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  • FIRST POST
    • oldwiring
    • By oldwiring 7th Apr 18, 6:41 PM
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    oldwiring
    Energy use question
    • #1
    • 7th Apr 18, 6:41 PM
    Energy use question 7th Apr 18 at 6:41 PM
    I'm playing around with figures of energy used in my home! This is out of a degree of curiosity, not a need to cut costs

    In my weather station area the mid point temperature, minimum to maximum, measured over many years is 10.4 deg C.
    I have a 180cm long bath, which is filled low to medium height. Our heating is by condensing combi.

    Using the info above, can anyone suggest what each bath run would consume in kWH to heat to 40 deg C?
Page 1
    • Hengus
    • By Hengus 7th Apr 18, 7:00 PM
    • 6,156 Posts
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    Hengus
    • #2
    • 7th Apr 18, 7:00 PM
    • #2
    • 7th Apr 18, 7:00 PM
    Two important pieces of information are missing from your question. What is the inflow temperature of your cold water and what is the boiler out flow temperature?

    http://www.clear-heater.co.uk/hot-water-calculations.html
    Last edited by Hengus; 07-04-2018 at 7:03 PM.
    • oldwiring
    • By oldwiring 7th Apr 18, 8:00 PM
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    oldwiring
    • #3
    • 7th Apr 18, 8:00 PM
    • #3
    • 7th Apr 18, 8:00 PM
    I'll assume the incoming water is at the figure mentioned in the OP.
    I'm not sure what you mean by 'boiler outflow temperature'. Where would I find that, please?
    • Hengus
    • By Hengus 7th Apr 18, 8:33 PM
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    Hengus
    • #4
    • 7th Apr 18, 8:33 PM
    • #4
    • 7th Apr 18, 8:33 PM
    I'll assume the incoming water is at the figure mentioned in the OP.
    I'm not sure what you mean by 'boiler outflow temperature'. Where would I find that, please?
    Originally posted by oldwiring
    What temperature is your boiler set at for hot water heating? You will use more kWhs if you heat to 70C rather than 60C.

    If it helps, my system boiler heats my hot water to 70C. A bath and a shower every day uses an average of 7kWhs of gas per day - about 21p plus the standing charge. I have a gas monitor that records gas usage every 15 minutes.
    • macman
    • By macman 8th Apr 18, 3:24 PM
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    macman
    • #5
    • 8th Apr 18, 3:24 PM
    • #5
    • 8th Apr 18, 3:24 PM
    You'll also need to factor into your calculations an allowance for each plastic toy you have in the bath, as these can absorb a lot of heat energy...this is known as the 'rubber duck adjustment'.
    More seriously, the figure you arrive at will only be a very rough average, as the cost will vary each time according to the incoming water temp, ambient house temp, etc etc; i.e. the energy needed to raise a certain volume of water (that you have not accurately defined) from XCto 40C.
    I'd suggest that an easier way to do this is simply to take a meter reading (with the CH and any other gas appliances switched off) before and after your bath, and then calculate the quantity of gas and it's price. This also means that you don't have to guess at the efficiency of your boiler, which could be anything from 70 to 90%.
    No free lunch, and no free laptop
    • oldwiring
    • By oldwiring 8th Apr 18, 5:57 PM
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    oldwiring
    • #6
    • 8th Apr 18, 5:57 PM
    • #6
    • 8th Apr 18, 5:57 PM
    Ha ha! @ macman You must have read/been read to the same kiddy book as me. "Bathtime toys on the water afloat; a goldfish, a duck and a wooden boat". My offcuts are nearing 50s now, and I still remember.
    Lawks!
    Thanks for advice too.
    • jk0
    • By jk0 8th Apr 18, 7:37 PM
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    jk0
    • #7
    • 8th Apr 18, 7:37 PM
    • #7
    • 8th Apr 18, 7:37 PM
    Energy to heat water= 4200 Joules/kg Kelvin.

    Say bath is 18dm by 5dm by 2dm deep, that makes 180 litres or 180 kg.

    If water is heated from 10 deg c to 40 deg c, that is 30 degrees.

    So energy required is 30x180x4200=22.7 MJ.

    1watt is one joule/sec, so 1kwh is 3600 secs x 1000watts=3.6MJ

    Therefore bath of water requires 22.7/3.6 kwh= 6.3 units.
    • oldwiring
    • By oldwiring 8th Apr 18, 7:53 PM
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    oldwiring
    • #8
    • 8th Apr 18, 7:53 PM
    • #8
    • 8th Apr 18, 7:53 PM
    Thanks for interesting calculation. Joules had me, as did mention of kg in calculations seen before. I hadn't connected water volume with its weight.
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