Welcome! Here are the website rules, as well as some tips for using this forum.

# Bladder tank air-side versus water-side pressures

Member Posts: 28
I think the answer to this question is 'YES", but I can't find it discussed anywhere. Do the air-side and water-side pressures on a bladder expansion tank always match? So if a gauge on the air side reads 12 psig at initial fill, the water pressure right there should also be 12 psig, correct? And as the temperature increases and we have expansion, both gauges would increase at the same rate, correct? Thanks for your replies.

• Member Posts: 1,709
Hopefully but not necessarily. Especially as the rubber ages.
• Member Posts: 17,022
Or to put jumper's comment another way -- if they don't, they should.

There two situations where this won't be true -- neither of them good. One, the waterside pressure is significantly low. Your system is underfilled, and the bladder has expanded to fill the whole tank. Not good. Two, the air side pressure is low. You've lost your air charge or -- just possibly -- the system pressure is way high and has fully compressed the bladder -- for one reason or another. You can try adding air -- but if the bladder has ruptured, that won't help
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 10,225
You need to decide what your system "static" water pressure needs to be, that's your fill pressure with no pump running. Most tanks come factory charged to 12-15 psi. That's enough pressure for a two story house. Got a 5 story office building with the expansion tank in the basement you may need 25psi. You need enough pressure to maintain 4-5 psi at the highest point in the building. that's the first thing.

With a factory charged tank not connected to the system at 15psi the bladder is pushed toward the water connection the bladder is fully expanded. If you install the tank& fill the system to 15 psi with water the bladder will be in the middle near where it should be. When the water is heated it expands and the water pressure increase will pressureize the air side of the tank.

System pressure and expansion tank pressure have to match or you will have no expansion "cushion". example: running the system at 30 psi with 10 psi air in the expansion tank you have no cushon, it will act like a waterlogged tank. Same thing if your system is filled to 30 psi and you have 60 psi air in the tank no cushion.

Find the static fill pressure you need. Put that psi into the air side of the tank (the water side must be open to the atmosphere and isolated) The fill water side to that psi.
• Member Posts: 603
edited May 2015
Like the experts said previously , most residential water boiler systems work at or around 12 psi with cold water . Just about every boiler we install we use a #30 Extrol expansion tank . That inital 12 psi boiler pressure rises to around 20 when the water reaches 180 degrees . Perfectly fine running pressure .

For the past few years I've been filling the tanks to 15 psi since they lose pressure over time .
• Member Posts: 3,815
edited May 2015
There was a Caleffi Coffee chat on this just last summer....