expansion tank - do you need to reduce house pressure when recalibrating?

[retroplumbing]

Had a new expansion tank installed. It needs to be calibrated to be the same pressure as the house. I've read online, and in expansion tank manual that to do this you have to turn the water to the house off, and leave a hot water faucet is open, then you can measure the pressure at the expansion tank valve. This reduces the water pressure so you get the true expansion tank pressure.

Someone I work with is saying this is incorrect. You can measure the expansion tank (and put more air in it) even if the water is still running and the house water pressure PSI is already at 80 PSI.

Any advice on this?

 

[GReynolds929]

The tank should be installed after the water heater shut off valve. Shut water off to the water heater, open a faucet or the relief valve and leave open, measure the psi in the tank and inflate to house pressure, turn water back on to the water heater, close faucet or relief valve.

 

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[Master Plumber Mark]

Generally, I set the tanks at around 88 psi and leave it at that..... Their is only so much you can do if
their is no pressure reducing valve in the system... their comes a point that if you dont have a prv valve
you cant expect the themral tank to do the job alone

 

[Reach4]

Had a new expansion tank installed. It needs to be calibrated to be the same pressure as the house. I've read online, and in expansion tank manual that to do this you have to turn the water to the house off, and leave a hot water faucet is open, then you can measure the pressure at the expansion tank valve. This reduces the water pressure so you get the true expansion tank pressure.

Someone I work with is saying this is incorrect. You can measure the expansion tank (and put more air in it) even if the water is still running and the house water pressure PSI is already at 80 PSI.

Any advice on this?

If the water pressure is higher than the air precharge measured at zero psi water pressure, then the air pressure at the tank will measure pretty close to the water pressure. That is because if the air pressure were less than the water pressure, more water will come in. The air pressure will therefore rise. So the steady state is they are equal (if we presume a perfectly limp diaphragm).

If the diaphragm is stretched, that will cause some difference.

If it is a city water system, I think the ideal precharge pressure would be the pressure at max, which might occur in the wee hours of the night. But who wants to stay up and measure that?

 

[Master Plumber Mark]

If the water pressure is higher than the air precharge measured at zero psi water pressure, then the air pressure at the tank will measure pretty close to the water pressure. That is because if the air pressure were less than the water pressure, more water will come in. The air pressure will therefore rise. So the steady state is they are equal (if we presume a perfectly limp diaphragm).

If the diaphragm is stretched, that will cause some difference.

If it is a city water system, I think the ideal precharge pressure would be the pressure at max, which might occur in the wee hours of the night. But who wants to stay up and measure that?

I agree with you on this..... they come precharged at 40psi which is way to low and I sort of wonder how
much stress they can actually take when I pump them up to 88??
I would think cranking them up would shorten the life of the
tank but you do what you got to do the best you can

I got my hands on 50 anode rods recently for dirt cheap and I have been throwing them into my
more difficult installs because I dont want to ever have to change any out under warranty if the tank
fails .....