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Water temperature range in a hot water cylinder

  1. Apr 14, 2013 #1
    I am thinking about solar water heating. The way that the system gathers heat is by moving any heated water into into the cylinder where it rises to the top and cold water is taken from the bottom of the cylinder and moved through the heating panel.

    Because the hot water will always rise there will always be a temperature difference between the water at the top and the bottom.

    But because of the motion of the water molecules the heat differential will disperse.

    So, for example, If the temperature is 50°C halfway up the cylinder, what will be the highest and lowest water temperatures in the cylinder?

  2. jcsd
  3. Apr 14, 2013 #2


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    I don't think it's that simple. If the tank is insulated there is no heat loss so the temperature of the whole tank could eventually approach that of the output from the solar panel. I've heard that pressurised systems can get hot enough to melt lead solder and that special solder as to be used!
  4. Apr 14, 2013 #3
    Everything I have read says that the water in the hot water cylinder will be hotter at the top than at the bottom of the cylinder according to the way the temperature affects the density of the water.

    I am looking for a way to estimate what the difference in temperature will be between the top and bottom of the cylinder.

    I guess it will be a balance between the rate at which the hotter water rises versus the rate at which the hotter water loses its heat energy through conduction to the cooler water.
  5. Apr 16, 2013 #4


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    Hot water rises yes but I don't know if that's the main reason why the top of a thermal store is hotter. I thought solar panels delivered hot water to the top of the tank like a boiler/furnace?

    My thermal store is heated by an oil boiler that way. Hot water goes in the top at about 75-80C and heats the top of the store until a thermost about half way down exceeds 60C. The return water taken from the bottom of the store seems to be about 50-55C (it's cooled by the return from the UFH). The boiler provides about 20-30C uplift.
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