Is it true that hot water pipes freeze quicker than cold water pipes?

Question

A friend of mine told me that hot water pipes in a house will freeze quicker than cold water pipes. This seems completely counter intuitive to me, and seem to violate laws of physics. It sounds like an urban legend to me. I tried to argue that it must have to do with the relative position of the pipes is some cases. He stated it had to do with an alteration on the property of the water (which I doubt). If it is true, I would like someone to explain to me the science behind it, which is why I am posting it as a science question rather than a plumbing one.

I really like the answer about Mpemba affect. Thanks for the link. It is interesting how many people have observed this to be true. I think I will try a little experiment tonight.

Answer 1

This seems like the misconception that hot water makes ice cubes faster than cold water. Hot water will cool more rapidly, but since it's temperature is elevated it actually takes longer.

In certain conditions (Mpemba effect), it can actually be that hot water freezes faster. You will not see the Mpemba effect for just any initial temperatures and certainly not for hot water (~90degC) versus cold (~10degC).

Nonetheless, the link below is very interesting and dispels plenty of myths regarding this effect. As it states, there are many proposed mechanisms for this effect (and a few have been stated in answers here), but none have been conclusively shown to be the dominant cause.

Regardless, hot water pipes should be insulated whereas cold water pipes are not.

Your friend is wrong.

Answer 2

The water heater is fed directly from the cold water line, so both systems recieve equal water pressure. As far as the hot water line reaching the same temperature as the cold water line, yes, depending on the insulation, the hot water pipe, if no water is drawn through it, will eventually reach ambient temperature. The local radio station has been advising people to leave their cupboard doors open to allow heat to get to the wall where the pipes are, but I am kind of taking this with a grain of salt. It is true that any little bit helps, but if cabinet doors were good insulation, we would be lining our walls with them. as far as one pipe freezing before the other, I would almost think that if a person is going to put pipe insulation on only ONE pipe, they would put it on the hot line and not the cold line, which would give it more freezing protection. I would think that with a new, well insulated house, in weather that is not unusually cold for the region, there would not be a great deal of danger, though you say that you are letting the interior temperature stay cooler than is usual for an inhabited house; and that invites the possibility of a freeze-up. sounds like you may be away from the house? I'd suggest if you have an emergency contact, you think about having them boost the temperature a bit, and run some water from each tap, just to check for a frozen pipe.

Answer 3

Does Hot Water Freeze Quicker

Answer 4

Your friend is absolutely correct - and you, too, are correct in stating that it is indeed counter intuitive. I find it very difficult to accept - and I know it is true.
My own personal experience with this phenomenon was in the middle of winter - after some minor mechanical repairs and in an ill conceived rush to get an automobile over to the paint shop (which was approximately 2 miles away) we filled the radiator with water and brought it up to teperature in the garage and headed out. Since we were going to leave it in the heated paint shop when we arrived, and we KNEW the water wouldn't freeze at ~160 degrees F in such a short run, it didn't concern us at all - you guessed it - we were wrong!
The engine overheated and we found the radiator was full of ice. The cold air rushing through the radiator had frozen the ~160 F water before we had arrived.
There are many theories as to why this happens, but none is totally accepted as the one and only conclusive fact. But you can believe it because it is indeed the truth.

Answer 5

City water often comes with lots of dissolved gasses (air).

When the pressure is let off (it comes out of the tap) the air slowly equilibriates.
If the water is Hot when it come out, the gasses are given off quite rapidly - this can be seen as a fog of tiny bubbles which rise slowly to the surface and also form bubble on the sides of the container.

Two samples - one from the cold tap and one from the hot - placed in a freezing environment will show the effect that the "hot" water freezes first. This is due to the low "air" content.

The notion is usually presented as "ice cube trays should be filled with hot water to make ice faster" I've never tested it!

I don't believe this effect will work as long as the water is still under pressure in the pipes.

Answer 6

I have been a plumber for almost 20 years hot water will most certainly freeze faster than cold water BECAUSE, the process of heating the water will remove certain impurities such as, calcium, magnesium, chlorine, etc etc. therefore making it a purer form of water allowing it to freeze faster.

I have done service calls where 2 pipes are running side by side both exposed to the same draft, the hot side freezes the cold side is fine. this is also anothher reason why in colder climates when you install a hot water heating system where alot of your pipes are on an outside wall (which are generally colder than an interior wall) it is always a good idea to put anti-freeze in the system.

Answer 7

Big time. The hot water freezes faster (period) , put warm water in an ice tray, then put a cold water tray in the hot water one will be ice before the cold water one is.
Also the problem with pipes is the Hot Water lines are constantly changing because of the Hot water sits and cools off, this Expansion and Contraction of the pipe and fittings takes only so much, then they burst.

Answer 8

From my understanding this observation of your friend is often true. A major reason why this very counter intuitive occurence is possible is due, I believe, to the fact that hot water has much more robust convection currents that, once a freezing temperture takes hold, will accelerate the cooling of the water. It is one of the same reasons that cups of warm water can freeze faster then cups of cold water when placed in a freezer.

Answer 9

Well then, explain why the rental I live in has frozen 6 times this winter? I've never had to thaw my cold water, just the hot????? It has become nothing for me to get up in the morning to no hot water but plenty of cold??? and it is always the hot water pipe leading into the water heater that I have to put the hair dryer on to get the water running again!!!! Pipes are insulated..I leave the cabinet doors open for heat flow, leave faucets on to a drip..and yet get up in the morning..no HOT WATER. My COLD WATER has never frozen once this winter. Hmmmmm

Answer 10

No, it's not true. If the pipes are open to freezing weather they will freeze. It doesn't matter whether they are hot or cold. If there is hot water in the pipe it will naturally take a little longer to get cold and to freeze. People will believe nearly anything these days.