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  • FIRST POST
    filigree
    Cheapest way to use the immersion heater
    • #1
    • 12th May 05, 11:04 AM
    Cheapest way to use the immersion heater 12th May 05 at 11:04 AM
    We have a temporary problem with the gas powered hot water and heating. In the short term we are using the electric immersion for hot water.

    Does anyone know if it is more economical to

    leave it on all the time
    switch it on as needed
    switch it on for a fixed period each day?

    There are three of us with the usual needs for baths and a washing machine.

    Years ago a plumber told me it was cheapest to leave it on all the time so the thermostat only kicked in occasionally to keep it hot. He said it used MORE fuel to heat up water from cold every time. I don't know about these things so does anyone have any ideas? Thanks :confused:
Page 4
    • Cardew
    • By Cardew 20th May 09, 7:41 PM
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    Cardew
    Hum.

    So if I were to heat up a tank of hot water, then drain that tank completely, then heat the tank fully again, then drain it, then heat it, are you saying this usage would not exceed 2KWh?

    No of course I am not saying that.

    I say it would and that is my point. In normal use (with, lets say, 1 bath per day, a couple of showers, other water usage) you are continually drawing cold water into the system. This requires a lot of energy to heat.

    Personally, I have my immersion almost permanently off.
    Originally posted by NowThenAgain
    Let us consider your "1 bath per day, a couple of showers, other water usage" and say for the sake of argument it will take 5kWh to heat that water.

    It will take 5kWh regardless of you having your immersion heater on timed, or on 24/7.

    In an ideal situation with a timed immersion heater you will use all the hot water in the tank, and it will be left full of cold water, and your total consumption for the day will be 5kWh.

    If you leave the immersion on 24/7, and use exactly the same amount of hot water, you will have used 5kWh for that hot water and in addition your tank will lose 2kWh, so your total consumption for the day will be 7kWh.

    In practice even if timed you will have some warm water in the tank for part of the 24 hours - and hence a small loss of heat - so the difference will be less than 2kWh.

    I am not in any way suggesting that you should leave the immersion(or any other form of HW heating) on 24/7, just pointing out that the differences are not as large as some people seem to think.
    • Magentasue
    • By Magentasue 20th May 09, 8:22 PM
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    Magentasue
    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?
    • Cardew
    • By Cardew 20th May 09, 8:41 PM
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    Cardew
    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?
    Originally posted by Magentasue
    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.
    • Magentasue
    • By Magentasue 20th May 09, 8:49 PM
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    Magentasue
    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. .
    Originally posted by Cardew
    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?
  • skelly01
    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.
    • Cardew
    • By Cardew 20th May 09, 9:21 PM
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    Cardew
    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?
    Originally posted by Magentasue
    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.
    • Magentasue
    • By Magentasue 20th May 09, 9:30 PM
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    Magentasue
    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.
    • Cardew
    • By Cardew 20th May 09, 9:30 PM
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    Cardew
    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.
    Originally posted by skelly01
    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?
    • Cardew
    • By Cardew 20th May 09, 9:42 PM
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    Cardew
    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.
    Originally posted by Magentasue
    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.
    • Magentasue
    • By Magentasue 20th May 09, 9:58 PM
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    Magentasue
    The problem is that people carry out 'trials' that are just meaningless. do you not use gas?
    Originally posted by Cardew
    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.
    • Cardew
    • By Cardew 20th May 09, 10:52 PM
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    Cardew
    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.
    Originally posted by Magentasue
    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)
    Last edited by Cardew; 20-05-2009 at 10:56 PM.
    • Magentasue
    • By Magentasue 20th May 09, 11:04 PM
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    Magentasue

    I suspect your experience was because you were using hotter water and/or more hot water when on 24/7.
    Originally posted by Cardew
    Yes, quite possibly. I have said before that in my experience, as a large family household, leaving the immersion heater on was very expensive, but I accept in a small household where no hot water is used for much of the day, it might not make much difference
  • skelly01
    Cardew,

    In answer to your question regarding what type of monitor I use, it is one of the digital energy monitors.It was supplied by Scottish Hydro (similar to an OWL). As I stated I also carried out my own monitoring by taking the meter readings over a period of time.
    Now on reading magentasues posts (unless I have missed something), it seems that we have both had the same experience in that there is significant savings having the immersion on timed versus on 24/7.
    I am not a scientist by any manner of means, but am trained electrically so I know enough to know that my electrics are as they should be. I tested all circuits last night and they are all correct with no current leakages etc.
    I also took the temperature of my water just after one of my timed periods had finished. I have the thermostat set at 55C. The thermometer recorded 54.2, so it is is also operating OK.
    The only other thing is that we have showers with the very occasional bath, there are 2 of us in the house. This I would suggest though does not have any baring on the savings. I put this in in case someone can come up with something that is being missed. Could it be the way that one of these tanks works? I wrongly stated it was a saniflo tank. It is in fact a Santon Premier Plus, which operates unvented via expansion tanks etc. It is lagged with 45mm polyurethane.
    I wouldn't post here with artificial findings as this does not benefit anyone, and goes against the goodwill of any forum. As I stated in my earlier post I am only reporting my true and honest findings.
  • espresso
    Cardew,

