Naked Science Forum
General Science => General Science => Topic started by: Alex on 29/09/2009 12:30:03
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Alex asked the Naked Scientists:
Hi
Someone once told me that it was more efficient (i.e. less electricity is used) if an immersion heater is left on, rather than switching it on to heat up water, then using some or all of the water, then turning it off. Then when hot water is needed again, switching it on to heat up another tank of water.Â
Which is most efficient? This may also be an interesting test over a week period to see which consumes most electricity?
Thanks, Alex
What do you think?
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I don't think there's a simple straightforward answer to this and it will depend upon how much heated water you want. The most efficient solution would be to only heat the quantity water that you need to use, for the energy used to heat water that is unused must be regarded as wasted (note that while excess water heated in one heating 'session' may still be hot enough to be used later, it will have cooled somewhat since heating, so it could be argued that if the water was still hot enough after cooling then you could have just heated it to the lower temperature in the first place. Following this logic loop means that you eventually end up at the point where the remaining water is too cold to use without being heated again).
The problem with an immersion type water heater here is that because they're typically fitted to a tank of water you can't really use it to heat just the required amount of water. There is some degree of control over the volume of water in the tank that's heated, of course, because the heated water will collect at the top of the tank and the colder water will collect at the bottom but, because of mixing between the two volumes of water, this will only become effective once an appreciable fraction of the water has been heated; I'd guess about 1/4 - 25% for a typical hot water tank (A very tall and narrow tank would reduce this figure, but would have a higher surface area and lose more heat for the same degree of insulation, and might be harder to physically accommodate too).
So unless you can work out the exact gradient between the heated water and the cold water in the tank, you'll probably have to heat more water than you need to guarantee getting the quantity required.
I think that what I would do then, is to find out how much hot water you need, over the period of time that the water stays hot enough after heating it, and then work on that basis.
If you can use all of the water heated by leaving the immersion heater on all of the time then that is efficient, so logically, if you don't use all of the water then you'll be better off just heating the amount you'll use, bearing in mind whether water heated in the morning, for example, will still be hot enough in the afternoon/evening, in which case you should heat more than you immediately need and sufficient for both morning and afternoon/evening, and then turn it off, or just heat enough for the morning, turn it off, and then turn it back on to heat more for the afternoon/evening.
The bottom line is that unused heated water will cool, so that energy is lost, or wasted, so what you need to do is find the optimum trade-off.
It will actually be fairly easy to find the most efficient way of using it because you can pretty safely assume that the current draw by the immersion heater will be constant regardless of the amount of hot water in the tank, so all you really need to do is compare the total length of time the immersion heater is switched on when you're comparing different strategies. Whichever strategy needs the immersion heater to be running for the shortest total amount of time, for the amount of water you need, heated to the minimum temperature you require, will be the most efficient for you.
Sorry for the rather rambling reply, but I think the last sentence above really sums it up.
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It will actually be fairly easy to find the most efficient way of using it because you can pretty safely assume that the current draw by the immersion heater will be constant regardless of the amount of hot water in the tank, so all you really need to do is compare the total length of time the immersion heater is switched on when you're comparing different strategies. Whichever strategy needs the immersion heater to be running for the shortest total amount of time, for the amount of water you need, heated to the minimum temperature you require, will be the most efficient for you.
I'm not sure that's going to work. The heater has a thermostat, so you can't tell whether the heater is drawing current or not. You'll need something a bit more sophisticated to tell. Or, if you don't use electricity to heat the house, you could run the experiment and read the electricity meter to try to determine if there was a significant difference, but the error with that approach could be rather large.
In the UK it was (is?) customary to provide a switch to let the homeowner turn the water heater on and off. Here in the US, no switch is provided. They are on all the time. The only way to turn them off is to flip a circuit breaker at the main distribution panel. We have an electric water heater. The tank is very well insulated, so I don't think we lose too much energy. I can hear when the tank is heating, and it only seems to turn on when we draw hot water from it.
As Lee says, it's a question of your usage pattern. However, if your tank is very well insulated, unless you are going to be away for at least a couple of days, I'd be inclined to leave it on all the time. For one thing, it's a lot less aggravating!
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True, the only "inefficiency" is heat-losses from the tank. All thermal heat loss is proportional to the temperature difference between the hot object and its surroundings, but are scaled down for good insultation.
By turning off the heater when not required you're reducing the average temperature of the tank (over time) and therefore, by definition, the heat losses - and therefore saving energy.
It's exactly the same argument as turning off the central-heating or air-con when you're not at home - always saves energy compared to leaving it on 24/7 because you're spending less time/effort "fighting the natural flow" of thermal energy.