I have a water expansion tank (5 gallons) installed on a new water heater (40 gallons) and my houses PSI is at 70-80.

The tank came precharged at 25psi.

I know that I am supposed to match my home water pressure and that of the tanks but I wonder if this does not have to happen immediately.

If I had to do it immediately, do I have to shut down the main and can I pump using an electric pump or do I have to disconnect the plumbing and remove the tank altogether before I can pump it?

  • Didn't instructions come with the expansion tank? If you decide to increase the pressure in the bladder in the tank, you would not have to turn anything off. I would turn on a cold water faucet on low and use a bicycle pump with pressure gauge to add air. Commented Feb 11, 2019 at 15:29

3 Answers 3


You must at least match the water pressure supplying your domestic water heater. The bladder is a bladder it is not made to handle the pressure it will stretch and stretch until it breaks. You can blow a tank in hours or days if you do not match the pressure.

For boilers, you must match whatever pressure you set your water regulator to, usually 14 psi. I usually charge the expansion tank to 16 psi for a water regulator set to 14 psi because I never want fresh city water entering the system. Each time fresh water enters, it brings chlorine and oxygen into the system that destroys the pressure regulator and Temperature & Pressure valve.


The tank needs to be pressurized to match the house line pressure. Do not leave it at 25 psi. The air acts as a spring; as the bladder fills, the air compresses and the pressure rises until it equalizes with the line pressure. At 25 psi, your tank may be half full (just as an example) which leaves two issues. First, you lose half the tank capacity. Second, the effective air "spring rate" increases, making it harder to fill as it fills. This reduces the overall effective cushioning effect of the tank, which is its primary virtue.

The tank needs to be confirmed empty in order to pressurize it, because it will read line pressure regardless of fill state. So yes, it should be disconnected.


The air pressure inside the tank needs to be set at least equal to the incoming pressure, with the air chamber inside the tank at full volume. You do not need to disconnect anything to do this. You can just 1) shut off the main water supply valve, and 2) open a tap somewhere to relieve any pressure in the house water piping, and then 3) pump up the tank pressure (using any sort of air pump you like) to equal normal house supply pressure. The purpose of the tank is to absorb the expansion volume of the water that happens when it is heated in your water heater, and in doing so will prevent any harmful excessive increase in pressure. In other words, it gives the expanding heated water somewhere to go.

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