Expansion tank pressure question!

[celiason]

I am trying to learn more about the plumbing industry and I have a quick question about expansion tanks.

I understand how they work but my question concerns how the pressure should be set when they are installed. Our water pressure is set at 55 PSI and we have a closed system with a 40 gallon gas water heater. The expansion tank that was installed is a Watts residential tank with a factory preset pressure of 40PSI. It was installed on the cold water side near our shut off valve in the basement...

I have read so many different things about equalizing the pressures vs. having the tank set lower than the house water pressure... I'm confused!

Should the pressure of the expansion tank and the house water pressure be the same? (I don't have the installation instructions...but I trust my Plumber...) I have been on the Watts site, but I haven't found the info I seek.

Should I be concerned about this or am I just creating one more thing to worry about?

Thank you for your patience while I get educated!

 

[Redwood]

Celiason,
I hope this helps you out.

Installation
1. Before beginning installation determine the system pressure.
a. Open a faucet to allow the system pressure to equalize.
b. Close faucet.
c. Read the system pressure at the pressure gauge (Fig. 1)
2. The expansion tank pre-charge must be set to the system pressure
as determined in Step 1. Pre-charge prior to installation in the system.
Caution: Pre-charge prior to installation in the system. Do not adjust
the air pre-charge of the expansion tank with the system under
pressure. The air pre-charge should only be adjusted under zero
system pressure.
Note: The normal pre-charge is 40psi. Do not exceed 80psi.

http://www.watts.com/pdf/1915356.pdf

The amount of precharge effects the amount of water entering the expansion tank. Too low allows more water to enter the tank and limits its expansion absorbing capacity. However the amout of water displaced by the thermal expansion in a water heater is a very small amount and the underpressure would probably not be a problem. The extra travel induced on the bladder may limit service life.

As far as location of the expansion tank anywhere after the PRV will see the same pressure. The tank must be tied in on the house side of the PRV. I like to tie them in on the system between the valve on the cold inlet of the water heater and the water heater itself. This assures that the expansion tank is protecting the water heater even when that valve is close. This is not necessary as if exoansion did ocurr while the valve was closed discharge would be about a coffee cup full, but, it is my preference.

 

[Sponsor]

 

[celiason]

Thank you!

Redwood,

Thank you for the information! I feel a little better about my expansion tank now... This site is great and I enjoy reading the advice you and the other Plumbers offer!

-celiason

 

[Redwood]

Your welcome! We enjoy giving that advice too! Thanks for bringing your questions to us!