I have a five gallon expansion tank installed for a 75 gallon water heater. After about a year, starting to get some 'singing'in the system.

Wondering if there is a way to check and top off pressure in an installed expansion tank (I know how to do a pre-charge.) My house water pressure is regulated to about 47 - 50psi. Not sure how to calculate the pressure (if doing so is possible) that I should put in the tank for an installed tank.

Also, is a 'singing system' a potential sign of too much expansion tank pressure, too little, or possibly either?

  • Since I don't know what you mean by "singing" no idea what that indicates, if anything. But jay is correct - you have to take all pressure off the water side to adjust air pressure correctly in any bladder tank. Generally they should not need regular adjustment - if set correctly, they should stay there until they fail, and at that point they won't hold air pressure anymore.
    – Ecnerwal
    Oct 2, 2022 at 14:31
  • If you mean you are hearing "boiling" type noises when the heater operates, might be time to clean it and remove hard water deposits.
    – Ecnerwal
    Oct 2, 2022 at 15:36

1 Answer 1


You can't precharge it in the presence of system water pressure. The objective is to have the air bladder full of air, ie occupying the entire steel tank, at system cold pressure. If there is any pressure on the water side while precharging, the calculated or targeted precharge will result in the air bladder being less than the full volume of the tank, thus reducing its capacity.

If the pre charge is too high or too low you'll get PRV bleeding when the system is hot and auto fill filling when it's cold. But you can't tell which problem you have ... Too high or too low because both result in reduced air volume for expansion.

If the tank is connected and charged too high you have to remove air and add water, or if too low you have to add air and remove water in an amount corresponding to the volume required for the bladder to completely fill the tank. There is no practical procedure to do either of those simultaneous adjustments, and unless the tank is made of glass you can't even know which one is necessary.

The only way to do this is with the water side of the tank open to the atmosphere.

This Answer and this article are good further reading.

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