    In answer to your question regarding what type of monitor I use, it is one of the digital energy monitors.It was supplied by Scottish Hydro (similar to an OWL). As I stated I also carried out my own monitoring by taking the meter readings over a period of time.
    Now on reading magentasues posts (unless I have missed something), it seems that we have both had the same experience in that there is significant savings having the immersion on timed versus on 24/7.
    I am not a scientist by any manner of means, but am trained electrically so I know enough to know that my electrics are as they should be. I tested all circuits last night and they are all correct with no current leakages etc.
    I also took the temperature of my water just after one of my timed periods had finished. I have the thermostat set at 55C. The thermometer recorded 54.2, so it is is also operating OK.
    The only other thing is that we have showers with the very occasional bath, there are 2 of us in the house. This I would suggest though does not have any baring on the savings. I put this in in case someone can come up with something that is being missed. Could it be the way that one of these tanks works? I wrongly stated it was a saniflo tank. It is in fact a Santon Premier Plus, which operates unvented via expansion tanks etc. It is lagged with 45mm polyurethane.
    I wouldn't post here with artificial findings as this does not benefit anyone, and goes against the goodwill of any forum. As I stated in my earlier post I am only reporting my true and honest findings.
    Originally posted by skelly01
    Does it not have a label with heat loss stated on it?



    If your temperature is set to 55, then your heat loss will be less.
    Blue text on this forum usually signifies hyperlinks, so click on them!..
  • skelly01
    Will check when I get home tonight espresso and report back.
  • skelly01
    Just took this from the website for my specific tank:

    Thermal Insulation:
    CFC/HCFC free (ODP Zero) fire-retardant expanded polyurethane (40-50mm thick). GWP 3.1 (Global Warming Potential)
    Standing Heat Loss Figures (kWh/24h)
    70ltr 1.10
    125ltr 1.49
    145ltr 1.65
    170ltr 1.77
    210ltr 1.97
    250ltr 2.09
    300ltr 2.25

    Mine is a 145lt tank.
    • Cardew
    • By Cardew 21st May 09, 12:17 PM
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    Cardew
    Just took this from the website for my specific tank:

    Thermal Insulation:
    CFC/HCFC free (ODP Zero) fire-retardant expanded polyurethane (40-50mm thick). GWP 3.1 (Global Warming Potential)
    Standing Heat Loss Figures (kWh/24h)
    70ltr 1.10
    125ltr 1.49
    145ltr 1.65
    170ltr 1.77
    210ltr 1.97
    250ltr 2.09
    300ltr 2.25

    Mine is a 145lt tank.
    Originally posted by skelly01
    So the maximum difference between on 24/7 and timed can only be 1.65kWh per day and that is with the water at a constant 65C.

    However the most important qualification to that statement is that you use exactly the same amount of hot water each day.

    In practice, even if timed, the difference will be less than 1.65kWh as there will be some hot water in the tank, or in your case you have the temperature at 55C

    With respect an Owl type meter cannot tell you the difference between 24/7 and timed. It will tell you when the immersion heater is on(if you can distinguish when it operates) however you would need to sit and monitor the Owl 24/7 for several days with a stop watch to check the periods it was on and off i.e. the thermostat had kicked in; and then monitor the same during timed periods as it is entirely possible that the thermostat will operate.

    As I stated above it is probable that with constant hot water you will simply use more hot water, than from a 'timed' tank where the some has been used.
    • Cardew
    • By Cardew 21st May 09, 12:26 PM
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    Cardew
    Just to clarify a point on my post above.

    If you are trying to monitor the difference, on the first day you switch to 24/7 you will get a false reading as it will consume the electricity to heat the tank of water, and then the electricity to heat the replacement water.

    However the next day you will have a full tank of hot water and it will only use the electricity to replace the loss(the 1.65kWh) so you would need to monitor 24/7 consumption over several days.
  • skelly01
    As I have stated, I have monitored over several days. I am well aware that the monitor cannot tell between the 2, however what it can tell you is how much energy you are using, or saving whatever way you want to look at it.
    I have done the comparisons both with the monitor and meter readings. I am saving what I have stated I am saving.
    My proerty is all electric, with a wet boiler. The heating is off at the moment for summer.
    With the immersion on timed I use 8 to 10 units per day, versus 16-18 I was using with on 24/7 on average.
    As I said the facts are there and the savings are being made so I am well happy.
    It can be debated until aliens land on earth, but the facts are that I save 8 units on average per day.
    • Cardew
    • By Cardew 22nd May 09, 1:09 PM
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    Cardew
    As I have stated, I have monitored over several days. I am well aware that the monitor cannot tell between the 2, however what it can tell you is how much energy you are using, or saving whatever way you want to look at it.
    I have done the comparisons both with the monitor and meter readings. I am saving what I have stated I am saving.
    My proerty is all electric, with a wet boiler. The heating is off at the moment for summer.
    With the immersion on timed I use 8 to 10 units per day, versus 16-18 I was using with on 24/7 on average.
    As I said the facts are there and the savings are being made so I am well happy.
    It can be debated until aliens land on earth, but the facts are that I save 8 units on average per day.
    Originally posted by skelly01
    Well given that the loss of heat is less than 1.65kWh, have you any theory how the extra electricity is being consumed?

    Or do you not agree that if you use exactly the same amount of hot water the difference cannot be more than 1.65kWh?
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