Hot water cylinder cools by 8 degrees in 24 hours - normal ?

WobblyDog
WobblyDog Posts: 512
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My hot water cylinder is currently heated once a day. In between those times, the temperature at one point on the cylinder drops from 53 to 45 Celcius. Hot water usage is very low in the house at present. It's a 140 litre tank, heated by a non-condensing gas boiler about 10 metres away.

Is that an acceptable rate of heat loss, or should I be trying to improve things?

This graph shows the cylinder temperature (Celcius) over 24 hours. It also shows that the temperature of the pipes going "in" and "out" of the boiler remain above ambient at all times, so presumably water heated by the hot water cylinder is convecting through the boiler in one direction or another while the pump is off.

https://i.imgur.com/ddxpctI.png
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  • xyz123
    xyz123 Posts: 1,662
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    What is the insulation on tank!?
  • WobblyDog
    WobblyDog Posts: 512
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    xyz123 wrote: »
    What is the insulation on tank!?

    There's about 40mm of rigid foam insulation bonded to the hot water cylinder, which is inside an airing cupboard, in the centre of the house. Most of the pipes between the boiler and the cylinder are inaccessible under floorboards, but the pipe sections in the airing cupboard are only partially insulated.
  • I_have_spoken
    I_have_spoken Posts: 5,051 Forumite
    The rate of flow of heat between two systems is measured in watts (joules per second).

    The formula for rate of heat flow is ∆Q/∆t = -K×A×∆T/x, where ∆Q/∆t is the rate of heat flow; -K is the thermal conductivity factor; A is the surface area; ∆T is the change in temperature and x is the thickness of the material (∆T/x is called the temperature gradient and is always negative because of the heat of flow always goes from more thermal energy to less).

    Simple for the OP to calculate, I hope
  • Based on an ambient of 18C around the tank, I get a temperature drop of 6.8C, 1 hour on 23 hours off.
  • coffeehound
    coffeehound Posts: 5,649
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    jack_pott wrote: »
    Based on an ambient of 18C around the tank, I get a temperature drop of 6.8C, 1 hour on 23 hours off.

    What is your tank's peak temperature? OP's appears to be only 53..
  • Jonesya
    Jonesya Posts: 1,824
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    If you're concerned, fit a hot water cylinder jacket over the existing insulation. They're not expensive and soon pay back.
  • WobblyDog
    WobblyDog Posts: 512
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    edited 15 July 2017 at 6:07PM
    The rate of flow of heat between two systems is measured in watts (joules per second).

    The formula for rate of heat flow is ∆Q/∆t = -K×A×∆T/x, where ∆Q/∆t is the rate of heat flow; -K is the thermal conductivity factor; A is the surface area; ∆T is the change in temperature and x is the thickness of the material (∆T/x is called the temperature gradient and is always negative because of the heat of flow always goes from more thermal energy to less).

    Simple for the OP to calculate, I hope

    Thanks, I hadn't thought of doing a thermal conductivity calulation. I've just done one, and I get a temperature drop of 8.3 Celcius over 23 hours. I'm a bit surprised that so much heat is being lost through the insulation, I thought convection around the loop through the boiler might be the biggest problem. Time to buy an extra insulated jacket maybe, but also possibly turn up the thermostat to avoid legionella.
    My input numbers were
    Height = 1m
    Diameter = 0.42m
    Insulation thickness = 0.04m
    Conductivity = 0.03 W/mK
    Air temp = 25 Celcius
  • coffeehound
    coffeehound Posts: 5,649
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    Another way of looking at it is the cost of heating it back up, which represents heat loss. Somebody please check my maths, but

    140,000 mL of water dropping by 8 degrees C
    Specific heat capacity of water is about 4.2 Joules / mL . deg C
    1 kWh = 3.6 MJ

    140,000 x 8 x 4.2 = 4.7 MJ

    4.7 / 3.6 = 1.3 kWh per day

    So not a great loss if that's right, though it would be worse with a higher thermostat setting. It sounds like it would be worth insulating the pipework.
  • WobblyDog
    WobblyDog Posts: 512
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    Another way of looking at it is the cost of heating it back up, which represents heat loss. Somebody please check my maths, but

    140,000 mL of water dropping by 8 degrees C
    Specific heat capacity of water is about 4.2 Joules / mL . deg C
    1 kWh = 3.6 MJ

    140,000 x 8 x 4.2 = 4.7 MJ

    4.7 / 3.6 = 1.3 kWh per day

    So not a great loss if that's right, though it would be worse with a higher thermostat setting. It sounds like it would be worth insulating the pipework.

    My boiler is using about 8kWh of gas per day to put about 1.3kWh into the hot water cylinder. Even allowing for heating up the boiler and 20 metres of 22mm pipe, that seems inefficient. Maybe I ought to get it serviced.
  • coffeehound
    coffeehound Posts: 5,649
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    WobblyDog wrote: »
    Maybe I ought to get it serviced.

    Either that or using the immersion heater might work out cheaper!
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