Why does "dripping the water" work?

Several people have mentioned this.

When I do the faucet drip thing (as I did last night), I noticed that when I first turn on the faucet, that first the water gurgles out for a couple of seconds, before producing the normal stream. Why does this happen?

Does it happen every time you turn on a faucet, or only when the weather’s really cold?

If the latter, your pipe may already be partly frozen when you first turn it on, disrupting the water flow; but as the warmer water flows through it, the ice melts and the stream goes back to normal.

The times I’ve had water do this it’s because the flow had previously been interrupted, though not necessarily by freezing – having had it shut off for any reason can do it; at least, if you’re describing what I think you are.

It only happens when I first turn a faucet on after a night of letting it drip. I don’t think the the pipes freeze, as I have a big house, so I do this at each sink (5 of them in the bathrooms and kitchen). And it never happens for the faucets that I didn’t let drip. For example, if it’s not going to be below 10 degrees F, I don’t do it with the basement faucets.

There can be fluctuations in pressure throughout the water system, especially if a fire hydrant is opened or large pumps are started and stopped, so that can suck a little bit of air back into your pipes from a slightly opened faucet. I suspect that’s what’s going on. Newer water systems have backflow preventers installed in customer service supplies to stop water flowing the wrong way and potentially contaminating the mains, but they’re by no means universal.

The simple answer is that a sufficient amount of flowing water above freezing temperature through a cold section of pipe adds enough heat in that area relative to the cooling rate outside the pipe to keep water in that section from freezing completely through, which would allow potential bursting via the expansion of freezing water in a closed system.

Flowing water does freeze; it freezes when sufficient energy/heat is removed, and can do so in a few forms not everyone has seen. Most people are familiar with surface ice forming on rivers when the atmosphere gets cold enough. You can also have conditions that allow frazil ice to form, which are patches of slush that form and keep moving with the flowing water. Anchor ice can form on the bottom of a stream, even when the surface is not frozen. An aerator keeps a small section of surface ice open by creating a slow steady flow of deep (relatively) warm water at 0-4C up to the surface and prevents or limits the cold -20C atmosphere from forming surface ice.

Once water starts freezing in a pipe or any other partially or fully enclosed area, it may or may not stick/expand/burst the confinement based on the shape of the vessel, how smooth the ice-wall interface is, how fast the ice forms, and how strong the confining material is. That’s why sometimes a cheap plastic outdoor spigot will burst when a very sudden cold snap comes through, while a strong metal well-insulated one can survive a mild winter with no issues. It doesn’t always happen at a certain temperature, and cracking the faucet may or may not work depending on variables.

The simplest experiment I know of that demonstrates this is a set of metal tubing in my yard. 3/4" pipe formed into an upward facing U-shape, open at the ends. During the summer it rains and the pipe partially fills with water. That same water froze during the winter. Despite being completely open (no-pressure) on both ends, a section of the pipe burst open when the water froze. The water froze in a way that created a plug which prevented it from expanding length-wise through the rest of the pipe.

Anyone who’s ever tried to just push a cylinder of ice out of a frozen a pipe knows how strong the friction is. Doesn’t matter if you leave a little crack in the faucet to reduce the pressure. Once ice forms across the full cross-sectional area of the pipe you’re pretty much screwed unless something warms it up before the material fails.

Dripping the water works by keeping enough of the pipe thawed in the cold exposed section via the heat of the incoming water to prevent it from freezing solid.

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.

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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.

None of this helps with pipes.

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.

I’ll be a bit more direct.

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.

No, you are quite right. I was confused writing this. Lapse of memory as well.

It isn’t as if I didn’t have a feeling I had got it wrong either, but I proceeded anyway. Mostly my first post is just plain wrong.

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.