LARGER VIEW of a heating boiler expansion tankHeating Boiler Expansion Tank Pressure Adjustment
When & how to set the air pre-charge pressure in an internal diaphragm type expansion tank (compression tank) used on hot water heating systems

  • EXPANSION TANK PRESSURE ADJUSTMENT - CONTENTS: for internal bladder type expansion or compression tanks used on hydronic heating systems, small adjustments to the air pre-charge pressure may be needed either to accommodate a heating system with a higher cold "start" pressure or when small amounts of air have been lost through the bladder.
  • POST a QUESTION or READ FAQs about heating system expansion tanks: their function, size, location, maintenance, and need for draining (on some models)

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Heating boiler bladder-type expansion tank / compression tank pressure adjustment:

For most installations the factory-set air pre-charge in an expansion tank is correct and should be left alone. But there are some circumstances in which an adjustment to the pre-charge may be needed: when the pre-charge was not to factory specifications, when the air pre-charge pressure needs to be increased to accommodate building height, or when there has been air lost from the system through the bladder.

In this article series we provide a heating system expansion tank / compression tank Troubleshooting & Repair Guide that will address just about any problem traced to this heating system component.

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Pressure Settings for Bladder-Type Heating Boiler Expansion Tanks

Heating boiler compression / expansion tank (C) Daniel Friedman The photo at left shows a modern Extrol(R) bladder-type heating system expansion tank mounted above the heating water line. Typically the internal bladder in this type of expansion tank is made of butyl rubber and accepts water from the hot water heating system. The area outside the bladder but inside the tank is where the tank's air pre-charge resides.

Typical factory air pre-charge settings is 12-psi, the same as the automatic water feed valve boiler set-pressure when the boiler is cold.

The red cap atop the expansion tank shown at left covers the air valve through which the expansion tank's air pressure is pre-set at the factory. Depending on the tank's installation position this plastic covered valve (not intended for homeowner use) may be on the tank's bottom rather than its upper surface.

[Click to enlarge any image]

I'm not sure about this position as shown, as Amtrol's installation instructions for the Amtrol Extrol models EX-15 through EX-90 state

Mount tank vertically in downward position only. Ensure the piping can support the entire weight of the tank when full of water.

Reader Question: how do I know the right PSI setting for my expansion tank?

Oct 5, 2014) Luis R. said: How I do know the correct PSI for my new expansion tank? Do I need a Fill Trol-valve in my new diaphragm expansion tank system?

I did eliminated the old horizontal expansion tank form my hot water boiler system and I decided to put a new diaphragm system , with the three part recommended , an air vent , air purge and the expansion tank. (The ET tank I did put in is not a Extrol Amtrol tank, is a Therm-o-Flex expansion tank).

I did not put a Fill troll valve, do I need one?, and do not know if I have to put more air into the new expansion tank that already came pre-charge with 12 PSI, and I do not know how much more will be need it .

My hot water heating boiler system is Gas; water 30PSI, min. relief valve cap. 144, water BTU/HR 125.200 input 180.000, output 144.000.


The tank you installed comes with a proper air pre-charge in it. Usually the manufacturer sets that air pre-charge at 12-psi cold, a pressure suitable for most residential installations.

The installer may check the air charge using an accurate gauge type pressure gauge, and as we explain in this article series, for taller buildings requiring a higher cold starting pressure in the boiler, the expansion tank may need to be pre-charged to that same pressure level, a setting above 12 psi, in which case your heating service technician may add a bit of air to the pressure tank through its normally-sealed air valve.

Don't change the pressure tank air pre-charge pressure yourself.

First check to see if the expansion tank has lost its charge

There are two valid checks that can be made on the pressure of the air charge in an internal diaphragm or bladder type expansion tank on a heating boiler:

  1. Disconnect the expansion tank completely from the heating system and then measure air pressure using an accurate air pressure gauge at the tank's air valve
  2. Cool down and drain the hot water heating system until its pressure reading is at zero psi and then measure air pressure using an accurate air pressure gauge at the tank's air valve

The only other time we might need to add air to an internal-bladder type compression / expansion tank on a hot water heating system is if over time the air pressure on a cold system's bladder tank has fallen below 12 psi. This can happen by slow air migration through the tank bladder into the heating system water.

As we explain
at EXPANSION TANK DIAGNOSIS, experts estimate the air loss through the tank bladder at about 1 psi per year - a very low rate. So you should not have to be adding air to a bladder-type compression tank frequently.

Also check the boiler's cold pressure setting. SOME but no all model of expansion tank include an automatic fill-trol valve while others do not - check the model that you installed.

OK enough theory, how do I know the right expansion tank air pre-charge setting:

Heating system pressure requirements to get heat to upper floorsFor most installations, the expansion tank pre-charge setting should be the same as the boiler's cold pressure setting. That's why the manufacturer ships the expansion tank pre-charged to 12 psi. That's the same default feed pressure found on hydronic boiler water feeders.

At WATER FEEDER VALVE, HYDRONIC BOILER we explain that because a hydronic heating system's circulator pump has little "lift" capacity (it just pushes the water around the loop of boiler and heating piping and radiators), the heating system depends on its internal water pressure to lift hot water to radiators (baseboards, or convectors) on upper floors in the building.

A higher building may therefore need higher heating system water pressure in order to be able to circulate heat adequately to upper floors. The sketch at left, courtesy of Carson Dunlop, explains that a three-story building will need heating system pressure set up to at least 15 psi. when the heating boiler is cold.

[Click to enlarge any image]

Watch out: never pressurize any tank at levels above the tank's rated pressure range. Over-pressurizing any enclosed container risks a burst container, severe injury and building damage. For pressure tanks on heating systems that need to operate at pressures over 30 psi - which would not be normal for a typical residential hot water heating system - an ASME-rated pressure tank must be selected and installed.

Hydronic Heating Boiler Expansion Tank Sources & Instructions

Some attic expansion tanks never needed a pressure adjustment nor air charge

In older hot water heating system installations an expansion tank was sometimes placed on the highest building enclosed space such as in the attic or under-roof space. These systems were often open to the atmosphere. An expansion tank overflow drain emptied out onto the roof or into a roof gutter or drainage system. When heating system pressures dropped and water level in the expansion tank dropped as well, air could be drawn back into the attic expansion tank through its drain opening.

Attic expansion tank (C) Daniel Friedman Attic expansion tank in Poughkeepsie (C) Daniel Friedman

In a few cases I (DF) have also found cistern-like designs for attic expansion tanks in which the tank in the attic had not top at all, simply an overflow drain. I'm not sure but I think the interesting open water receptacle shown below, and something I observed while inspecting the Justin Morrill Smith Homestead in South Stratford, Vermont in the U.S., was a heating boiler overflow system. It was quite small, certainly not large enough to be used as a water supply cistern. Its piping was incomplete.

Possible attic expansion tank system, Justin Morrill Smith House (C) Daniel Friedman

See ATTIC EXPANSION TANKS, HEATING for details about these systems.

Article Series Contents


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