many people went with this option was that it was cheaper than installing a central water heater and then running the pipes all throughout the house. This meant that the majority of houses built only had the cold water pipes integrated with the house and running through the foundation. But at some point it became popular, or practical, or cheaper, to have a central water heater to service the entire house. This meant that now the house had to be retrofitted with hot water pipes, much like the older houses had to be retrofitted with standard pipes to get indoor plumbing. The problem was that now the plumber had to get the pipes from one point in the house to every bathroom in the house. Typically it was impossible to run the pipes right alongside the cold water pipes (or the existing water pipes) because some times they were in the foundation or in other inaccessible places that were not inaccessible when the house was being built. This meant that the new "hot water" pipes had to be put in a location where it was easy to install, because it was cheaper to run the pipes through the attic or under a floor board than it was to completely remodel the house and put the hot and cold water pipes close to each other (like is standard now in the US).
While this may not have been the case for every house, this is probably the way it was done in just enough houses to start the urban legend. Because the cold water pipes were integrated with the house they got more insulation than the hot water pipes which were in the attic or some other place that didn't have as much insulation. Thus in the middle of the winter the cold water pipes were protected from the cold unlike the hot water pipes which were not. Thus when the residents of the house woke up in the morning they go to turn on the water for their shower or in the kitchen and they only get cold water, no hot water. They call a plumber or track down the problem themselves and they find that the hot water pipes are frozen (because they have been sitting idle and exposed all night) and thus they scratch their heads and say, "Huh? That doesn't make sense, why would the hot water pipes freeze but not the cold water pipes?" not realizing that the pipes were actually in two separate places and therefore were exposed to different temperatures. And thus began the great urban legend of the hot water pipes freezing before the cold water pipes, and it was all because it was cheaper to retrofit the house for the hot water pipes than it was to remodel the whole house and run the hot and cold water pipes together, as is standard now.
Now this did not happen in every house, but my guess is that it happened in just enough houses to start the legend (I grew up in Arizona where nothing freezes and I still heard about it). But we do not hear so much about the legend now, and that is largely do to the fact that modern houses are all built so that the hot and cold water pipes are together and thus if one freezes they both freeze, and people will no longer scratch their heads about the hot water pipes freezing, and not the cold water pipes. Also most of the people who read this probably grew up in the era of the new houses with modern plumbing standards, but there were just enough houses left over from before to give the rumor legendary status.