View Full Version : Expansion tank air pre-charge pressure?

09-15-2005, 06:18 PM
Hi Guys,

What air pressure to pre-charge an expansion tank?

- Tank comes pre-charged to the 40psi (this looks to low).
- I would like to maintain 50psi in my house plumbing.
(Is this OK? City water pressure is 90psi so I have room to play).
- "Watts" recommendation is to pre-charge tank to the working
pressure (this seems to be very logical and makes sense).
- Master_Plumber_Mark recommended pumping it to about 60psi
(why the higher air pressure would be better then equal to the
system working?)

So, what is the correct answer? Mark?

Thank you,

- Vitaliy

master plumber mark
09-15-2005, 06:40 PM
I sort of feel its better to charge the tank
about 10 --15 lbs higher than you want your system to be...

my reasoning is that the bladder inside the tank only needs to expand
at times of " higher" water pressure.....like when the water heater is
heating at night....

what we have run into is where the bladders are full to capacity
already all day long,

cause they are set at the same
pressure that is comming into the house and eventually they just
give way and water expands into the bladder...and stays there...full up
all the time....

and then they seem to wear out quicker...
the reubber inside expands and finally leaks

if you set your pressure a little higher then the water pushes up into
the tank ---and the tank is basically empty of water so their is plenty of
room still in the bladder for it to expand into...then when you open a faucet it all goes back out

I have noticed a slight pressure rush at first when you open a faucet
but it seems to be the best way for it to function ...you get used to it.

its better than the bladder being water logged all the time with no place for higher pressures to go. IMO

09-15-2005, 06:47 PM
Hi Mark,

Thank you,

Your explanation sounds very reasonable and it
comes from experience this adds a lot of value to it!

- Vitaliy

09-15-2005, 08:49 PM
Because the tank is a device to handle "extraordinary" events, rather than to store water for future use, the initial pressure is immaterial, as long as it is more than zero, and less than the 150psi the heater relief valve is set for. Between "more than zero" and the system's set pressure the tank will immediately read the system's set pressure when it is turned on. Above the set pressure it will accept any overpressure up to the point where the heater's relief valve opens and prevents any pressure over 150 psi.

Gary Slusser
09-16-2005, 06:26 AM
I tend to agree and yet the main cause of failure of all precharged captive air tanks is incorrect precharge air pressure; the bladder is stretched, leading to premature failure. IMO, I'd follow the manufacturers' instructions, they certainly have tested their product to know what is best.

Quality Water Associates

09-16-2005, 11:17 AM
The bladder is not "stretched". It is the proper size to "fill" the tank with even a 1 psi charge, and always fills the tank when the system pressure is zero or the tank is not connected to any piping. Water entering the tank "folds the bladder so it takes up less space. But if there is no air pressure and the water enters the tank, the bladder is inverted so that it fills the top of the tank.

master plumber mark
09-16-2005, 04:13 PM
It looks like I am wrong about
the proper charge to make the expansioin tank

according to AMTROL it should be set at
incomming city water pressure , and I assume they
mean BEFORE it is kicked down through the PRV valve...

or do they mean after its gone through the PRV valve??

I have always set mine well above above the pressure
of the PRV valve....

check out the last page...of this spec sheet

figure it out for yourselves, I am going to ask my
sales rep on Monday just for fun

notice the diagram of the expanded water
pushing into the diaphram in the tank.......

what I understand thia is only supposed to do this when
their is thermal expansion,


09-16-2005, 06:03 PM
Hi Mark,

No, they meant system pressure AFTER the PRV.
Tank will be doing its job as soon as water pressure
will become grater then air pressure. Before this
point tank will do nothing. Water is not compressible
(well it is a little bit compressible) but air is.
As soon as water pressure will try to exceed pre-charged
(and this is a key point!) air pressure, air will begin
to compress and free up space for the expanded
water. Up to certain point (this depend on tank size)
pressure in the system will be stable.

Here is a good related link:

- Vitaliy

Gary Slusser
09-16-2005, 07:24 PM
The bladder is not "stretched". It is the proper size to "fill" the tank with even a 1 psi charge, and always fills the tank when the system pressure is zero or the tank is not connected to any piping. Water entering the tank "folds the bladder so it takes up less space. But if there is no air pressure and the water enters the tank, the bladder is inverted so that it fills the top of the tank.

hj I don't think so... The bladder only "fills" the tank when there is no water in the tank and the precharge is 1 psi or set correctly.

When the tank is in operation, the bladder never touches the inside of the tank except where it is anchored to the tank. At 1 psi air pressure plus the line pressure, the bladder is already over (past) its designed operational position inside the tank.

More line pressure due to thermal expansion over extends the operational position of the bladder that much more.

That shortenes the life of the bladder and thereby the tank. You can see the actual construction supporting this in Amtrol's pdf file that Mark posted.

Otherwise the manufacturers wouldn't say "equal to line pressure" if the precharge pressure could be anything above 1 psi.

09-16-2005, 07:49 PM
Look at a tank cutaway, or look inside the tank's inlet. The rubber bellows will be right there until water starts filling the tank The diaphragm is secured at the middle of the tank at the joint. When it is empty any air pressure pushes it to the bottom. and if there is no air pressure the water will push it to the top.

master plumber mark
09-17-2005, 05:28 AM
thank you for the post...

it pretty much says to use the bigger tank
like we do here all the time, the smaller one is ust
too wimpy to do the job... and the larger one is all we use too

the note at the botom of his article about supporting the
weight of the tank is pretty important too...

we run into a lot of lop sided wobbley tanks that are under charged
and full of about 4 .5 gallons of water nd thats quite a bit of weight
for the pipes to tolerate.. around about 35+lbs just hanging there

that is one of the reasons we decided to increase
the pressure 10 --20lbs higher in those tanks a lot higher than the house pressure...
I might even start go higher now that I think more about it.

finding a fairley new expansion tank totally water logged pretty
much told me it wasnt the right way to install these puppies.

Also when you shut off your water , it takes a few extra minutes for that
thermal expansion tank to drain out too , and you about think your
stop isnt working...

their is another plumber in town that installs a ball valve
to the thermal expansion tank to isolate it when turning off the
city water... I think that is overkill..