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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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Replies

  • skelly01skelly01 Forumite
    186 posts
    Have too disagree with you Cardew regarding leaving your immersion heater on.
    I live in a brand new house, with a very modern saniflo hot water tank, which is lagged to the hilt.
    I used to leave the immersion heater on all day long, but have since changed to timed. I am saving around 8kwh (units) per day.
    So there is proof that the savings are in not leaving it on 24/7.
  • djohn2002ukdjohn2002uk Forumite
    2.3K posts
    skelly01 wrote: »
    Have too disagree with you Cardew regarding leaving your immersion heater on.
    I live in a brand new house, with a very modern saniflo hot water tank, which is lagged to the hilt.
    I used to leave the immersion heater on all day long, but have since changed to timed. I am saving around 8kwh (units) per day.
    So there is proof that the savings are in not leaving it on 24/7.

    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? Youre figures just defy logic and physics.
  • CardewCardew Forumite
    28K posts
    Part of the Furniture 10,000 Posts Name Dropper Rampant Recycler
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    skelly01 wrote: »
    Have too disagree with you Cardew regarding leaving your immersion heater on.
    I live in a brand new house, with a very modern saniflo hot water tank, which is lagged to the hilt.
    I used to leave the immersion heater on all day long, but have since changed to timed. I am saving around 8kwh (units) per day.
    So there is proof that the savings are in not leaving it on 24/7.

    It is not 'proof' I am afraid! and you are not disagreeing with me but the people at British Standards who test hot water cylinders.

    Unfortunately it will cost you £42 to buy the British Standard:

    http://www.bsi-global.com/en/Shop/Publication-Detail/?pid=000000000000150928

    However you can look on the internet and find lots of statements like this:
    All hot water cylinders lose energy even if no hot water is being used. The technical term is called "standing loss". Standing losses will always occur and a well insulated electric hot water cylinder may have heat losses of about 2kwh/day

    Or this:

    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


    Does your Saniflo tank have a heat loss figure on a label?

    Have you looked at the tested heat loss from your Saniflo tank? Or contacted them to ask what is the loss per day?
  • david17589david17589 Forumite
    22 posts
    yes thats ok turn it down from thermostat and you will save loads and if you have a boiler turn it down from there aswell...
  • skelly01skelly01 Forumite
    186 posts
    I am only stating what I have found. I use the same amount of water, have not touched the thermostat and by switching from always on to timed only (3 x 2 hour slots throughout 24 hours)
    So how that is not proof I do not know, its bare faced facts in my case.
    I wasnt arguing with Cardew over the heat loss element. As for the lables etc no I have not looked. I was stating what I have saved by switching from always on to timed only.
    There is nothing wrong with the element, thermostat or any of the workings. I have hot water at the temperature I need and want and as I stated I use the same amount roughly.
    The thread is about either keeping on all the time or switching to timed. My personal experience is I save by switching to timed.
    I might read the standard, although find them all very boring, it wouldn't cost me as my employer subscribes to the BSi site.
    No debate in my personal experience.
  • I think the disagreement here is due to the difference between:

    1. Maintaining a tank full of water at a required temperature for 24 hours. NO WATER USAGE. The Cardew / boiler label situation.

    2. Using some of the heated water so causing cold water to be brought into tank and be heated to the required temperature. The skelly01 situation.

    Of course, situation 2 is the reality and therefore the savings from having the tank mostly off may well reach to 8 KWH/day.
  • CardewCardew Forumite
    28K posts
    Part of the Furniture 10,000 Posts Name Dropper Rampant Recycler
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    I think the disagreement here is due to the difference between:

    1. Maintaining a tank full of water at a required temperature for 24 hours. NO WATER USAGE. The Cardew / boiler label situation.

    2. Using some of the heated water so causing cold water to be brought into tank and be heated to the required temperature. The skelly01 situation.

    Of course, situation 2 is the reality and therefore the savings from having the tank mostly off may well reach to 8 KWH/day.

    I am sorry but you are totally missing the point.

    1. If I have a tank full of hot water, and use none of that water, it will consume, say, 2kWh to maintain that water at the required temperature for 24 hours. That is the loss from the tank.

    2. That is a worse case scenario.

    3. If you use xxx litres of hot water, it will have used yy kWh to heat that water - that is nothing to do with heat losses.

    So for a given amount of hot water(at the same temperature) used, the difference between having your immersion on 24/7 or timed cannot be more than 2kWh. How can you save 8kWh a day when the maximum losses are only 2 kWh?

    Indeed the average annual consumption of energy to provide domestic hot water is under 3,000kWh, so to 'save' 8kWh a day( 2,920kWh pa) by having water timed is simply not supportable.
  • 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?

    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.
  • edited 20 May 2009 at 5:20PM
    paceinternetpaceinternet Forumite
    355 posts
    edited 20 May 2009 at 5:20PM
    The explanation is that you will not loose more than 2kwh per day through heat loss from the tank however many times you fill it.
    This is not the same as how many kwh you might use in different timed scanarios to heat the water. You are most likely wasting higher temperature water when it is permanently on, and accepting lower temperature water when it is timed.

    The saving on timed heating can only be because part of the tank is at lower temperature for parts of the day, so the heat loss is less due to temperature differences. Simple example being if half the tank was cold water, and half hot, you would loose 50% of the 2kw heat loss compared to if it was all hot. Yes?

    So, I don't doubt you can achieve savings by using timed water, but it is due to the way you use the water, and you are not comparing the same useage. If you are comfy with what comes out of the taps on timed water, then it is the best solution for you.
  • CardewCardew Forumite
    28K posts
    Part of the Furniture 10,000 Posts Name Dropper Rampant Recycler
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    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.

    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.
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