A more mundane explanation is that hot water evaporates faster than cold, decreasing its volume and thus the time it takes to freeze. Cold water also could contain more dissolved gases, which lower its freezing point.
One explanation of the effect is that as the hot water cools, it loses mass to evaporation. With less mass, the liquid has to lose less heat to cool, and so it cools faster. With this explanation, the hot water freezes first, but only because there's less of it to freeze.
Another interesting thing with freezing water: The water in the hot-water lines freezes faster. This is insanely counterintuitive but true. You can easily demonstrate this by pouring two equal cups of water, heating one of them up, and putting both cups into the freezer.
A trickle of hot and cold water might be all it takes to keep your pipes from freezing. Let warm water drip overnight when temperatures are cold, preferably from a faucet on an outside wall. Adjust the thermostat. Keeping your thermostat set at the same temperature day and night may help prevent frozen pipes.
Water expands as it freezes into ice. In a pipe, ice forms first on the inside wall of the pipe and grows radially inward until there is a solid plug of ice blocking the pipe.
Never pour hot water on frozen plumbing or try to thaw pipes with a blowtorch. The hot water usually freezes on the outside of the pipe, and the torch can cause frozen pipes to explode if steam is produced. Also, do not strike pipes in an attempt to break up the ice, as hammer blows can cause pipes to crack.
Any of those circumstances could lead to pipes freezing – especially when the outside temperature is 20 degrees or colder. Generally, keeping your home at 55 degrees or warmer will prevent pipes from freezing.
When the water freezes, it expands and can split or break the pipes. This can cause a major problem when the pipes thaw. When they thaw, the water at full pressure comes through and can flood your home which can cause significant further damage.
"It all depends on how fast the cooling occurs, and it turns out that hot water will not freeze before cold water but will freeze before lukewarm water. Water at 100 degrees C, for example, will freeze before water warmer than 60 degrees C but not before water cooler than 60 degrees C.
If you haven't already, shut off the water to your home immediately. Do not attempt to thaw pipes without turning off the main shut-off valve.
In fact, turning up the heat might lead to a burst pipe or other damage. Call an expert plumber immediately if you suspect the pipes in your home have frozen.
Pipe freezing kits make pipework repair and maintenance a doddle without needing to drain down entire systems. You can save up to 85% on time and save money with minimum disruption and reduce the potential of water spillage and property damage.
It's important to note that pipes do not always burst once they're frozen or while in the process of freezing. It's the process of thawing that can be the biggest problem.
Pipes that freeze most frequently are: Pipes that are exposed to severe cold, like outdoor hose bibs, swimming pool supply lines, and water sprinkler lines.
We all know that freezing begins at 32° F or 0° C, but at what point do pipes freeze within our own homes? Temperatures only need to drop to about 20° F for a few hours to put exposed pipes at risk.
The temperature of the pipe
If the temperature falls to 32 degrees or lower, your pipes are likely to freeze.
The rule of the thumb is that it takes roughly 6-hours for water in your pipe to freeze after left in an unheated area. Meaning if you lose power at your home and the weather is reaching below freezing point, you have approximately 6-hours until the pipes will begin to freeze.
Pipes are most likely to burst when exposed to below-freezing conditions for six hours or more, which can happen throughout winter and early spring. Burst pipes could lead to a disaster, bringing severe water damage to your home.
The lower the temperature is inside of the home, the slower the rate of thermal energy loss. To achieve optimal comfort, it is recommended for homeowners to set their thermostats between 68 to 72 degrees while there are people inside of the home.
Risk of bursting
Frozen pipes will eventually thaw on their own, but it could take days or even weeks. If New York City is experiencing a brutal cold snap, the temperature could stay below freezing for many days. Meanwhile, the water in the pipes could be expanding.
If the pipes have frozen and burst, a plumber can fix them. If the pipes are frozen but have not burst, a plumber can remedy them by applying a heat gun to thaw them. For an interim solution, a homeowner could use a hair dryer to start the thawing process.
Use a space heater, heat lamp, or hair dryer to thaw the frozen length of pipe. Wrapping freezing pipes with thermostatically controlled heat tape (from $25 to $61, depending on length) is also an effective way to quickly thaw a trouble spot. Don't thaw pipes using a propane torch, which presents a fire risk.
DON'T use a blowtorch or open flame to thaw a pipe
In addition to being dangerous, using a blowtorch to melt a frozen pipe may actually damage the pipe, Davis explained — making matters much worse.
Do Frozen Pipes Always Burst? Thankfully, no. But ice does build up inside pipes, blocking unfrozen water from flowing. Pipes made of copper, steel, PVC, PEX and other plastics are all susceptible to freezing, with copper being the most vulnerable to ruptures when frozen.
If the temperature continues to stay below freezing, the liquid will continue to expand causing it to rupture the pipe at its weakest point. But do frozen pipes always burst in this situation? No – but without some attention and maintenance, you increase the risk that it will happen.