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Expansion Tank Pre Charge Pressure
The precharge pressure should be a small amount below the system fill pressure. Some diaphragm tanks will damage the bladder if the bladder is forced against the inlet for long periods. Ideal is for the bladder to be approximately 90% extended in normal operation. If the charge presure is higher than system pressure, then the bladder will be fully inflated and the tank will take on no water until the system pressure rises above the pre-charge pressure. Going in the other direction, if the precharge pressure is lower than the system fill pressure some of the acceptance volume is wasted; could have used a smaller tank.