This is a big question from people who live in areas that don’t see a lot of cold weather – and even in someplace where cold weather happens less frequently.
At what temperature do pipes freeze?
Sadly, there isn’t just one answer for this. Because it depends on a lot of factors. Is it a hot water pipe or cold water pipe? Is it a PVC pipe or copper pipe? How far underground is it? How much pipe is exposed to the outdoor temperature? Is it insulated? And so on…
Just why do pipes burst, anyway?
Jeffrey Gordon at the University of Illinois conducted research to answer the pipe-freezing question. And his research attempted to take into account, design temperature, pipe composition (copper or PVC), insulation level, pipe diameter (1 /2″ or 3/4″), and water source (cold or hot water tap).
The physics of how water freezes is the reason a rupture occurs – not just that liquid is now ice. From Jeffrey Gordon’s research: “Water is denser than ice. At 0°C, pure water has a density of 62.42 lb./cubic foot, while the density of ice is 57.5 lb./cubic foot. These relative densities imply that as water freezes to ice, it expands by about 8 percent by volume.“
Gordon’s research found that the elevated pressures where the ice freezes isn’t the only problem. The pipe section downstream of the ice blockage experiences the same increased pressure, so a burst can occur in a part of the pipe where there is ice or in a section where there is no ice. Even within a heated area of a home or building.
So, if you don’t protect the spigot on the side of your house or insulate the pipes under your home, you can still get a pipe burst in your house – not just where it is below freezing.
An interesting note, that Gordon performed tests on both copper and PVC pipes. And found that no PVC pipes burst during testing.
Preventing the burst
Let ‘er drip!
A lot of times it is recommended to ‘drip your faucet’ as means to prevent a pipe burst. This isn’t necessarily because flowing water will stop it from freezing (just look at any river in Canada in January) but rather that opening a faucet relieves the pressure that may build up if and when ice forms.
And if you let a faucet drip overnight on cold nights and wake up and it is still open but not dripping, continue to leave it open until you’re certain there isn’t blocked up by ice. A simple test would be to let a bit more water come out. If water flows freely (and just as fast as normal) it is likely safe to close. But if you open it up – full boar! – and nothing is coming out, or very little water is trickling, it may be important to leave it open.
Insulation
Gordon’s research looked at the effectiveness of pipe insulation. Turns out that that foam insulation that is put on pipes doesn’t keep the water warmer initially. Again, due to the phyiscs of how water freezes, it actually works to delay the time it takes to change from supercooled liquid to frozen ice.
Did’ja know: When water freezes it can only form ice at 32 degrees. Even if liquid water is super cooled to below 32 degrees, and stays a liquid, it has to warm back to 32 degrees to form into solid ice.
You can see that on the graph above. The liquid cools past the freezing line, but must be at 32 degrees when it freezes. This creates an interesting situation, as explained by Jeff Haby, a meteorologist that runs Haby’ Hints a great meteorological resource:
“Liquid water has more energy than frozen water. When water freezes it gives up some of the water’s energy. This energy that is given up is the latent heat of freezing. When the water was freezing latent heat of freezing energy was being released. Heat energy was actually being released.“
And if a pipe is insulated, that insulation will trap that energy from escaping. And it will slow the accretion of ice within the pipe – even if the water temperature is well below freezing. This also means that when you open the tap and pour some of the water out, it may crystallize and turn to ice on contact. That happened in south Mississippi back in January of 2018 for a lot of people.
MATH TIME!
Another ‘cool’ thing is that it turns out that pipes feel a “wind chill” too. We know this because moving air conducts more heat than stagnant air. And we know that because a cool breeze in the summer feels amazing. And a hair dryer works better at heating things up than simply just standing in a sauna.
It also turns out that non-insulated pipes feel this wind chill a lot.
BTUs (British Thermal Units) is a good way to get a measure of energy needed for water because it is based on changing one pound of water one degree.
And it just so happens that 12ft of 1/2-inch pipe holds about one gallon of water. So if you have 12ft of pipe, exposed to the air, if you want to change the temperature by five degrees, it takes about 41.65 BTUs.
According to that table, that can – almost – be accomplished with a 1mph wind in a 20-degree temperature in just two hours. And that would be on top of the cooling that is already occurring with an exposed pipe at that temperature.
In those two hours, a 12ft, 1/2″ pipe, in a temperature falling from 24 degrees to 21 degrees, will lose about 3 degrees of water temperature per hour. In stagnant air. But add 8mph breeze and it will lose up to an extra 15 degrees per hour.
That means it could drop that 45-degree water to just 30 degrees (super cooled) in an hour. That is pretty crazy.
Again, in 21 degrees, with an 8mph breeze, an exposed pipe with 45-degree water in it will cool to just 30-degrees in one hour.
Yeah, but Nick, how do I know if my pipes will burst?
You’re right! I’ve been bombarding you with science and math. I apologize.
The short answer is: It depends. On a lot.
Research shows that as the temperature approaches 20 degrees – in the South (or places that aren’t as accustomed to cold) – that is when it is important to take steps to mitigate frozen pipes. So, if the outside temperature approaches the 20-degree mark, it is time to start dripping the faucet and insulating pipes.
But the length of time below freezing also matters. If the temperature is going to be below 32 for an extended period of time – say, for three days – supercooling and ice development can also occur. But the air temperature around the pipe has to be below the freezing mark, at all times, for ice development to occur. Otherwise, supercooling is all that happens.
If the temperature around a 3/4″ pipe – with no insulation – is 20 degrees, it will take about three hours for the water to begin to freeze. It takes, roughly, another three hours for the water to fully freeze within pipe blocking the flow of water. Lab tests showed that it took another three hours for the pipe to burst.
So a non-insulated 3/4″ pipe, with an ambient air temperature of 20 degrees, can burst in a single night.
The good news is that once you insulate pipes once, you’re done. So long as the pipes stay insulated, you don’t have to apply new insulation every year.
For insulated 3/4″ pipe (with 1.5″ insulation), water begins to freeze in the same amount of time – three hours. But in this case, it can take 12 hours for the water in the pipe to freeze completely, blocking the flow.
Per Gordon’s research:
An 1/8″ of insulation delayed ice blockage by 3 hours, and 5/8″ of insulation provided an eight-hour delay. The relationship between hours to complete phase change and the conductivity of the insulation level is approximately inverse. The benefit of pipe insulation is clearly in its ability to slow the phase change within a water pipe, and thereby reduce the likelihood of an annular ice blockage before a warming period.
Keep in mind, that some homes have 1/2″ pipe. And since 1/2″ pipe holds half as much water as a 3/4″ pipe, the freezing time is – roughly – half as long.
To read more about this topics / read the research conducted, check this out!
Mitigating the freeze
The best way to keep your pipes from freezing is to insulate the pipes. Adding 1.5″ insulation was shown, within the research above, to slow down the freezing process by a lot. As it says above, even 1/8″ of insulation can slow the freezing process by three hours.
The second-best step is to drip your faucets. Often it is recommended to drip the faucet that is the farthest down the line from the water source (well or city/county water supply line) and drip any faucet that runs along the inside of an exterior wall.
The last resort is to open any cupboards below any sink to help the heat from your home try to slow any freezing.
Doing all three steps should help to mitigate any freezing potential for pipes in your home.
