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  • FIRST POST
    • WobblyDog
    • By WobblyDog 15th Jul 17, 12:43 PM
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    WobblyDog
    Hot water cylinder cools by 8 degrees in 24 hours - normal ?
    • #1
    • 15th Jul 17, 12:43 PM
    Hot water cylinder cools by 8 degrees in 24 hours - normal ? 15th Jul 17 at 12:43 PM
    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
    Last edited by WobblyDog; 15-07-2017 at 12:52 PM.
Page 1
    • xyz123
    • By xyz123 15th Jul 17, 12:56 PM
    • 1,504 Posts
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    xyz123
    • #2
    • 15th Jul 17, 12:56 PM
    • #2
    • 15th Jul 17, 12:56 PM
    What is the insulation on tank!?
    • WobblyDog
    • By WobblyDog 15th Jul 17, 1:02 PM
    • 437 Posts
    • 281 Thanks
    WobblyDog
    • #3
    • 15th Jul 17, 1:02 PM
    • #3
    • 15th Jul 17, 1:02 PM
    What is the insulation on tank!?
    Originally posted by xyz123
    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
    • By I have spoken 15th Jul 17, 3:18 PM
    • 4,963 Posts
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    I have spoken
    • #4
    • 15th Jul 17, 3:18 PM
    • #4
    • 15th Jul 17, 3:18 PM
    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 = -KA∆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
    • jack_pott
    • By jack_pott 15th Jul 17, 5:00 PM
    • 4,291 Posts
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    jack_pott
    • #5
    • 15th Jul 17, 5:00 PM
    • #5
    • 15th Jul 17, 5:00 PM
    Based on an ambient of 18C around the tank, I get a temperature drop of 6.8C, 1 hour on 23 hours off.
    • coffeehound
    • By coffeehound 15th Jul 17, 6:38 PM
    • 1,253 Posts
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    coffeehound
    • #6
    • 15th Jul 17, 6:38 PM
    • #6
    • 15th Jul 17, 6:38 PM
    Based on an ambient of 18C around the tank, I get a temperature drop of 6.8C, 1 hour on 23 hours off.
    Originally posted by jack_pott
    What is your tank's peak temperature? OP's appears to be only 53..
    • Jonesya
    • By Jonesya 15th Jul 17, 7:04 PM
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    Jonesya
    • #7
    • 15th Jul 17, 7:04 PM
    • #7
    • 15th Jul 17, 7:04 PM
    If you're concerned, fit a hot water cylinder jacket over the existing insulation. They're not expensive and soon pay back.
    • WobblyDog
    • By WobblyDog 15th Jul 17, 7:05 PM
    • 437 Posts
    • 281 Thanks
    WobblyDog
    • #8
    • 15th Jul 17, 7:05 PM
    • #8
    • 15th Jul 17, 7:05 PM
    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 = -KA∆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
    Originally posted by I have spoken
    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
    Last edited by WobblyDog; 15-07-2017 at 7:07 PM.
    • coffeehound
    • By coffeehound 15th Jul 17, 7:17 PM
    • 1,253 Posts
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    coffeehound
    • #9
    • 15th Jul 17, 7:17 PM
    • #9
    • 15th Jul 17, 7:17 PM
    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
    • By WobblyDog 15th Jul 17, 7:29 PM
    • 437 Posts
    • 281 Thanks
    WobblyDog
    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.
    Originally posted by coffeehound
    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
    • By coffeehound 15th Jul 17, 7:44 PM
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    coffeehound
    Maybe I ought to get it serviced.
    Originally posted by WobblyDog
    Either that or using the immersion heater might work out cheaper!
    • jack_pott
    • By jack_pott 15th Jul 17, 8:39 PM
    • 4,291 Posts
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    jack_pott
    What is your tank's peak temperature? OP's appears to be only 53..
    Originally posted by coffeehound
    I don't have one, the calculation was for the OP.

    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
    Originally posted by WobblyDog
    Based on those data, I get a temperature drop of about 4.2C.

