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Cheapest way to use the immersion heater

edited 30 November -1 at 1:00AM in Energy
150 replies 308.9K views
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  • MagentasueMagentasue Forumite
    4.2K posts
    But if you heat your hypothetical tank of water and use 5kwh to do so, running a bath will use some of that hot water. If your immersion is on 24/7, won't it then fill with cold water and heat that using more energy to do so?

    Or have I missed the point here. I thought leaving an immersion on meant that it would refill tto have a tank full of hot water ready for the next bath. Is it not like that? Does it empty completely before it refills?
  • CardewCardew Forumite
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    Magentasue wrote: »
    But if you heat your hypothetical tank of water and use 5kwh to do so, running a bath will use some of that hot water. If your immersion is on 24/7, won't it then fill with cold water and heat that using more energy to do so?

    Yes it will use more energy to reheat that replacement cold water; let us say hypothetically it takes 5kWh!

    If you have the immersion on 24/7 you will use that energy straight away HOWEVER you will then have that hot water in the tank sitting and waiting for you to use tomorrow. All it will take is up to 2kWh to keep it at the same temperature.

    On timed you will still use 5kWh the next day to heat that water.

    So by any way of looking at the issue, whenever you reheat the replacement cold water - immediately or the next day - it will still take 5kWh. The only difference will be how much heat the water will lose from the tank.
  • MagentasueMagentasue Forumite
    4.2K posts
    Cardew wrote: »
    Yes it will use more energy to reheat that replacement cold water; let us say hypothetically it takes 5kWh!

    If you have the immersion on 24/7 you will use that energy straight away HOWEVER you will then have that hot water in the tank sitting and waiting for you to use tomorrow. .

    Not in my house. There'd be someone having a shower. More cold water to heat. Then a couple of bowlfuls to wash up ... and there's still four more of us going in the bathroom. Surely, if the immersion is on all the time, it's kicking in to heat up water every time another sinkful or bowlful is run?
  • skelly01skelly01 Forumite
    186 posts
    Since I posted a few days ago, I have been monitoring my useage (I have a monitor). I am definately saving 8kwh per day since I swtched over from the immersion heater being on 24/7.
    I have also checked my meter readings to be sure and they tie with the monitor.
    Other than that I have not changed my useage patterns.
    It works for me, would be good to hear from others with their findings.
  • CardewCardew Forumite
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    Magentasue wrote: »
    Not in my house. There'd be someone having a shower. More cold water to heat. Then a couple of bowlfuls to wash up ... and there's still four more of us going in the bathroom. Surely, if the immersion is on all the time, it's kicking in to heat up water every time another sinkful or bowlful is run?

    Yes of course it will be kicking in to reheat the water, but in the scenario you describe the differences between timed and 24/7 will be LESS than 2 kWH.

    I can't really see how else to make the point, but I will try one more time!

    However much hot water you use, and however often you use it, it will use energy.

    There are only 2 ways to use that energy, in heating the hot water, and to replace the heat lost from the tank.

    The very worst case scenario for loss of heat is to have the tank at maximum temperature without using any hot water; and in 24 hours it will lose 2kWh.

    Now surely you can see that if you are using xxx litres of hot water it will take y kWh to heat that water.

    It doesn't matter if you use 1 litre of hot water at a time or 20 litres of hot water at a time or even empty the whole tank at one go; the energy taken to heat that hot water will still be the same. The only difference will be the heat loss from the tank - and we surely have agreed that the worst case scenario(as per the British Standard for heat loss) will be 2 kWh.
  • MagentasueMagentasue Forumite
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    Okaaaaaaaaaaay, I can see what you're saying but my own experience was that there was much more than a couple of kwh in it.
  • CardewCardew Forumite
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    skelly01 wrote: »
    Since I posted a few days ago, I have been monitoring my useage (I have a monitor). I am definately saving 8kwh per day since I swtched over from the immersion heater being on 24/7.
    I have also checked my meter readings to be sure and they tie with the monitor.
    Other than that I have not changed my useage patterns.
    It works for me, would be good to hear from others with their findings.
    djohn200uk Then I sugest that there is either something wrong with the thermostat or you are using less hot water since you switched to "timed".
    If, as you say, the tank is "lagged to the hilt" then where was/is all the heat going? Your figures just defy logic and physics.

    What method do you use to monitor?

    If your tank can only lose in 24 hours, say, 2kWh where is the other 6kWh going?
  • CardewCardew Forumite
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    Magentasue wrote: »
    Okaaaaaaaaaaay, I can see what you're saying but my own experience was that there was much more than a couple of kwh in it.

    The problem is that people carry out 'trials' that are just meaningless. do you not use gas?

    You need to carry out controlled trials with controlled conditions to work out savings.

    Fortunately we have the organisation that writes British Standards that does just that for us(provided we pay £42!!) on Hot water Cylinders.

    Now if they say that a tank will only lose, say, 2 kWh, in 24 hours, where else can the heat go?

    Besides 2 kWh is still a lot of heat to lose - the biggest domestic electric kettle at 3 kW boiling water for 40 minutes.
  • MagentasueMagentasue Forumite
    4.2K posts
    Cardew wrote: »
    The problem is that people carry out 'trials' that are just meaningless. do you not use gas?

    I do use gas. In my present house and the last, we had to use an immersion heater before we put gas CH in. Prior to that we lived in an all electric house with an immersion heater and found not having the immersion on timer was prohibitively expensive. Can't remember the details, and my experience is anecdotal not scientific, but more than a couple of units in it.
  • edited 20 May 2009 at 10:56PM
    CardewCardew Forumite
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    edited 20 May 2009 at 10:56PM
    Magentasue wrote: »
    I do use gas. In my present house and the last, we had to use an immersion heater before we put gas CH in. Prior to that we lived in an all electric house with an immersion heater and found not having the immersion on timer was prohibitively expensive. Can't remember the details, and my experience is anecdotal not scientific, but more than a couple of units in it.

    Well another anecdotal story.

    I discovered my(gas) Hot Water had been left on 24/7. It transpired, that when my son stayed here some weeks earlier while we were away, he had turned it on.

    I monitor my consumption quite carefully and hadn't noticed the difference in consumption.

    I suspect your experience was because you were using hotter water and/or more hot water when on 24/7.

    What I am saying is that the heat losses will not be more than the 2 kWh(or whatever your cylinder is rated at)
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