Science Questions

Turn off the immersion to save energy?

Thu, 5th Sep 2013

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Rob Bernstein asked:


I have heard that people can save large amounts of electricity by switching their immersion heaters off and I really would like to understand how this works.


One needs to understand that the geyser trip switch does not switch the geyser on, what it does is make electricity available to the thermostat which then can turn the heater on once the temperature drops to the point where heating is required. So why would turning it off have an effect, assuming minimal heat loss from the container?



Kind Regards,

Rob Bernstein



Dave - So essentially, with something like a hot water tank, the hotter it is, theHot water tank more energy itís going to be losing. So, if itís not very well insulated, the effect is even bigger. Essentially, if you keep it hot all the time, then you'll be losing lots of electricity and lots of energy. You're going to have to put more electricity in there to keep it hot. If you turn off the electricity, it will cool down and the more it cools down, the less heat it will lose. Now, almost the best solution would be to improve the insulation but if you can't do that or thatís expensive or difficult, turning it off will save energy. I can't say off the top of my head how much itís actually going to lose.

Dominic - Itís interesting if you look at the typical electricity usage of a house. I think heating is one of really big costs you tend to have and thatís water heating, but also, space heating. If you look at an airing cupboard where you've got a hot water tank in there and you feel it, it feels hot. So, that heat is leaking out of that tank into your house. So, if youíve got that on all night then you're basically heating your house with your hot water tank all night.


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On the face of it, switching off will save electricity because the thermostat will then be irrelevant - no electricity will be used at all.

But assuming he doesn't mean that the power is off permanently, won't insulation be the key factor? Under thermostatic control, the temperature remains around its maximum, with continual maximal heat loss, regardless of demand.

When powered off, the rate of heat loss declines with temperature to a minimum. When powered back on, the energy goes mainly to heating the water and maximal heat loss only occurs once maximum temperature is reached and only continues until power is switched off again, when it once more declines.

dlorde, Fri, 2nd Aug 2013

People are talking about switching off for a few hours e.g. overnight or while at work during the day.

Obviously a badly insulated geyser will lose a significant amount of heat, but I suspect that a modern geyser would be efficiently insulated to cut heat loss, after all that's what it is designed for.

robbie, Sat, 3rd Aug 2013

When it's off, it's off. Minimum energy is expended by heating only the water you need, when you need it. Minimum cost may require heating and storing water at a different time from time of use, so you need to experiment with your particular system and power tariff to get the optimum to suit your needs. alancalverd, Sat, 3rd Aug 2013

I suspect many systems combine thermostatic control with on/off timers, so the water is kept hot only when demand is most likely. dlorde, Sun, 4th Aug 2013

That is the standard installation for all but "on demand" heaters. alancalverd, Sun, 4th Aug 2013

Turning of devices can never fail to save energy but whether you always save money is a mute point, if you have a water heater and it is well insulated always run it when power is cheapest but storage type room heaters are a different matter.
If you store up heat at night when power is at its cheapest they have an annoying tendency to release it during the day when it tends to be warm enough anyway (in the UK) I have generally found them to be of little use. syhprum, Thu, 8th Aug 2013

I came accross a very interesting website with more info on this product:
ingedata, Mon, 7th Jul 2014

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