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  • FIRST POST
    • WobblyDog
    • By WobblyDog 15th Jul 17, 12:43 PM
    • 383Posts
    • 241Thanks
    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 2
    • dggar
    • By dggar 16th Jul 17, 9:23 AM
    • 520 Posts
    • 260 Thanks
    dggar
    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.
    Originally posted by WobblyDog

    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.
    • jack_pott
    • By jack_pott 16th Jul 17, 11:08 AM
    • 3,776 Posts
    • 4,816 Thanks
    jack_pott
    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.
    Originally posted by WobblyDog
    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
    • By getmore4less 16th Jul 17, 12:46 PM
    • 29,138 Posts
    • 17,424 Thanks
    getmore4less
    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.
    • jack_pott
    • By jack_pott 16th Jul 17, 1:42 PM
    • 3,776 Posts
    • 4,816 Thanks
    jack_pott
    Insulting all the pipework off the cylinder will help.
    Originally posted by getmore4less
    Stop wasting so much energy, you f*cking useless piece of pipe.
    • WobblyDog
    • By WobblyDog 16th Jul 17, 1:45 PM
    • 383 Posts
    • 241 Thanks
    WobblyDog
    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.
    Originally posted by getmore4less
    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
    • By Alex1983 16th Jul 17, 2:05 PM
    • 312 Posts
    • 165 Thanks
    Alex1983
    Think your worrying far to much about it. How much is your gas bill currently?
    • coffeehound
    • By coffeehound 16th Jul 17, 2:06 PM
    • 1,067 Posts
    • 1,747 Thanks
    coffeehound
    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.
    • jack_pott
    • By jack_pott 16th Jul 17, 2:31 PM
    • 3,776 Posts
    • 4,816 Thanks
    jack_pott
    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.
    Originally posted by coffeehound
    The heat capacity of the tank is only about 1% of the heat capacity of the water.
    • Gloomendoom
    • By Gloomendoom 16th Jul 17, 4:08 PM
    • 12,404 Posts
    • 16,580 Thanks
    Gloomendoom
    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.
    Originally posted by dggar
    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.
    Advice; it rhymes with mice. Advise; it rhymes with wise.
    • coffeehound
    • By coffeehound 16th Jul 17, 6:50 PM
    • 1,067 Posts
    • 1,747 Thanks
    coffeehound
    The heat capacity of the tank is only about 1% of the heat capacity of the water.
    Originally posted by jack_pott
    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
    • By getmore4less 17th Jul 17, 6:16 AM
    • 29,138 Posts
    • 17,424 Thanks
    getmore4less
    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.
    Originally posted by WobblyDog
    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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