Freezing Point Of Water

Factual Questions
1

Can Someone Tell Me Why Hot Water Pipes Freeze Before Cold Water Pipes? To A Non-scientist, This Seems Counter Intuitive. Yet Reality Reigns.

Thanks.

2

First, you don’t need capital letters for each word. One per sentence usually is enough.

Second, why do you think that hot water pipes freeze before cold water pipes? That certainly isn’t my experience.

3

Cecil did a column related to this:

Which freezes faster, hot water or cold water?

4

I think he typed it in all caps–the board software converts it so that each word only has the first letter capitalized.

To address the OP, it probably depends on how your pipes are run. Although there may be some effect on freezing rate due to the fact that water, when heated, gives up dissolved gasses which in turn will have some effect on freezing rate, at least in theory, this won’t be really noticeable in practice. More likely, some of your hot water pipes are run up an outside wall somewhere for some reason, whereas your cold water pipes are all in the interior walls.

5

All else being equal, they don’t. The Heat transer (loss) rate is proportinal to the difference in temp. but as the difference narrows the rate slows and equalizes. Two possible things that could cause you to perceive that the hot pipes are freezing faster: 1. Ice crystals may begin to form on the interior walls of the hot pipes before they do on the cold pipes, due to the accelerated tranfer rate. These could break loose if you draw water, causing a valve or other restriction to become plugged. 2. The hot pipes may, at some point in the system, be more exposed to the cold ambient air.

6

The OP posed this question in a way that is answerable, or at least conjecturable, which is rare for this question.

The OP discussion is limited to hot and cold water pipes. I will assume them to be in a residential building in an area of the building that is exposed to sub freezing temperatures. This usually indicates a crawl space under the house or water piping run in an exterior wall.

Three of the typical answers usually given have been posted above -

  1. Not likely to be the case - Potentially raised freezing point of the hot water due to losing dissolved gasses in the water heater. But water heaters in the US are not vented, so there would be no place for the gasses to escape to, ergo this is unlikely to be the case.

  2. Good possibility - Hot water pipe run in a more exposed location than cold. Or, even if it is not an exposed location it could be positioned just right to hit the cold while the cold water pipe just avoids the cold draft.

  3. Is not possible. Accelerated heat transfer rate from higher temp - There is initially an accelerated heat transfer rate, but once the hot water in the pipe cools down to approach the cold water temp their heat losses will be similar.

Another minor answer usually given-

  1. Minimally contributory - Potentially raised freezing point of the hot water due to losing dissolved minerals in the water heater. Especially with electric heaters, the intense heat of the water next to the heating element does cause calcium and other minerals to precipitate out of solution. The amount lost is so minor that this has only a tiny effect of freeezing temp.

And the two answers MOST responsible for this question -

  1. Hot water pipes are not used as often as cold water pipes. It is far more likely that the hot water pipes remain stagnant and freezes while the cold water sees just enough use (glass of water, toilet flush, etc) to keep water flowing such that it doesn’t freeze.

  2. Plumbers do not understand heat transfer or the problem with anectdotal evidence and faulty memory, and thus continue to propagate this myth. Normal people may have their water pipes freeze once or twice in their life. With this myth circulating around, people whose hot water pipes freeze pass the myth off as proven fact based on their anecdote. People whose cold water pipes freeze write it off as an anomoly.

7

This is only an opinion, but it IS related to the OP, and does deserve consideration. I think #5 has a good deal of validity in some circumstances.

8

This seems most likely the answer to me as well. Hot water pipes are not heated…only the water in the water heater is heated. Doubtless most people have noticed when you turn on the hot water you need to let it run awhile till you actually get hot water. The reason of course is you are drawing the now room temperature water through the pipes and are waiting for the hot water from the water heater to make it to you. If the water pipe is in an external wall and exposed to subfreezing temperatures it may freeze between uses and as mentioned it gets used less frequently than cold water pipes.