Why is the precharge pressure so important to check?
A closed expansion vessel with precharge pressure will slowly lose its precharge pressure due to the compressed air between the rubber bladder and the outer edge of the vessel, diffusing through the rubber bladder. This means that the precharge pressure in the vessel drops, as well as the system pressure. Due to this, the possibility of expansion also reduces.
A common action to take when the pressure in the system drops, is to fill it up with fresh water to get the pressure up. This is the wrong measure to start with. You should always check the precharge pressure first.
To be able to check the precharge pressure in a correct way, you have to be able to close/isolate the expansion vessel from the system and then make the vessel pressureless on its fluid side. When this has been done, you can check the precharge pressure with an air pressure gauge through the expansion vessel’s air vent.
HL Hydronics can help you to choose the right precharge pressure
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