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How much is inside hot water? - part 1

How should we heat the water before cooking pasta?

Marriages have been torn apart, house-mates have been turned into axe-murderers, and friendships have been terminated by arguments about this question.

There are two main opinions.

Figure 1

On the left, the gas advocates say `simply boil the water in a saucepan on the gas hob'.

On the right, the kettle advocates say `no, that's inefficient, you should boil the water in the kettle, then put it in the saucepan and bring it back to the boil on the gas'.

Who is right? Which is best? Does it depend on the time of year? I had to find out.

1-Litre bottle Safety First
Figure 2

I assembled some equipment. First, two identical saucepans (figure 1); second, an accurate water-measuring device (figure 2a) third, since this science experiment was going to involve potentially lethal boiling water, safety goggles were essential (figure 2b).

The rules for both methods were that we would start with water from the cold tap, and the goal was to end up with a pan full of boiling water. I started with the gas method.

Experiment 1

I measured the energy consumption of the gas method by pouring one litre of water in the pan, reading the meter, then setting the gas going. I put a lid on the saucepan. The first four digits on the meter give the gas used in hundreds of cubic feet; the next two (red) digits give cubic feet, and the dial reads the fraction of a cubic foot. The initial reading was 515316.18 cubic feet.

The heat of the flame was amazing, but the kettle-advocates clearly had a point, when they say that the gas method is inefficient: hot exhaust gases surged up the side of the pan.

After about 5 minutes, the water was boiling. I read the gas meter again. The new reading was 515317.10 cubic feet. Now, each cubic foot has a chemical energy of 0.3232 kWh. So, subtracting and multiplying appropriately, the energy used by the gas method turned out to be

0.3 kWh

Continue reading part 2 of How much is inside hot water...


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To find out more energy numbers, you might like to read my book Sustainable Energy - Without the Hot Air.

David MacKay
Last modified: Tue Jun 24 00:56:10 2008