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Heating boiler expansion tank / compression tank air adjustment valve & air vent valve troubleshooting guide:
This article explains the difference between the air pressure adjustment valve on an internal-bladder compression/expansion tank and the a automatic air vent valve found on both bladder type and bladderless type expansion tanks on hot water heating systems. We explain how each of these valves may be leaking but the implications, cause, and repair are quite different.
This article series provides a heating system expansion tank (compression tank) troubleshooting & repair guide.
Reader Question: water coming out of the top brass venting/valve atop the boiler's expansion tank
I found information on the expansion tank noted in the subject line on your web page. This is the kind I have on my gas furnace. There is water coming out of the top brass venting thing above the tank. My water pressure cold is almost down to zero.
The unit runs and heats the house but wondering what is going on here something is off. Any suggestions on what to do would be greatly appreciated.
At left we illustrate a new automatic air bleed valve.
[Click to enlarge any image]
Reply: First let's distinguish leaky automatic air bleed valves from a leak at the air pressure adjustment valve on an internal-bladder heating system expansion / compression tank
Water Leaks at an Automatic Air Bleed Valve
The most likely cause of leakage at the automatic air bleeder valve is that the valve core or stem itself has failed or its seat has debris on it, preventing the valve from closing when it should, or the float inside the valve has failed and no longer opens and closes the valve properly. That leaky automatic air bleed valve should be replaced.
If the part that is leaking on your boiler looks somewhat like the device shown at left, that's an Automatic Air Eliminator - Auto Vent No. 67 from Maid O Mist in our example. (Your heater may use a different brand or product).
If that's the leak source the part replacement is usually a minor repair.
Temporarily you can usually stop the leak at an automatic air vent by screwing closed the valve cap atop the valve. But that will also prevent the valve from venting air and thus can lead to an air-bound heating system and thus to loss of heat in one or more areas of your home.
Therefore while you want to stop any immediate leak to avoid making a mess or dripping onto electrical components, the proper repair is to replace the automatic air vent promptly.
Leaks at an air pressure adjustment valve on an internal-bladder compression / expansion tank
However if the leak is not at the automatic air bleed vent (shown in our photo above), but is at the air valve mounted on an internal-bladder type heating boiler expansion tank, then we have a different problem.
Our page top photo shows an internal-bladder type expansion / compression tank used on a hot water heating boiler. In the page top photo a small blue cap on the under-side of the tank covers its air pressure adjustment valve.
[Click to enlarge any image]
At left, that little red cap atop the heating boiler compression tank hides the air pressure adjustment valve. Manufacturers typically ship these tanks pre-charged to 12 psi cold - suitable for most circumstances. For some heating systems operating at higher pressures the service technician may use this valve to adjust the pre-charge air pressure in this little compression / expansion tank.
What does it mean if water squirts out of the air adjustment valve on an expansion tank?
If the expansion tank you are using is an internal-bladder type and water is coming out of the air charge valve as you describe, I suspect the bladder is burst - it's time for a replacement tank bladder or if that's not an option on your tank, a new expansion tank is needed.
Watch out: before even considering expansion tank replacement, if the water leak you are describing is not at an air charge vent on the expansion tank itself (a part that is indeed not present on most bladder type expansion tanks for heating systems) then you are probably talking not about a problem with the expansion tank but rather a problem with an automatic air bleeder vent - a separate part intended to bleed air from a hot water heating system.
Our photo below illustrates water squirting out of a water tank air pressure adjustment valve. This particular tank is in use on a potable water supply system pressure tank, it is not a heating system expansion tank. But this is what you might see when pressing the center pin of the air adjustment valve on an internal-bladder-type heating system pressure tank if its bladder has leaked and the tank is waterlogged.
Watch out: unlike the cold water squirting out of a water pressure tank with a burst internal bladder, the water that may squirt out of a heating system's expansion tank will be hot and nasty.
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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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The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
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