What is net capacity of an unvented cylinder (posts moved )

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Do Megaflo and other makers of UVCs with a internal air bubble state the net HW available, I reckon that if you don't want the cylinder pressure to rise above say 4.5bar when heated to 65C that the air space is 25% of the total volume so a 150L is 112L net, 200 is 150L, 300 is 225L etc.
 
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Do Megaflo and other makers of UVCs with a internal air bubble state the net HW available

Gledhill seem to. They knock off about 10%.

Platinum is a range of unvented hot water
storage cylinders with internal expansion,
manufactured in the latest high quality duplex
stainless steel.

1709932932336.png
 
The expansion relief valve is set at 8.0bar, the only one above that I see that won't lift it when a full cylinder is heated from 15C to 60C is the 300 which has a expansion volume of nearly 12% and will reach 7.5bar, all the others have a expansion volume of 10% or very slightly less and will lift the expansion valve as they would reach 9.9bar if no expansion valve was installed. This assumes the pressure reducing valve is set to 3.0bar.
 
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