May 09

Food Science Question: Boiling Water

FT29 comments • 1,168 views

So I was in the pub the other day and suddenly one of our number asked: why don’t we use water from the hot tap in the kettle, or in the water when we need boiling water. Its already hotter, so it will use less energy, and if the alternative is said water in the tank going cold, it would be more environmentally friendly.

My kneejerk reaction, was something to do with stagnancy and hot water tanks. But then, we’re boiling the water – so that should kill all the microscopic nasties off. And I’ve never seen macroscopic or even just scopic nasties in my bathwater (at least not before I get in it, where I BECOME the macroscopic nasty). So would we be saving the planet more by using up water from the hot tank. And does it taste worse? Food scientists, I leave it to you, as I don’t drink hot drinks that often.


  1. 1
    c on 27 May 2009 #

    Water from the hot tank is technically non-potable water. They’re classed in grades, i think: grade one being potable, the stuff you get through your direct feed cold water tap in the kitchen, and any water that’s stored in tanks in a conventional system tends to be grade two (except in some tall buildings where you have to have intermediary direct-cold-water tanks else you’d have no water pressure high up). But grade two is very wide – it goes from stored hot water to stuff that’s dirty but still isn’t toxic/waste water. I think basically it’s anything that’s gone through some kind of process subsequent to being purified, even if that process is just being boiled.

  2. 2
    a tanned rested and unlogged lørd sükråt wötsît on 27 May 2009 #

    Yes, my gran was always trying to teach us to put water from the hot tap in the kettle on the (true) grounds that it requires less electricity to boil it — rather surprisingly, given my family’s approach to the “peck of dirt you will eat in your life”* this never took hold as a sukrat-practice

    Actual routine quote from ma sukrat: “Dirt is good for you”

  3. 3
    cis on 27 May 2009 #

    oh and also there can be nasties in water that aren’t germs and won’t be killed at higher temperatures (or be safely deposited out of the way on the walls of a boiler or kettle)!

  4. 4
    a tanned rested and unlogged lørd sükråt wötsît on 27 May 2009 #

    our main line of resistance to gran’s argument was “but hot water tastes yucky” (in shropshire in the 70s there was a marked taste difference: to be honest i haven’t been where lately where this was anything like so evident…)

  5. 5
    Tracer Hand on 27 May 2009 #

    I do this. Perhaps I am already dead from it and simply haven’t realized yet.

  6. 6

    When I first moved to London there was a nine-day scandal when all the cold tapwater from a particular reservoir (I think south of the river) had tiny little dancing threadlike animals in it

    I reported this to mum who told me (with tremendous pleasure) that the Shropshire name for these beasts is “SNIGS”

  7. 7
    Pete Baran on 27 May 2009 #

    Walls of kettle in not being nasty filter shock.

    Eli, after your hand washing admission on the radio at the weekend, I am not sure if we can use you as a model of
    a) hygiene
    b) the average intestinal tract. Hard water for a hard man!

    oh and also there can be nasties in water that aren’t germs and won’t be killed at higher temperatures

    Wh wha what? (Though these MUST come from the tank because before water gets into the tank its Grade 1 water amirite?)

  8. 8
    marna on 27 May 2009 #

    Too many tales of dead pigeons in hot water tanks for me to be terribly happy glugging it.

    The actual science answer, I think, is that heated-several-times, kept-hot water has less oxygen in it, and gives a muddier taste as a result. So you should also boil fresh water every time.

    Dirt *is* good for you. Soil contains tasty and essential vitamin B12.

  9. 9
    cis on 27 May 2009 #

    ugh i shouldn’t keep saying ‘tank’, with indirect hot water it’s a cylinder. (the ‘hot water tank’ generally means the tank in the roof that contains hot water from the central heating system.)

    direct cold water is quite controlled: the system is set up with check valves so that water can’t backflow, and the distance from the entry to yr property to the kitchen tap is short. So the likelihood of encountering contaminants is low and the likelihood of those contaminants mixing in the system is as minimised as possible.

    the cold water that gets stored in your tank is necessarily a little bit open to the air (there’ll be normal expansion, contraction, movement of water, etc), even though it’s usually covered over. This is the water that comes through in eg bathroom taps and it’s the water that goes into your hot water cylinder. So if you had something nasty floating about in your attic (or a contaminant somewhere along the system) – whether living bacteria or some airborne particle like disturbed asbestos – it could get into this cold water tank, then travel down into the hot water cylinder, get warmed up a little (don’t think the water in the cylinder boils, that’d be crazy unsafe, pressure-wise), and then if you used it in the kettle it would get boiled. Boiling would kill e.g. legionella but I’m not sure it’d be much help if there was lead in the water.

    all this despite, i’ve drunk warm water from the hot tap quite a few times.

