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

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  • [Deleted User]
    [Deleted User] Posts: 0 Newbie
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    WobblyDog wrote: »
    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.

    That figure of 1.3kWh you're quoting from coffeehound is just the heat lost through the lagging, you need to add on the energy required to heat the water you use too.
  • getmore4less
    getmore4less Posts: 46,882 Forumite
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    Where are you measuring the cylinder temp?

    Any use of water sucking in cold will make a massive difference.

    I have my temp gauge at the normal place(near the bottom) and a small draw off drops that quickly and it goes back up when you stop drawing.

    Insulting all the pipework off the cylinder will help.
  • [Deleted User]
    [Deleted User] Posts: 0 Newbie
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    getmore4less wrote: »
    Insulting all the pipework off the cylinder will help.

    Stop wasting so much energy, you f*cking useless piece of pipe.
  • WobblyDog
    WobblyDog Posts: 512 Forumite
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    getmore4less wrote: »
    Where are you measuring the cylinder temp?

    Any use of water sucking in cold will make a massive difference.

    I have my temp gauge at the normal place(near the bottom) and a small draw off drops that quickly and it goes back up when you stop drawing.

    Insulting all the pipework off the cylinder will help.

    I'm measuring temperature near the top of the tank. I'm assuming that convection and conduction within the tank over a period of hours will cause the temperature at the top to drop proportionally after cold water is introduced at the bottom, but I've never drawn enough hot water to test that.

    In answer to another question, my electric kWh cost about 4 times as much as my gas kWh, so if I had an electric immersion heater it would be a viable option.
  • Alex1983
    Alex1983 Posts: 958 Forumite
    Think your worrying far to much about it. How much is your gas bill currently?
  • coffeehound
    coffeehound Posts: 5,657 Forumite
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    Thinking about it, the 1.3 kWh figure doesn't include heating of the copper tank itself so 1½ kWh is probably nearer the mark.
  • [Deleted User]
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    coffeehound wrote: »
    Thinking about it, the 1.3 kWh figure doesn't include heating of the copper tank itself so 1½ kWh is probably nearer the mark.

    The heat capacity of the tank is only about 1% of the heat capacity of the water.
  • Gloomendoom
    Gloomendoom Posts: 16,550 Forumite
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    dggar wrote: »
    Have you estimated the input of kWh using the electric immersion heater (I assume it should still be approx 8kWh).


    If you have how does the cost of 1kWH of gas compare with 1kWh of electricity for your energy tarriff.

    The immersion heater will heat the water in the tank directly so should be more efficient than a gas boiler that heats itself up, then the water in it and then has to transport that water some distance to the tank with all the associated heat losses from the pipes along the way.
  • coffeehound
    coffeehound Posts: 5,657 Forumite
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    jack_pott wrote: »
    The heat capacity of the tank is only about 1% of the heat capacity of the water.

    Well given the pipework and water therein leading off the tank is taking some of the heat too, and that the calculation was only to one significant figure, it seems reasonable to round it up to the half kWh.
  • getmore4less
    getmore4less Posts: 46,882 Forumite
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    WobblyDog wrote: »
    I'm measuring temperature near the top of the tank. I'm assuming that convection and conduction within the tank over a period of hours will cause the temperature at the top to drop proportionally after cold water is introduced at the bottom, but I've never drawn enough hot water to test that.

    I just checked my water temp from Sunday when we were out all day with small draw off around 12:00am

    heating cycle at 15:20-15:40pm
    peaked at 44c 16:44
    dropped to 33c 7:31 about 15hr.

    I insulated all the pipes in the airing cupboard, the ambiant temp in there dropped a lot and the tank heat loss reduced significantly(did not record the old profile)

    25mm foam tank, just realised missed the important pipe the cold inlet.


    if you are using very little hot water you don't need to heat a full tank,
    may be worth trying a top emersion and only heat around demand time.
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