    Are those graphs of yours from a data logger?
    • WobblyDog
    • By WobblyDog 15th Jul 17, 9:00 PM
    • 437 Posts
    • 281 Thanks
    WobblyDog
    I don't have one, the calculation was for the OP.



    Based on those data, I get a temperature drop of about 4.2C.

    Are those graphs of yours from a data logger?
    Originally posted by jack_pott
    Having just checked my numbers, I had the area calculation wrong. I now get a 3.9 Celcius drop due to loss through the tank insulation in 23 hours. I think I need to buy some pipe insulation as well. The graphs are from a Raspberry Pi, and a bunch of DIY Python and Java.
    Last edited by WobblyDog; 15-07-2017 at 9:03 PM.
    • silverwhistle
    • By silverwhistle 15th Jul 17, 10:39 PM
    • 1,675 Posts
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    silverwhistle
    An observation not based on any figures at all, but I heat my hot water using an immersion and excess energy from my PV panels using a diverter which identifies when there is spare energy available.

    My tank has integral insulation but the other day I was in a well known chain for other reasons and decided to add a 10 insulating jacket to my order. I've fitted it and the airing cupboard is now a lot cooler, and (anecdotally) I seem to be losing less heat. I've a monitor which tells me how much I've put into the tank each day, and that appears to be reducing.

    Gas boilers are a fairly inefficient way just to heat a little water. My motivation was more to extend the period during which I don't need to use the boiler at all (currently mid March - end of September), but additional insulation should help your situation as well.
    • Geoff1963
    • By Geoff1963 15th Jul 17, 10:56 PM
    • 1,056 Posts
    • 679 Thanks
    Geoff1963
    Instead of lagging the cylinder, why not lag the inside wall of the airing cupboard, then it will run warmer ? If you don't use the airing cupboard, you can fill it with insulation.
    • Geoff1963
    • By Geoff1963 15th Jul 17, 11:02 PM
    • 1,056 Posts
    • 679 Thanks
    Geoff1963
    Check the temperature of the pipes. Any that are colder as they enter the tank, is probably a return convection.
    • FreeBear
    • By FreeBear 15th Jul 17, 11:52 PM
    • 1,394 Posts
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    FreeBear
    Gas boilers are a fairly inefficient way just to heat a little water.
    Originally posted by silverwhistle
    Agreed. I only use a couple of litres of hot water each day. Much more efficient to boil a kettle for that amount. Also have an electric shower, and only fire up the gas boiler when I want a tank of hot water for a bath during the summer months.
    Her courage will change the world.

    Treasure the moments that you have. Savour them for as long as you can for they will never come back again.
    • stator
    • By stator 16th Jul 17, 12:24 AM
    • 5,908 Posts
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    stator
    My tank is from the 90s. It has some foam lagging and an extra jacket that I fitted.
    After 24hrs I would not expect the water to still be hot or even warm, so 45c would be good.
    Changing the world, one sarcastic comment at a time.
    • ComicGeek
    • By ComicGeek 16th Jul 17, 8:42 AM
    • 251 Posts
    • 197 Thanks
    ComicGeek
    Cylinder loss of 1-1.5 kWh per day is actually really good, and comparable with new cylinders. If water usage is very low then you definitely need to use the immersion during non heating months, it's so inefficient using an older non condensing boiler to do just a small load.

    Ideally you would insulate the primary pipework from the cylinder all the way back to the boiler, but that may not be possible in terms of access. Even just doing the first 1m length from the cylinder (which is probably accessible within the cupboard would have an impact.

    But at what point is it not worth doing any more, as you won't get a return on your investment. Sounds like you should ultimately get a condensing combi boiler as a replacement whenever you need to replace the boiler, so any payback period for additional insulation needs to take this into account.
    • ComicGeek
    • By ComicGeek 16th Jul 17, 8:56 AM
    • 251 Posts
    • 197 Thanks
    ComicGeek
    But you should also be heating the cylinder up to at least 60 degC once a week to kill any nasty bugs etc. Particularly important if you're not using enough hot water each day to turn over the cylinder.
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