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    • CommentAuthortony
    • CommentTimeOct 5th 2010
     
    My wife is thinking that a kettle on the induction hob will be more efficient than the kettle is

    I told her probably not but that it would make more difference how much too much water was boiled than any difference in efficiency

    who was nearer?

    I only drink cold water.
  1.  
    Do you mean who was right technicley,

    or who was right for the sake of the marriage?
    • CommentAuthorbrig001
    • CommentTimeOct 5th 2010
     
    I can't imagine the hob being more efficient than the kettle because of the losses in the electronics used to generate the high frequencies used in an induction hob.

    On the other hand, my wife is almost never wrong :bigsmile:
    Brian.
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 5th 2010 edited
     
    If the electric kettle was a plastic one it probably has better thermal properties than a metal one on the hob.
    Put a litre in each, note the time they take to boil and look at the base of the kettle for the kW rating, find similar for the induction hob (or get an amp clamp). Do the sums. Be a very interesting study. You can do the same with a microwave (may need a special jug).
  2.  
    Check out: http://withouthotair.blogspot.com/2008/10/how-to-boil-water.html
    and:
    http://withouthotair.blogspot.com/2009/11/how-to-boil-water-sequel.html

    Intuitively, I would have to think that the kettle would be more efficient since it is essentially resistive and there are virtually no losses. In comparison an induction hob will have a substantial metal pan to heat up, (significant thermal mass compared to a plastic kettle!) which is probably wasted heat. However, a modern induction hob will be able to output more power (albeit less efficiently) and will be faster.

    Making sure you only boil as much as needed is more likely to minimise energy usage, rather than how the heat is delivered.
    •  
      CommentAuthorfostertom
    • CommentTimeOct 6th 2010 edited
     
    I recommend a huge aluminium kettle on a coal fired Aga - can't beat that boiled-socks flavour (we forget how far we've come already).
    • CommentAuthorStuartB
    • CommentTimeOct 6th 2010
     
    Both are inefficient, the Tefal one cup uses the minimum amount of energy to 'boil' exactly what you need.
  3.  
    Posted By: StuartBBoth are inefficient, the Tefal one cup uses the minimum amount of energy to 'boil' exactly what you need.


    We tried one of these but it didn't produce hot enough water to make tea to the required quality (not me, I don't drink tea), annoyingly needed your attention for too long if you wanted more than a cupful, was noisy, needed dedicated filter cartridges and died within a couple of weeks.

    Maybe the new ones are better, but we weren't at all impressed by our first generation model and were actually quite happy that it broke so we could get a refund.
    • CommentAuthorRobinB
    • CommentTimeOct 6th 2010 edited
     
    I use a Philips kettle that can switch off at 40-60 or 80 degrees or boil. I find 80 degrees takes literally half as long as boiling and is hot enough for my coffee with milk. Expect that's half the energy too. I only wish it had a 90 degree option for cafetiere coffee. I have since seen and covet a Cuisinart stove top kettle with a temperature dial. My sister tells me I should use 93 degrees for coffee.

    I imagine the induction hob would be about the same for the same amount of water.
    • CommentAuthorevan
    • CommentTimeOct 6th 2010 edited
     
    I posted this before:

    "Some more data: a 2.3kW plastic jug kettle (with flat base) required 102Wh to boil one cup, and a 3kW Tefal QuickCup hot water dispenser required only 28Wh for the same amount of water.

    However, the water in the cup ended up 10 degrees C lower using the quick-cup machine. It was on the edge of being acceptable for a cup of tea. OTOH drinking scalding hot tea is pretty bad for you."

    And yes, it broke after a couple of weeks, whereas the kettle is still working 2 years later.
  4.  
    Posted By: evanAnd yes, it broke after a couple of weeks, whereas the kettle is still working 2 years later.


    Hmm, seems like a trend. What's the embodied energy cost of a new QuickCup every two weeks compared to an extra 74Wh per cup? Discuss :devil:
    •  
      CommentAuthorDamonHD
    • CommentTimeOct 6th 2010 edited
     
    In my 1kW small flat-bottomed travel kettle it takes ~35Wh to boil just enough water for a mug of tea (and maybe 6Wh for its share of the dishwasher energy at the end of the day to wash it up). I put just enough in, there are no 'elements' to cover, I stop it as soon as it starts to boil...

    As has been pointed out elsewhere, the embodied energy of any milk you add to that tea may well exceed the energy to boil the water, so be sparing with the milk.

    http://www.earth.org.uk/food-and-CO2.html#cuppaCO2

    Rgds

    Damon
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 6th 2010
     
    Heating water (or anything) follows an exponential curve. Hence it can take twice as much energy to raise it the final few degrees compared to the first (large) number of degrees.
    This is also why lowering your internal house temperature by 1 C can save 20% on fuel (those using the C scale plays a part here).
    •  
      CommentAuthorDamonHD
    • CommentTimeOct 6th 2010
     
    Not exponential ST: at worst polynomial.

    Plus latent heat of vapourisation boiling water just to trigger the auto-switch-off when the water itself cannot get any hotter!

    Rgds

    Damon
    • CommentAuthorevan
    • CommentTimeOct 6th 2010 edited
     

    This is also why lowering your internal house temperature by 1 C can save 20% on fuel (those using the C scale plays a part here).


