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A question about probably the most mundane subject in cooking: boiling water.

For cooking techniques where you drop ingredients in simmering or boiling water - such as for vegetables, pasta, many rice recipes - I have often seen the recommendation that you start by putting cold water into a pot, then bring it to a boil. Why would you not start with hot water from the tap? It's going to be quicker than heating cold water, and your water heater is going to be way more energy efficient than your stove top at heating the stuff.

In particular, is there any physical or chemical process that starting from cold water encourages or prevents from happening?

(To reiterate: in the case where you add stuff to cold water and then start heating it, there clearly is a difference with starting with hot water; this question is about the case where you drop your ingredients in pure water that's already boiling.)

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    Cold water boiling faster is one of those persistent myths that proves that many people are unable to think critically. Feel free to laugh at people who haven't thought it through. :-p
    – ceejayoz
    Commented Oct 24, 2010 at 20:52
  • Have you considered performing a simple test measuring how long it takes your cold water to boil versus warm water?
    – Zoredache
    Commented Nov 13, 2010 at 4:57
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    I always start from hot water. But then I have a newer hot water heater that I know is free of sediment.
    – renegade
    Commented May 19, 2011 at 20:04
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    @ceejayoz I'm late to this party, but recall there is also the counterintuitive Mpemba effect (en.wikipedia.org/wiki/Mpemba_effect), which was also ridiculed at first. I'm not claiming there's such a thing a reverse Mpemba effect, but maybe the myth persists due to misremembering which way it's supposed to go.
    – Todd Trimble
    Commented Mar 14, 2015 at 12:05
  • Re: cooking.stackexchange.com/questions/21471/… in the sidebar… never drink tanked hot water. Hot water is only potable if it comes directly through an on-demand heater directly off the mains supply.
    – Tetsujin
    Commented Dec 11, 2020 at 8:56

7 Answers 7

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Some people say cold water boils faster than hot water, this is false, found here and here.

One reason might be (from the first link): "Some water heaters may introduce additional sediment into the water, giving you another reason to consider starting with cold—at least, if time is not of the essence."

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    I haven't read all the links, but what IS true is that if you start with boiling water and make ice out of it, you'll get ice faster than starting with cold water. But you'll get LESS ice from the boiling than the cold water - it's faster because some evaporates.
    – Ward - Trying Codidact
    Commented Oct 25, 2010 at 2:19
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    The idea that cold water boils faster than hot is total nonsense. If you try to boil cold water, it will take some time to reach the temperature of the hot water. From that point, it will take exactly the same amount of additional time to boil as the hot water would have done if you'd started with it.
    – David Richerby
    Commented Aug 14, 2014 at 17:46
  • @ward Sooooo not buying
    – paparazzo
    Commented Jul 2, 2018 at 15:07
  • @Rob Not sure what you mean, that's exactly what the answerer is saying too.
    – John Montgomery
    Commented Feb 24, 2020 at 22:34
  • @JohnMontgomery I have GOT to quit reading these things so early in the morning.
    – Rob
    Commented Feb 24, 2020 at 22:45
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There have been plumbing systems in which the hot water was likely to have dissolved more [toxic|unsightly|unpleasant-tasting] material from the pipe walls or joints than the cold. In particular any system that uses lead-based solder, can leach minute{*}, but detectable amounts of lead into the drinking water, and the hot water is more efficient at this.

In this case that advice amounts to "use the clean water".

{*} Really minute. Like "Use this water all you life and not suffer any ill-effects" minute. But it can be detected, and who wants to chug down a glass of lead solution...

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    If a bear chugs a glass of lead solution while alone in the woods and doesn't suffer any ill-effects, ...
    – Roger Pate
    Commented Oct 25, 2010 at 1:53
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    'minute' depends on the water system. There was an issue that years ago a chemical was added to the water systems in the Washington, DC area that caused the solder to start breaking down giving off significant amounts of lead. This has resulted in the recommendation that you run the cold water for a minute before consuming it. (we have signs on every water fountain at my work). Hot water would've been stored in a tank and can't easily be flushed out like the cold water can.
    – Joe
    Commented Mar 14, 2015 at 15:16
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My hot water tap is supplied by a combi boiler which heats the water on-demand, supplied by the same cold water source as my cold water tap. As a result, I'm confident that the water is reasonably fresh and clean. I don't use it for brewing tea or coffee, but I'm happy to boil vegetables and rice in it. It saves a couple of minutes bringing the water to the boil.

My parents' hot tap is supplied from an insulated immersion heater tank. The same water can sit in there for days, and it may heat and cool several times in that period. It is supplied by a header cistern in the attic. Last time I looked at the header cistern, there was a crop of dead flies floating on the surface, and some unidentifiable gunge settled at the bottom. This hot water is suitable for bathing and cleaning; it's not suitable for cooking.

If you don't know the details of your plumbing, and you're not sure it's safe, don't cook with water from your hot tap.

