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Hot Water Cylinder Temperature Gradient

  • 01-06-2020 3:50pm
    #1
    John.G
    Registered Users, Registered Users 2 Posts: 3,563 ✭✭✭


    I have a 150 litre dual coil cylinder, the top coil heats 100 litres (from oil fired boiler) the bottom coil (solar) heats the bottom 50 litres or the whole cylinder for ~ 6 months/year. I have numerous PT1000 (very accurate) probes installed in the cylinder.
    Assuming no solar effect if I heat up the cylinder to 60C with the oil fired coil, SP 70C, I never see any variation in the temp rise through the cylinder, bottom probe is installed about 1/4 way up from the coil bottom (and controls the motorized valve), second probe is about "20 litres" above the top coil and the top probe very near the cylinder top. Examples from cold.......
    time(mins) bot probe second probe Top probe
    0 14C 14.5C 14.5C
    15 28.7C 28.5C 28.6C
    30 40.5C 39.4C 39.4C
    45 48.9C 48.0C 48.0C
    60 54.7C 52.8C 53.4C

    I thought that one of the reasons for coil flow/return to be top/bottom was to heat the water from the top to the bottom and to give the hottest water first at the top, the above doesn't show this and makes me wonder if there is any huge difference if the coil was pumped bottom to top apart from possibly having a hotter return to the boiler which will affect the efficiency a bit.


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