    That always amuses me. If you followed that advice every time you read some "energy saving tips", you'd end up with your thermostat set to minus several hundred :)
    •  
      CommentAuthorDamonHD
    • CommentTimeOct 6th 2010
     
    The rule of thumb I understood was 1C to save 10% presumbably on the the basis of an average delta-T/difference between inside and outside of 10C on heating days and a nice olde worlde linear releationship between deltaT and heat demand.

    Rgds

    Damon
    • CommentAuthordjh
    • CommentTimeOct 6th 2010
     
    Posted By: SteamyTeaHeating water (or anything) follows an exponential curve. Hence it can take twice as much energy to raise it the final few degrees compared to the first (large) number of degrees.
    This is also why lowering your internal house temperature by 1 C can save 20% on fuel (those using the C scale plays a part here).

    Eh?!

    Do you have a reference, for all three statements?

    Yours enquiringly, Dave
  5.  
    Posted By: DamonHDThe rule of thumb I understood was 1C to save 10% presumbably on the the basis of an average delta-T/difference between inside and outside of 10C on heating days and a nice olde worlde linear releationship between deltaT and heat demand.


    That's it exactly. When there's a delta-T of 40 or 45C over here I don't think about turning the thermostat down - if anything, it goes up a degree as the radiant cold from the windows feels uncomfortable when it's -25C or lower outside. Always be wary of any relationship expressed in percentages.

    For those worrying about the energy required to make a cup of tea, so long as you only make your tea when it's cold outside, then all that "waste" heat will be useful anyway. As for the temperature of coffee, I believe the optimum is around 84C - but maybe that's just for espresso.

    Paul in Montreal.
    •  
      CommentAuthorDamonHD
    • CommentTimeOct 6th 2010
     
    Still much better to heat my house with gas at 0.19kgCO2/kWh rather than waste electricity heat at over 0.5kgCO2/kWh in winter... (And indeed cheaper.)

    Rgds

    Damon
  6.  
    Posted By: DamonHDStill much better to heat my house with gas at 0.19kgCO2/kWh rather than waste electricity heat at over 0.5kgCO2/kWh in winter... (And indeed cheaper.)
    True - but it's probably better to use an electric kettle rather than one that sits on a gas hob. Really, I was just trying to point out that not all waste heat is really wasted. Personally, I don't drink tea anyway.

    Paul in Montreal.
    •  
      CommentAuthorfostertom
    • CommentTimeOct 6th 2010 edited
     
    English/Turkish/Russian-style fermented black tea does I believe require water that's actually boiling as it goes in. Other more delicate, semi-or unfermented teas specify water that's been boiled and allowed to cool a bit. Unboiled 80C hasn't a hope of making a good pot of any kind - and the idea of doing it in the cup! Whatever next - tea bags?
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 6th 2010 edited
     
    DJH

    http://www.ugrad.math.ubc.ca/coursedoc/math100/notes/diffeqs/cool.html

    Needs a bit of modification to make is applicable to heating because as something is heating it is also loosing heat, whereas if it is cooling it is only loosing heat. It can be described with a polynomial in the real world as there are set limits on it (freezing and boiling).

    I use http://www.degreedays.net to calculate HDD at different base temperatures.
    I will agree that 20% was a bit optimistic (I just plucked it from the air to highlight the issue).
    But did a comparison between between 15C and 16C at Heathrow Airport and the total saving should be about 13% Reduction or 15% increase. Depends which way you want to work it. It also depends on other factors such as number of occupants, electrical goods generating heat i.e.Fridge/Freezer, solar gain etc.
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 6th 2010 edited
     
    Posted By: Paul in MontrealPersonally, I don't drink tea anyway


    Not even Yorkshire Tea. Tut Tut :bigsmile:

    Was told that coffee should be made between 88 and 94 C by the buyer at Pelican Rouge.
  7.  
    I am looking at getting one of the instant water heaters that gives you boiling water from the tap. My thinking was that you only boil exactly what you need. Does anyone have any opinions on these? I love tea and coffee. Diolch yn fawr.
    •  
      CommentAuthorfostertom
    • CommentTimeOct 6th 2010
     
    I don't think they deliver genuinely boiling water therefore make a rotten pot of tea. For all other purposes, great.
    • CommentAuthortony
    • CommentTimeOct 6th 2010
     
    they are far less efficient than most other ways of heating water as the losses are so great -- convenient and as Paul points out all the "wasted" heat goes into the house so long as it isn't sent down the drain.
    • CommentAuthorHairlocks
    • CommentTimeOct 6th 2010 edited
     
    The only instant hot water I would consider is the quooker, it is pressurized so does come out at 98 degree, so hot enough for tea, and the water is held in a vaccum flask so losses a small amount of heat 10W. It would need to be used a several times a day, by people who can't be trained to boil what they need (teenager?) to be more efficent though.
    • CommentAuthorHairlocks
    • CommentTimeOct 6th 2010
     
    I did some tests about 6 months ago which invovled heating 1 litre of water in a rapid boil kettle using a watt meter, and 1 litre in a saucepan on a gas hob watching the meter. I calculated that the electric Kettle used 50% +/- 5% of the energy of the gas hob and saucepan.
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 7th 2010 edited
     
    Posted By: Hairlockselectric Kettle used 50% +/- 5% of the energy of the gas hob and saucepan.

    So cheaper and environmentally better to use saucepan on gas then.
    •  
      CommentAuthorDamonHD
    • CommentTimeOct 7th 2010
     
   
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