Heating 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.
Pressure Settings for Bladder-Type Heating Boiler Expansion Tanks
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:
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
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:
For 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.
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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.
Follow the manufacturer's installation instructions about the air pre-charge setting that came with the boiler compression / expansion tank. Standard factory air pre-charge is 12 psi. measured with the tank disconnected from the system - see our next warning.
When measuring expansion tank air pressure the boiler pressure must be at zero - which really means you will need to shut down the heating system, cool it, and drain all pressure off of the system before measuring expansion tank pressure,
you need to measure the expansion tank's air pre-charge pressure before the tank is installed.
Measuring the pressure in an installed pre-charged internal bladder type expansion tank under any other circumstances doesn't give you an accurate factory air pre-charge pressure reading because pressure in the tank may be affected by pressure (or even vacuum in a few cases) exerted on the tank by the heating system to which it is attached.
To check other expansion tank air pre-charge levels for bladder or diaphragm type expansion tanks,
see PRESSURE & TEMPERATURE SETTINGS, CONTROLS where you will see advice on setting boiler pressure and temperature.
To compare your expansion tank's pre-charge pressure with the effects of building height, also
see WATER FEEDER VALVE, HYDRONIC BOILER for a neat boiler water pressure setting vs building height guide courtesy of Carson Dunlop Associates.
Hydronic Heating Boiler Expansion Tank Sources & Instructions
AAtanks, the company's NLA Series ASME expansion tanks, - original source http://www.aatanks.com/expansion-tanks.php
Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
Bell & Gossett (Xylem Brand) ASME Expansion Tanks, Series HFT Pre-Charged Tanks,Xylem Inc.
8200 N. Austin Avenue
Morton Grove, Illinois 60053
Phone: (847) 966-3700 , - retrieved 19 March 2015, original source: http://bellgossett.com/tanks/asme-expansion-tanks-series-wta-pre-charged-tanks/ - HFT tanks are for use on heating water in closed hydronic heating systems (hot water heat) not for potable water. Excerpting from the company's installation instructions:
After initial venting and purging of air from the system, more
air will be released from the water as it is heated. Therefore, it
is recommended that a B&G air separator be installed on the
If the system has multiple loops or zones, the supply water for
all loops and zones must pass through the air separator for
complete and continuous air removal.
In case the piping
arrangement does not permit the installation of a single air
separator on the main, air separators should be installed on
each loop or zone. In this event, only one expansion tank is
required for the system.
Even with a B&G air separator installed on the main or mains,
it is recommended that B&G air vents be installed on high
points in the system.
It is also recommended that manual (key or coin type) air
vents be installed at higher points on the radiation.
ExpanFlex, by Calefactio, 1120 Blvd Michele-Bohec
Our opening hours are from Monday to Friday, 8:30 am to 4:30 pm EST.
Phone 450-951-0818, ASME tanks and accessories including thermal expansion tanks (for domestic hot water) and ExpanFlex ASME expansion tanks (for heating systems), Website: http://calefactio.com/
REFERENCES contains research citations and more how-to references for this topic.
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.
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.
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Holohan, Dan, "Why compression tanks waterlog", HeatingHelp.com (June 2014) - retrieved 19 March 2015, original source: https://heatinghelp.com/systems-help-center/why-compression-tanks-waterlog/
Etherton, Mark (contractor), "Expansion Tanks 101: the facts and myths", Contractor Magazine, contractormag.com, (March 2000), retrieved 19 March 2015, original source: http://contractormag.com/hydronics/cm_column_75
Research on permeability of rubber used in internal bladder-type heating system compression tanks or expansion tanks
Bodell, Bruce R. "Distillation of saline water using silicone rubber membrane." U.S. Patent 3,361,645, issued January 2, 1968.
Ciesielski, Andrew. An introduction to rubber technology. iSmithers Rapra Publishing, 1999.
Fuller, James, and David M. Stedham. "Expansion tank with a predictive sensor." U.S. Patent 8,633,825, issued January 21, 2014.
Martinello, Ermanno, and Mark Weih. "Membrane coating for a water pressurization Bladder." U.S. Patent Application 12/498,128, filed July 6, 2009.
Stern, S. A., F. J. Onorato, and Charles Libove. "The permeation of gases through hollow silicone rubber fibers: Effect of fiber elasticity on gas permeability." AIChE Journal 23, no. 4 (1977): 567-578.
Terashita, Fumihiro, Shingo Takagi, Shinzo Kohjiya, and Yasutoshi Naito. "Airtight butyl rubber under high pressures in the storage tank of CAES‐G/T system power plant." Journal of applied polymer science 95, no. 1 (2005): 173-177.
 B&G / ITT Reducing Valves, Instruction Manual V55999: Reducing Valves Installation, Operation, & Service Instructions, Bell & Gossett Air Separators and other heating system components, Bell & Gossett, 8200 N. Austin Ave., Morton Grove IL 60053, USA - Tel 847 966-3700 Fax 847 965-8379. Original source www.bellgossett.com/literature/files/610.pdf
Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
"Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
Installation Guide for Residential Hydronic Heating Systems
Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
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