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Will top coil heat entire cylinder ?

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  • 22-10-2007 9:45am
    #1
    ennisjim
    Closed Accounts Posts: 85 ✭✭


    Hi,
    Does anyone know if heat going to top coil of a dual coil cylinder will heat the all the water in a cylinder. I am installing solar and have dual coil 300 litre cylinder with solar driving bottom coil and central heating driving upper coil. In times when there is a little solar energy (lots of the time will the upper coil be able to heat the entire cylinder or will the upper part only be heated ?
    Thanks
    ennisjim


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    #2
    10-10-20
    Registered Users Posts: 7,165 ✭✭✭10-10-20


    No. The colder denser water falls and the hot rises. The same principal as with an electric element.


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    #3
    Owen
    Registered Users Posts: 12,683 ✭✭✭✭Owen


    Eventually it will, but it would take a long long time. Water has a high specific heat capacity, which means it takes a lot of heat energy to warm it up, and that heat in the top section will consequently have a warming effect on the water below, but for the heat in the top section to overcome both convection currents, and the colder relative temperature in the lower part of the boiler, would take a long time, and a lot of electricity.


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    #4
    GreeBo
    Registered Users Posts: 27,164 ✭✭✭✭GreeBo


    ned78 wrote: »
    Eventually it will, but it would take a long long time. Water has a high specific heat capacity, which means it takes a lot of heat energy to warm it up, and that heat in the top section will consequently have a warming effect on the water below, but for the heat in the top section to overcome both convection currents, and the colder relative temperature in the lower part of the boiler, would take a long time, and a lot of electricity.

    Actually, I dont think it ever will.
    The element will keep shutting off as the water in the top gets to the set temperature. If the element was on constantly then it would eventually heat the entire thing, but these elements are thermostatically controlled.


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    #5
    JamesM
    Registered Users Posts: 1,775 ✭✭✭JamesM


    ennisjim wrote: »
    Hi,
    Does anyone know if heat going to top coil of a dual coil cylinder will heat the all the water in a cylinder. I am installing solar and have dual coil 300 litre cylinder with solar driving bottom coil and central heating driving upper coil. In times when there is a little solar energy (lots of the time will the upper coil be able to heat the entire cylinder or will the upper part only be heated ?
    Thanks
    ennisjim

    It will heat to the bottom connection of the upper coil - and maybe slightly below.
    Jim.


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    #6
    ennisjim
    Closed Accounts Posts: 85 ✭✭ennisjim


    Hi,

    Sorry I need to clarify. The coils are not electric - bottom coil is from solar (water heating type) and top coil is from a solid fuel stove back boiler. We have a good supply of solid fuel so the stove/back boiler will be active quite a bit (also supplies space heating into a largish space) so I was wondering if the fact there is a plentiful supply of hot water from back boiler would mean that the lower part of buffer tank would also heat up eventually (ie. in a reasonable period of time, eg. hour or two) ?

    Thanks
    ennisjim


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    #7
    JamesM
    Registered Users Posts: 1,775 ✭✭✭JamesM


    See my post above - it will probably not heat much below that point.
    Jim.


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    #8
    Owen
    Registered Users Posts: 12,683 ✭✭✭✭Owen


    GreeBo wrote: »
    Actually, I dont think it ever will.

    Mine has, several times.
    GreeBo wrote: »
    The element will keep shutting off as the water in the top gets to the set temperature. If the element was on constantly then it would eventually heat the entire thing, but these elements are thermostatically controlled.

    You see, you're making the assumption that everyone's house has a thermostaically controlled boiler, and that's not true. Mine certainly doesn't. My house was built in the 60's and has an immersion, it has 2 elements, one on the top, and one on the bottom. It's an extremely simple circuit designed to be turned off by human intervention alone when the required volume of water has reach the required temperature.

    Now that the OP has clarified which system he has, it looks unlikely that his back boiler can produce enough heat energy to do the task in a mere hour or two, but it is very possible indeed for people with older bath/sink immersion systems.


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    #9
    ennisjim
    Closed Accounts Posts: 85 ✭✭ennisjim


    Since the consensus is that the bottom of the tank would not be heated by lower coil would it be true to say then that as we continue to burn fuel in the solid fuel stove (could do so for many hours a day during winter for example) it's more likely that boiling water will be vented from system than cold water at bottom of tank will be heated ?


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    #10
    GreeBo
    Registered Users Posts: 27,164 ✭✭✭✭GreeBo


    ennisjim wrote: »
    Since the consensus is that the bottom of the tank would not be heated by lower coil would it be true to say then that as we continue to burn fuel in the solid fuel stove (could do so for many hours a day during winter for example) it's more likely that boiling water will be vented from system than cold water at bottom of tank will be heated ?

    possibly (or should that be hopefully?!)
    you will also probably end up with a really, really noisy cylinder as the water bubbles/boils around.


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    #11
    GreeBo
    Registered Users Posts: 27,164 ✭✭✭✭GreeBo


    ned78 wrote: »
    You see, you're making the assumption that everyone's house has a thermostaically controlled boiler, and that's not true. Mine certainly doesn't. My house was built in the 60's and has an immersion, it has 2 elements, one on the top, and one on the bottom. It's an extremely simple circuit designed to be turned off by human intervention alone when the required volume of water has reach the required temperature.
    No I am not. I a making the assumption that the immersion has a thermostat inside it. Are you 100% sure that if you remove the cap from your immersion that you will not see a little tiny screw dial that is used to set the temperature?
    Are you also sure that this is not broken and thus your immersion heats indefinitely?

    There is a difference between a thermostatically controlled boiler and an immersion with a thermostatic shutoff inside it.
    One is a boiler and one is an immersion.


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    #12
    Owen
    Registered Users Posts: 12,683 ✭✭✭✭Owen


    GreeBo wrote: »
    No I am not. I a making the assumption that the immersion has a thermostat inside it. Are you 100% sure that if you remove the cap from your immersion that you will not see a little tiny screw dial that is used to set the temperature?

    Oh, I'm positive. Not only did I study E-Eng in College and a have a good grounding in the theory behind the workings, I've witnessed my house, and a neighbours hosing banging and clanking as the immersion was forgotten about and the water was left to heat beyond a useful temp.


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    #13
    John32c
    Registered Users Posts: 78 ✭✭John32c


    Bit of a novice at this but just wondering do I need to install a new water tank as part of the Installation of solar panels etc for Hot water supply only or will my existing tank suffice?


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    #14
    GreeBo
    Registered Users Posts: 27,164 ✭✭✭✭GreeBo


    ned78 wrote: »
    Oh, I'm positive. Not only did I study E-Eng in College and a have a good grounding in the theory behind the workings, I've witnessed my house, and a neighbours hosing banging and clanking as the immersion was forgotten about and the water was left to heat beyond a useful temp.

    so your immersion (not the cylinder) in ~68 years old and you have opened the top of it and had a look inside?

    The water can often keep heating if the thermostat is broken (like mine!)


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