I have just discovered 'ask Yahoo'—mostly answers to questions I never cared to ask, but with a sense of humor. Some questions, though, I really was curious about the answers:
- Does hot water really freeze faster than cold water? (answer: yes, sometimes, which I find really odd)
- How many vacation days has George W. Bush taken to date as president? How does that compare with Clinton? (what you'd expect)
- Why are legal pads yellow (because they've been yellow since 1888)
- Did Jessica Lynch receive the Bronze Star? Why? (Nobody knows why)
- Why do people hang pairs of shoes from power lines? (This one is almost as mysterious)
That’s true about the hot and cold water and pipes freezing. We have a crawl space that it poorly insulated, copper pipes, and it is the hot water line that always freezes before the cold water line. We have a lot of minerals in the water also, so it the hot water heater hasn’t been flushed for awhile there may be a lot of accuumulation of minerals for the water to crystallize on.
Someone’s got to let the secret out. They hang shoes off the power lines so people like me will know which innercity blocks to turn down, where I can buy drugs out my car window. Really.
The hot water pipes in your crawl space will freeze on a cold night when the cold water pipes do not for a very simple reason. At bedtime both pipes contain water at the same temaperature(called ambient). During the night any water used will be cold water…(flushing toilets, getting a drink or taking a pill). This movement in the cold pipes brings in water about 20 F degrees warmer than a pipe that is about to freeze. The hot water pipe will burst also, because it has been through a million heating/cooling cycles which cause a loss of ductility(the abilitiy to stretch). hence the slighest expansion cracks the metal.
There is a great explanation of why a hot water pipe bursts first here.
I also think that frank c is correct on his first point about the usage of more cold water during the night (or leaky faucets) keeps it from freezing.
But he is wrong on the million heating/cooling cycles causing a loss of ductility. All metals except Aluminum have what is called an ‘endurance limit’. If you do a number of tests to find the number of cycles it takes to break a metal versus the amount of stress applied each time you get a graph of a curve that would fit within the shape of the letter ‘L’ (a smoothed out L shape). At very high stress it breaks within a very few cycles. At very low cycles there is a point at which the number of cycles to failure becomes infinite. This is the ‘endurance limit’. Any stress below this level no matter how many cycles it is applied will NEVER break the part. The stress induced in Copper in a straight pipe by heating/cooling cycles induced by carrying hot water is so small it is well under the endurance limit so it is not a factor. Believe me, I’m a Metalurgical Engineer. Rather frank c’s first point plus the affects of particles in hot versus cold water outlined at the above URL are the cause.
Interestingly, I mentioned that Aluminum does not have an endurance limit. This is why airplane landing gear and other critical parts must be replaced after a certain number of flights regardless of the observed condition of the part. Its impossible to design the part never to break. It will break but the number of cycles are known so it can be predicted with some precision and replaced well before that. That’s why airplane maintenance records are such a big deal.