  10. 10
    ledge on 27 May 2009 #

    Hang on – the water from the hot tap isn’t just magically hot, or piped in from a thermal spring, it’s been heated by a separate process, probably yer gas boiler. So the question is, which would win in a straight out efficiency fight between a kettle and a gas boiler? And the answer is… I don’t know, it’s probly v complicated.

  11. 11
    ledge on 27 May 2009 #

    Alright maybe not that complicated. using gas to directly heat the water is of course more efficient than using a fuel to generate electricity to heat the water. But what happens when the gas runs out, eh?!


  12. 12
    Mark M on 27 May 2009 #

    Re 10: But in most cases the hot water is already hot, waiting for you to wash the dishes or whatever. You’re not heating up additional water to do the job.

  13. 13
    ledge on 27 May 2009 #

    If you have a lot of hot water sitting round in your boiler all day it might be wiser to change your heating schedule than quibble about the odd kettleful… but I have an on-demand boiler anyway.

  14. 14

    I wonder if the idea (of preheated water in the kettle) derives from an old-grandma’s-tale derived from public-services suggestions made during WW2, when there was rationing: though my grandad was a civil engineer, so i assume had a reasonably on-the-money grasp of electricity use and water potability, and etc


  15. 15
    cis on 27 May 2009 #


  16. 16
    Ben on 27 May 2009 #

    Bizarrely, without any knowledge of this article having been posted, my cow-orkers were already engaged in an afternoon debate about George Orwell’s ‘A Nice Cup Of Tea’. I raised the suggestion that they could try boiling the kettle using water from the hot tap, to save electricity, but the idea was met with derision, as, apparently “it doesn’t taste as good”. Therefore Marna (#8) is clearly telling the truth. Actual science fact.

  17. 17
    CarsmileSteve on 27 May 2009 #

    there is only one response to this question and it’s the punchline to one of my favourite jokes ever:


  18. 18
    koganbot on 27 May 2009 #

    What I read in several places that I trusted (but have subsequently forgotten the places, though not the trust) is that unwanted chemicals are much more likely to leach from pipes into hot water than into cold water, so you should use cold water even if it takes longer to heat. Boiling the water isn’t likely to undo the toxicity of the chemicals.

  19. 19
    Matt DC on 28 May 2009 #

    I use hot water in my kettle semi-regularly without thinking about it, it’s never done me any harm. Mmmm pigeon omnomnomnomnom.

  20. 20
    Steve Mannion on 28 May 2009 #

    It takes about 10-20 seconds for the water from my hot tap to heat up but once it does it feels like Actual Boiling itself. I am unsure about trying to adjust boiler/temp setting or if that’s even doable.

  21. 21
    Steve Mannion on 28 May 2009 #

    What about microwaving the water? I don’t approve of this. Microwaved tea is the second most disgusting thing in the world (after cold tea).

  22. 22
    Martin Skidmore on 29 May 2009 #

    I happened to be watching DVDs of season 4 of Dad’s Army last night. In one episode (called something like ‘Boots, Boots, Boots’ I think), Private Pike calls to his mum in the night and asks her to fetch him a glass of water. As she sets off he calls her back, and says “Kitchen water, not bathroom water.”

  23. 23
    Prince Florizel on 1 Jun 2009 #

    My mum told me not to drink the water from the bathroom. Something to do with those mysterious tanks again. Not a problem in all houses apparently.

    Also, microwaved tea tastes a lot more disgusting than cold tea, although I wonder what cold microwaved tea would taste like and whether it would taste the same as microwaved cold tea, but just colder.

    Private Pike, I mean, uh, Prince Florizel, of course.

  24. 24
    SANDRA OAKLEY on 27 Nov 2009 #

    does boiling water kill the parasite .Helicobacter.

  25. 25
    Mark C on 29 Jan 2010 #

    My plumber told me that in the past, the hot tap workings (no idea what, somewhere in the system) included solder containing lead, while the cold tap solder didn’t as it was to be used as drinking water.

  26. 26
    Mark C on 29 Jan 2010 #

    Also, doesn’t boiling water have the most heavenly smell? And I mean proper, honest-to-god odour. Everyone says I’m mad when I say this but I’m not. THEY’RE mad!

  27. 27
    tom larson on 6 Jan 2011 #

    I only have a question regarding freezing water and an e-mail response will do just fine. I was once told that boiling water will freeze quicker than cold water?? Thanks Tom Larson

  28. 28
    thefatgit on 6 Jan 2011 #

    @27…this sounds similar to how to achieve a truly clear ice cube. Most water contains particulates which will interfere with the forming of ice crystals and make a cloudy ice cube. Filtered or distilled water is particulate-free and should form a clear ice cube. Whether the freezing process can be achieved quicker than say, normal tap water, is a matter for someone else to answer.

  29. 29
    tom larson on 8 Jan 2011 #

    perhaps I wasn’t clear with the question. Using two ice trays,,one filled with cold water and the other with boiling water, put into the freezer at the same time,, which one will freeze first.. Thanks Tom Larson

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