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The Sheriff

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I have been trying to get my better half to fill the kettle just after it has boiled, so the water gets to room temp ready for the next brew and thus save electricity; or so I thought !

Last night I measured 2 pints of water into the kettle and left overnight. The next morning I timed it to boil and auto shutoff - 2 minutes 41 seconds.

Later I remeasured same amount of water straight from the tap and timed it to auto shut off - 2 minutes 43 seconds !!!

Amazed at how little difference, I was expecting it to be much longer.

ANY THOUGHTS / COMMENTS ON MY FINDINGS
 

Sporky McGuffin

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I think someone should do the sums. Not me though. I've done enough sums today.

I reckon the water comes out of the tap at 5-10°C. Room temperature is for the sake of argument 20°C, and let's say the kettle takes it all the way to 100°C. If anyone can remember the SHC of water and knows the power output of a typical kettle...
 

Woody2Shoes

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You will save vastly more energy only boiling the water you actually need.

As suggested above, letting mains tap water warm to room temp probably takes it from 8C to 18C - the national grid (at your, and to a lesser degree the polar bears' expense) takes it from say 18C to say 98C. You've perhaps saved an eighth of the energy needed to get the water from underground temp to boiling. If you boil twice as much as you need, three times a day, any 'saving' you get from pre-warming pales into insignificance.

A better approach might be to fill a large jug with water and let that warm to room temp, then fill the kettle as needed from that (best of luck!!).

I find it hard enough to persuade my co-habitees to only boil what is required!!
 

Yojevol

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These arguments about 'only boiling what you need' doesn't apply in the winter when you've got the heating going. Any heat left over in the kettle will transfer to the house environment and, if you've got thermostat control, the CH output will reduce in accordingly.
Brian
 

Terry - Somerset

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Google - the fount of all knowledge says:

The energy required to change water from a liquid to a solid is 333.7 kJ/kg while the energy required to boil water is 2257 kJ/kg. The amount of energy needed to change the phase of water to a gas from a liquid is 540 times the amount of energy needed to raise the same amount of water 1° C.

Boiling involves a phase change from liquid to gas. So most of the energy is the last degree, not the total temperature change!
 

John Brown

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These arguments about 'only boiling what you need' doesn't apply in the winter when you've got the heating going. Any heat left over in the kettle will transfer to the house environment and, if you've got thermostat control, the CH output will reduce in accordingly.
Brian
While that is true, if you have gas central heating, it's still way cheaper than electricity per therm, or whatever measurement is currently used.
So I suppose the idea could work, to the extent that the initial heating of the water to room temperature would be at gas rates, as opposed to the kettle, at electricity rates.

I tried to persuade my wife to leave the bathwater to cool before emptying the tub. She thinks I'm crazy, but my father used to talk about having heat exchangers in the sink wastes, to capture all the wasted heat from the vegetable boiling water... Those Yorkshire genes are strong...
 

Fitzroy

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Measurement accuracy, voltage differences or both. Internal heat transfer in the kettle will be fixed in both cases. A 10
Google - the fount of all knowledge says:

The energy required to change water from a liquid to a solid is 333.7 kJ/kg while the energy required to boil water is 2257 kJ/kg. The amount of energy needed to change the phase of water to a gas from a liquid is 540 times the amount of energy needed to raise the same amount of water 1° C.

Boiling involves a phase change from liquid to gas. So most of the energy is the last degree, not the total temperature change!
Only if you boil the kettle dry. In reality only a tiny fraction of the water will change phase before the kettle turns off
 

Droogs

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Iced tea uses boiled water, the tea would take about 4 days to diffuse otherwise, before using even more energy to become cold.
 

Krome10

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Does it require the same amount of energy to heat water throughout the range of temperatures? So does 20 to 30c take the same energy as 90 to 100c?

Sorry if that's an obvious or stupid question but I really don't know!

I'm curious because I've got one of those kettles where you can select the desired temp...
 

Suffolkboy

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Iced tea uses boiled water, the tea would take about 4 days to diffuse otherwise, before using even more energy to become cold.
Tea bags in a bottle of water, leave it in the fridge for four days. Four bottles on rotation, bottle a day.

Sell your kettle on ebay and buy a second hand plane with the proceeds then just watch the energy savings pile up.
 

Droogs

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Does it require the same amount of energy to heat water throughout the range of temperatures? So does 20 to 30c take the same energy as 90 to 100c?

Sorry if that's an obvious or stupid question but I really don't know!

I'm curious because I've got one of those kettles where you can select the desired temp...
The below graph from the great courses plus shows it is fairly linear in terms of energy needed to rise in temp until you get to boiling point and then it is an exponential jump to get a change of state phase

1635409191133.png
 

stuart little

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While that is true, if you have gas central heating, it's still way cheaper than electricity per therm, or whatever measurement is currently used.
So I suppose the idea could work, to the extent that the initial heating of the water to room temperature would be at gas rates, as opposed to the kettle, at electricity rates.

I tried to persuade my wife to leave the bathwater to cool before emptying the tub. She thinks I'm crazy, but my father used to talk about having heat exchangers in the sink wastes, to capture all the wasted heat from the vegetable boiling water... Those Yorkshire genes are strong...
I use the veg h2o to make gravy!
 

hairy

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1 We have a plastic kettle because it's light. I bought a similar kettle for my Nan years back and was amazed at the difference in kettle weights available. She could then make tea much more easily. More weight means more material needs heating as well as the water.

2 I bought a new thermos earlier this year, a Thermos Ultimate series. It's the best flask I've ever had. If I leave it in the shed overnight I can still make tea on day two, I'm fussy with tea too. But more to the point, it's supposed to be more energy efficient to fill the kettle totally up then use it from a flask whenever you want a brew. I started doing this a wee bit, and also discovered that you are obviously no longer waiting for the kettle to boil, you just pour the flask and there's your tea. Weirdly satisfying to not be waiting.

3 Also, kind of almost related, I had always thought it was a thing that an upright fridge or freezer was wasteful because the air you had paid to chill falls out every time you open the door. I read someones experiment to see if that was the case and it was so tiny as to not be bothering with. They calculated, for comparison, after a warm or hot bath do you pull the plug? You would save many times more money than the fridge loses by leaving the water in the bath when you have finished until it had given up it's excess heat to the house then drain it, or you are paying to heat your drains! You do get more condensation though.
 

Vann

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...I tried to persuade my wife to leave the bathwater to cool before emptying the tub. She thinks I'm crazy...
Same here. I tell her that we've paid for all that heat - leave it to warm the house. But no, she's got to drain all the water down the plug hole and then turn on a heater. I don't know if she doesn't understand (or if she thinks I'm an old tight-a$$).

Cheers, Vann.
 

Fitzroy

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@Krome10 to all intents and purpose yes it is constant (roughly 4.18 kilojoules per kilograms per degree Celcius)*. In theory it varies with temperature, between a minimum of 4.1796 and a maximum of 4.2199, less than a 1% variation.

You can actually get a clever shower drain that is a heat exchanger, heating the cold supply to your shower such that it takes less hot water to achieve the desired temperature.

* this is actually the definition of a kilocalorie, the heat require to raise 1 kilogramme of water by 1 degree celcius.
 
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