The one simple reason we wouldn’t need an over limit stat on a cylinder with a heat pump on it, is because there is a physical limitation with R410a whereby we cannot achieve high water flow temperatures
The critical temperature of a refrigerant is the temperature at which the refrigerant in vapour form cannot be liquified via condensation, irrespective of pressure.
Heat pumps work by transferring the heat energy across to the water cycle using a heat exchanger. The refrigerant runs in and out on one side of the heat exchanger and the water runs in and out on the other side of the heat exchanger.
Water runs in at a lower temperature than the refrigerant and causes the Vapour refrigerant to condense and reject heat energy into the water circuit)
(2nd Law of thermodynamics refers to the spontaneous flow of heat from a hot body to a cold body)
The Critical Temperature of R410a refrigerant is 72.8 degrees Celsius.
So from the critical temperature of R410a, we can deduce that if the water was coming back to the heat pump at 72.8 degrees C (Or above), the refrigerant cannot reject any more heat into the water circuit due to its inability to condense into a liquid.
This would mean that in practice, you will never see the water reach above 72.8 degrees, rendering any high limit stat set above this point obsolete.
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