The problem I have with the pressure explanation is that nothing about it would actually keep the water from freezing. Yet the result is that the faucets just work. They don’t act at all like how the water flows when your pipes are partially frozen. (And, no, slush doesn’t come out of the faucets because faucets have mesh filters to break up the water tension.)
I think people are just underestimating the sheer amount of heat being added into the system from the flowing water. The water doesn’t get below freezing except when the pipes are above the frost level and exposed to the freezing temperatures.
The smallest leak in your pipes can do a number on your water bill. And water even just a few degrees above freezing has a lot more heat energy.
Just want to re-iterate a point that seems to be missed a few times.
Cooling ice does not expand - it contracts. Once ice has frozen, the lower you take the temperature the more it shrinks again. All of the expansion occurs in the gap from 4C to 0C, when it is still a liquid.
The problem with ice itself isn’t that it expands, but that it blocks a pipe. Then the free water in the blocked section may be able to expand and rupture the pipe. That water does not need to freeze.
The pressure relief effect of a dripping tape makes sense in one possible setup.
Take a length of pipe with a tap at one end and connected to the mains supply at the other, and filled with water. Now start to cool the pipe from the supply end - as if perhaps at the outermost extremity of a house. Eventually an ice plug may form which blocks the pipe. As the pipe continues to cool, it gets colder from the outside in and more of the liquid water in the pipe cools to 4C and below, and its expansion increases the pressure in the pipe. The pipe so cools the plug of ice also extends in towards the tap end.
If the tap is closed, the pipe may eventually rupture. If the tap is open, the pressure can be relieved, and the pipe doesn’t rupture.
Water expands about one part in 10000 when it drops from 4C to when it freezes. So the amount of water needed to flow from the tap from the time the initial ice plug forms is very low.
Clearly this doesn’t work if there are multiple points along the pipe an ice plug may form. But if the pipe is simply cooling from one end, the tactic should work fine.
Why does the idea that running water doesn’t freeze exist?
Liquid water has the property that it expands just before it freezes. This has the outcome that if you cool a body of water from above - such as when the air temperature drops over a body of water, the water at the top expands and forms a stable layer of colder water - which then freezes. Any other liquid would tend to develop convection cells and as the colder liquid became more dense it would fall and warmer liquid would rise, with this circulation acting to churn the liquid, evening out the drop in temperature through the body, which would delay freezing, especially in deep bodies.
Moving water is already churning, and this will act to even out the temperature in the body of water as well. So it won’t freeze until the entire body of water is cold enough.
So it isn’t that moving water is resistant to freezing. It is that a still body of open water will freeze over much earlier.
So the observation that say a pond of water will freeze whilst a running creek won’t freeze is easy to make. Obviously, eventually the cold will win. But the cold will need to freeze the entire body of moving water, whilst only the top layer of a still body needs to be frozen to be observed as “frozen.” With the irony that the still body now has a useful insulating layer of ice on top of it which will tend to prevent the water below from freezing.
This. When a pipe freezes it doesn’t burst at the point it freezes. It bursts at the weakest point.
Water doesn’t compress. When it freezes in a pipe the expansion puts a tremendous amount of pressure on the entire line. A dripping faucet relieves that pressure. You would need to let a faucet drip on any line that can freeze.
If the pressure explanation was true, then why is it that the taps still work? Simply releasing the pressure will not keep ice from forming. But running the water does.
I think there’s some confusion about what does what. Dripping water helps prevent freezing because of the movement of water combined with its high latent heat. Dripping water helps prevent burst pipes because there’s pressure relief. These explanations about how dripping water works are not in conflict, but complementary.
If pressure relief is the/a reason, then shouldn’t pipes be just as likely to burst far away from the freeze? Why wouldn’t the pressure just break open a sink faucet in a random bathroom, blow out a toilet fill valve, or rupture the rubber hoses feeding the washing machine? In older homes without a check valve or pressure reducer on the supply main (which also acts as a check valve) there could not be a break before the ice plug because the pressure would just back up into the water main. This is why in such installations you don’t need an expansion tank on the water heater. However there’s always something downstream (unless it’s a pipe that’s capped off) that seems like it would be easier for a pressure buildup to breach through washers, seals, and packings if that were actually the mechanism at play.
I’m trying hard to figure out if there is a confusion of definitions of “cooling” vs “freezing” or if this refers to what happens to ice once it’s formed, but:
Yes, as water is freezing it does expand, - with great pressure - which is the force that ruptures the pipes. This is a basic fact we learn in physics. From the link:
“When water freezes, it increases in volume (about 9% for fresh water). The effect of expansion during freezing can be dramatic, and ice expansion is a basic cause of freeze-thaw weathering of rock in nature and damage to building foundations and roadways from frost heaving. It is also a common cause of the flooding of houses when water pipes burst due to the pressure of expanding water when it freezes.”
