UVCs like Megaflo and Gledhill Premium have a air bubble  in the top to allow for expansion, these have a dip tube which extends down to just below the level of the T&PRV which is dimensionally ~ 34% down from the top of the cylinder with the dip tube say  36% from the top, which in a nominally sized 150L cylinder only leaves 96L of usable HW?. When the cylinder is pressurized to its normal 3.0bar via the pressure reducing valve there will then be 136.5L of cold water in the cylinder, if this cold water say at 15C is then heated to 60C, the expanded volume of 2.21L will give a total (hot) volume of 138.7L (cylinder pressure 3.8 bar) but only 96L of this is available as once this vol is used,  cold water will then enter the bottom of the dip tube and there will be 43L of HW above this which cannot be utilized.

Gledhill figures suggest that the capacity of these cylinders is 90% of the nominal capacity which it ~ is but its certainly not available as HW IMO.

Has anyone noticed that these cylinders run out of HW sooner than might be expected I wonder?

 

 

depending on probe placement and hysteresis values, shouldn't reheat be ocurring by then?

Almost the entire volume (92%) is/can be reheated but the volume above the bottom of the dip tube cannot be used?.

1 hour ago, John Carroll said:

Almost the entire volume (92%) is/can be reheated but the volume above the bottom of the dip tube cannot be used?.

The cold inlet is at the bottom, and water will take the path of least resistance to exit the cylinder, ergo the hot water at the top will make its way out. Me, personally, I don't like the "bubble-top" arrangements, as these need to be drained down periodically to regenerate the "bubble", and I would have an expansion vessel without a moments thought. Then you are at true '100%' useful capacity vs space / size.

I'd be a little surprised if this happens. What drew by attention to this bubble was that years ago I recharged it for someone who used up all the hot water immediately before I recharged it and to my surprise, when I shut the cold feed and opened the T&PRV, very hot water kept draining out until the T&PRV started gurgling.