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Unvented HW Cylinders with Internal Expansion (Air Bubble)

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  • 14-06-2024 5:58pm
    #1
    John.G
    Registered Users Posts: 3,490 ✭✭✭


    Has anyone ever noticed that they seem to run out of hot water sooner than expected using one of these?, on another website someone using a 170L cylinder in a new house is a bit surprised that the HW runs out "fast".

    I looked at the Megaflo, it says the "170SB" has an empty weight of 48kgs and a full weigt of 230kgs which = 182L, (total cylinder volume) and states that its nominal capacity is 170L which is correct by my calcs once the cylinder is pressurized to 3.0 bar (cold). By my calcs, at the end of the heating cycle to 60C there will be 173L of HW in the cylinder but only 134L of this, due to the position of the HW dip pipe is available, the 39L of HW above the bottom of the Dip Tube is IMO unavailable. So this nominal volume of 170L does not mean that 170L of HW is available.

    Any comments?



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    #2
    10-10-20
    Registered Users Posts: 7,023 ✭✭✭10-10-20


    Hi John,

    Could the coil(s) be taking up a considerable volume, especially if they have a quick-recovery coil?



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    #3
    John.G
    Registered Users Posts: 3,490 ✭✭✭John.G


    No, thats not the reason but does seem a bit daft that you require a 182L capacity cylinder to give a effective HW volume of 134L.



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    #4
    Wearb
    Moderators, Home & Garden Moderators Posts: 6,258 Mod ✭✭✭✭Wearb


    I suppose that amount of expansion is required to allow for the way the air dissolves over time.

    Please follow site and charter rules. "Resistance is futile"



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    #5
    John.G
    Registered Users Posts: 3,490 ✭✭✭John.G


    If that 182L cylinder had no HW dip pipe but had a external EV then it would be sized to 10% (or nearest) of the cylinder capacity or a 18L EV, this is then precharged to 3.0bar, the cold water mains is also controlled to 3.0bar and will result in a final pressure of 3.78bar after a full reheat from 15C to 60C. If the EV had no precharge pressure, ie 0.0bar, then the final pressure will be (theoretically) 10.6bar but of course the cylinder expansion relief valve will have lifted well before that at 6.0bar or if that didn't lift then the cylinder temperature and pressure relief valve (TPRV) will lift at 8.0bar. That EV would have to sized to ~ 26.3% or 48L to give a final pressure of ~ 3.5bar at the end of heating, this is exactly the same expansion vol that must be allowed in the bubble type cylinder. I would be quite (or quietly) confident that in this cylinder while there will be 173L of HW in the cylinder that only 134L (HW)is available and that the remaining 39L above the HW dip pipe bottom is not available. Someone on another website suggested that this 39L will be pushed out through the dip pipe by the rising cold water but I just can't imagine this, IMO, as soon as the cold water reaches the bottom of the dip pipe then its the cold water that will flow out with the remaining HW staying in the top of the cylinder?.



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