If ice contracted or became more dense than liquid water, it would sink. But it floats… because it expanded while freezing and has become less dense than liquid water.
If the bolded “free water” means “liquid water”, this is incorrect. Liquid water changing temperature from 4C to 0.1C and expanding 0.01% while doing so does not rupture pipes. There is a larger expansion of water between 4C and the say 50C in a heater, and this does not burst pipes.
The water changing phase from liquid to solid and expanding 9% at a force between 25,000 and 114,000 psi will crack a pipe. Copper piping may start cracking at 3,000psi.
You see this when you put a can of pop/beer in the freezer. The can may burst and you’ll see a big bulge from the ice expansion. This doesn’t happen when you have them sitting in an ice water bath where they will chill down to 4 then 0 degrees; the can don’t burst.
It does. it will burst at the weakest point in the pipe. It won’t burst a rubber hose because they expand but it will burst a copper line. I didn’t click on the video above but the one from This Old House demonstrated it. My apologies if it’s the same video. They froze a section of pipe and measured the water pressure. It went from 60 lbs to beyond the 200 lb gauge in a few seconds.
The video described above showed how a section of freezing pipe can compress the adjacent liquid water behind a closed valve with that same 27,000psi pressure I mentioned. The dialoque was a little confusing, because the plumber at first said the water turning to ice it’s self wasn’t enough to crack the pipe, and implied you needed a closed portion that gets over compressed (which an open valve prevents). Later though, the hosts says (and the plumber agrees) that the pipe can burst either at the ice expansion section its self, or at a sealed point somewhere else downstream affected by the forming ice. The experiment was set up to very quickly form a plug of ice which grew down the pipe, which is only one way it can happen.
In my example of a pipe open on both ends, the ice formed in such a way that it couldn’t expand along from point A to point B. A section froze all at once, and still burst the pipe despite having no valves constraining the pressure at all.
In addition, it’s almost a tautology. Obviously frozen water can’t run, so running water by definition doesn’t freeze. If it froze, it wouldn’t be running any more! QED.
One thing that needs to be considered in the running water freezing examples is that the conditions of water in a pipe are different from that in an open body of water. In something like a river, ice can form at the edges of the river or in eddies where the flow is minimal. Then more ice can build from the edges of that ice and eventually freeze the whole thing. In addition, the pressure of freezing water in the open can easily go in various directions without impediment. The same is not true in a pipe. A pipe is uniform in shape and filled with water, the water is under pressure, any ice that forms will have forces pushing on it, etc. All those factors will affect the development of ice in the pipe and may mean the conditions to get water to freeze in a running pipe are much different than in a moving body of water.
According to the plot at this page, liquid water reaches its max density at ~4C, as you state. However, only a small portion of the decrease in density happens as the water transitions from 4C to 0C; the vast majority of the density increase happens right at 0C as the liquid water transitions to solid ice.
Keep in mind that water will freeze along the pipe and not necessarily expand the pipe where freezing starts. Where it often causes problems are outside faucets that are of cast metal. they will not have much stretch available. It will crack the faucet.
Many of these outdoor faucets come into the house under sinks in kitchens and bathrooms. That means you can get to them easily. In an emergency you can put rags soaked in hot water on the pipes to quickly melt the ice.
I proposed a thought experiment upthread where you have a freestanding circular pipe filled with water, and you either run a pump to make it move in a circle or you don’t.
Assuming we correct for the heat the pump adds, will the moving water freeze at a different rate than the still water? I claim that it will not (though I do not have the means to perform the experiment). This is consistent with the idea that the only relevant thing that moving water does to prevent freezing is move from someplace warmer, bringing heat with it.
For anyone who thinks that moving water will freeze slower for other reasons, what’s the reason? What’s keeping it from freezing?
I’d think it would freeze slower for the same reason you can have supercooled water, i.e. the “turning solid” part of freezing requires still points, but maybe there’s something counter to my intuition that makes that untrue.
Super cooled water tends to only exist when it is very still. Movement causes a nucleation site somewhere and the entire body freezes. You need very pure - or at least particulate free water - as well, to avoid nucleation sites. Which would rule out both ordinary tap water and pipes with any sort of texture or irregularity. There is little hope you could supercool water in household pipes.
Overall all else being the same, moving water is more, not less, likely to freeze.
A household water line with no water movement exposed to freezing temperatures (outside wall, etc…) will freeze and possibly burst.
A household water line with some water movement is bringing water from below the frost line at around 50 deg F so this warmer water prevents the line from freezing.
The movement of water itself may delay freezing but won’t prevent it if is part of a closed loop. The movement of warmer water from under the frost line and exiting from the faucet prevents the freezing.