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  • Vegetables and rice? You are supposed to put grains in cold water and bring to boil slowly, because they profit from the soak. I can understand starting with hot clean water for recipes where you add the ingredients to boiling water, like pasta and short-cooking veggies, but not for rice.
    – rumtscho ♦
    Commented Dec 15, 2011 at 12:29
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    My basmati recipe is 1 part rice, 2 parts boiling water, salt, simmer covered for 10-12 minutes until absorbed. Brown rice might benefit from a soak though.
    – slim
    Commented Dec 16, 2011 at 13:04
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    To explain the attic cistern -- it's a UK thing : youtube.com/watch?v=HfHgUu_8KgA ... taller buildings in other countries frequently have water tanks on their roof-top, and those feed both hot and cold water : nytimes.com/2014/01/27/nyregion/…
    – Joe
    Commented Mar 14, 2015 at 15:25
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There has been a lot of discussion over the years in cooking circles about whether cold water or hot water comes to a boil faster, and the people that wrote those recipes are passing on their determination on to you. There is zero functional difference between one pot of boiling water and another. Once a roiling boil has been achieved, the water will always be the exact same temperature. That is one of the reasons that we use water as a cooking medium.

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    Assuming the two pots of boiling water are at the same altitude, of course.
    – Michael Hoffman
    Commented Dec 15, 2011 at 21:36
  • assuming pressure to close all the loopholes.
    – sarge_smith
    Commented Dec 16, 2011 at 1:17
  • I think you need to assume the same isotopic ratios in the hydrogen and oxygen atoms, too, if you really want to close all the loopholes. (Oh, and pure water, of course.)
    – David Richerby
    Commented Aug 17, 2017 at 0:17
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No, there are no different physical processes. The only thing to be wary of is that water heaters can be pretty icky places so, depending on your system, you might want to avoid drinking that water. On the other hand, if you have an electric kettle then by all means use that to heat the water: it's clean and, like your water heater, much more efficient than heating water on the stove.

The idea that cold water boils faster than hot water is complete and utter nonsense. Suppose you have a pan containing a litre of water at 40°C. That pan will take some amount of time to boil – let's say five minutes. Water has no "memory" of its past. It does not know how it got to 40°C so how it got to 40°C cannot possibly influence how long it takes to boil from that point. In other words, any litre of water at 40°C will take five minutes to boil in that pan. In particular, if you put a litre of water at 20°C in the pan and start heating it, it will take some amount of time to come to 40°C. After that, it will take the standard five minutes to boil. In other words, surprise!, cold water takes longer to boil than hot water because, first, the cold water has to be turned into hot water and then the hot water has to be boiled.

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The specific heat capacity (SHC) of water is 4.184, which is a measurement of how much energy (in kilojoules) it takes to raise 1 liter of water by 1 degree celsius. Your stovetop is constantly generating heat energy and transferring it through the pot and into the water, which absorbs it and raises its temperature. Once the temperature reaches 100 degrees celsius (212f), the water begins absorbing that heat energy to convert to steam, which converts to a gas at the bottom of the pot and rises to the surface, causing boiling.

So let's say the average recipe uses 4L of water. Tapwater comes out around 4 degrees C, and your water heater has water at about 60 degrees C. In both cases, the temperature will have to be raised to 100 degrees C, and while we don't know how much heat our stove generates, we know that it will be the same.

The amount of heat required to raise an amount of water by a certain temperature can be shown with the formula:

Q = mc(T2-T1)

Where Q is the total energy required, m is the mass(4L of water = 4kg), c is the SHC of water (4.184), and T2 and T1 being the difference in temperature. So if we plug in our respective numbers:

Q = (4kg)(4.184)(100-60) = 669.44 kJ

Q = (4kg)(4.184)(100-4) = 1606.656 kJ

So as you can see, the cold water requires 2.5x more energy than the hot water. Your stove will be outputting a certain number of kilojoules per second of heat energy and once it has put in the required amount the water will reach 100 and start to boil. These numbers may vary based on different temperatures of hot and cold water, but warmer water will always boil faster.

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    Welcome! While this is probably correct, it's not actually addressing the question of why recipes recommend you start with cold water. You're really just explaining why hot water boils faster... which isn't the question at all. If you look at the other answers, particularly the top rated answers, you'll see a good explanation for why this is the general recommendation.
    – Catija
    Commented Mar 13, 2015 at 17:20
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My high-school chemistry teacher claimed that hot water is inferior at storing dissolved gasses, and that hot water has probably been sitting in the plumbing system for some time, so a great deal of the dissolved oxygen gas has been expelled. On the other hand, cold water from the taps is more "fresh" and enriched with oxygen gas. Even after boiling water, the trapped gases will take some time before they escape. So if you start with cold water, whatever you're cooking with the water will become more enriched with oxygen gas than if you started with warm water.

Dissolved oxygen gas = tasty?

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    What foods are enhanced by dissolved oxygen? And in what way?
    – slim
    Commented Dec 15, 2011 at 11:05
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    This is incorrect. Boiling water has almost zero dissolved gas in it so, from that point of view, it doesn't matter whether the water started hot or cold.
    – David Richerby
    Commented Aug 14, 2014 at 17:30

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