Expansion Tank / Compression Tank Diagnosis
How to troubleshoot the expansion tank on hot water heating systems
EXPANSION TANK DIAGNOSIS - CONTENTS: Heating System Boiler Expansion Tanks, Waterlogged tanks, How to Drain an Expansion Tank - Troubleshooting & Repair Guide: Diagnostic Tests for Waterlogged Heating Boiler Expansion Tank - How to troubleshoot an internal-bladder type expansion tank or How to troubleshoot a traditional bladderless expansion tank or compression tank. HOW TO diagnose trouble with internal bladder Extrol type expansion tanks - What goes wrong with bladderless expansion tanks - waterlogging & its causes
POST a QUESTION or READ FAQs about heating system expansion tanks: their function, size, location, maintenance, and need for draining (on some models)
Signs of Trouble with a Heating Boiler Expansion Tank
Expansion tanks on hot water heating systems can be divided roughly into two groups:
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Older type bladderless heating system expansion
tanks (atmospheric expansion tanks) and
Newer type internal bladder-type expansion tanks such as those sold by Amtrol Inc.
The behavior, maintenance, and sources of leaks or trouble are not identical with these two different types of compression / expansion tanks on boilers.
A modern internal bladder type expansion tank like this Amtrol Extrol tank (photo at left) is generally trouble free, and it's unusual for these tanks to fail. The Ex-Trol® is properly referred to as a pressurized diaphragm-type expansion tank.
Both older non-bladder type expansion tanks and even modern (and much smaller) internal bladder expansion tanks can get in trouble.
But on occasion the internal bladder in one of these tanks could rupture or develop a leak. If that happens, because the tank is not field-repairable, the "fix" is to install a new expansion tank.
But far more often, it is older non-bladder type expansion tanks on boilers, like the one below) that become waterlogged.
What Happens if an Expansion Tank Becomes Waterlogged?
When the expansion tank is no longer able to absorb this initial pressure increase, it is possible that the heating system's internal pressure would exceed
30 psi - the typical point at which a heating boiler pressure/temperature relief valve will open to spill excess pressure. If the relief
valve is forced open in this manner the heating system will first lose water each time a heating cycle starts by heating up
Then the heating system will take in makeup water (through the automatic water feed valve)
each time the system cools down. The result would be recurrent loss and then inflow of water through the boiler, increasing
the risk of system corrosion as well as wasting water and possibly causing other damage or operating problems.
Watch out: spillage at the pressure temperature relief valve is potentially dangerous: eventually minerals in the water can clog a leaky relief valve, causing it to
stop leaking - which might look ok but this means that the relief valve has become clogged - the boiler is operating without this critical
safety device and an explosion could occur.
Shown above, an older style ceiling mounted atmospheric expansion tank on a hydronic heating boiler in Two Harbors, MN. The drain valve shown at the left end of this expansion tank is a special valve that makes it easier for replacement air to enter the tank when the tank's water is being drained - a standard procedure during annual heating boiler maintenance. We describe that procedure separeately at EXPANSION TANK DRAIN & AIR RE-CHARGE .
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Trouble signs that could be traced to a water-logged boiler expansion tank include:
Water drips or spills from the pressure/temperature relief valve on the boiler, typically at every boiler run cycle. This means that if you see dripping at the pressure/temperature relief valve on a heating boiler, one thing to check is whether or not
the expansion tank is working properly. We discuss relief valve leaks caused by expansion tank problems just below.
See RELIEF VALVE LEAKS Watch out: a dripping or frequently spilling T&P valve is dangerous because those very leaks can eventually cause the valve to clog and then to fail to open when it should.
Boiler pressure gauge reads higher than normal pressure. More than 30 psi on a typical residential hot water heating boiler is abnormal. Usually in a typical residential home the boiler operating pressure when it's up to temperature will be 20 to 22 psi.
But before blaming the expansion tank, take a look at both the temperature on the gauge (it should be under 200 °F) and also the temperature settings on the aquastat.
See AQUASTAT CONTROL FUNCTIONS
If the temperature is over 200 °F. then don't rush to blame either the expansion tank or the relief valve - they're doing their job and something else is wrong, probably the aquastat setting.
Next check the pressure-reducer water feed valve for the boiler. If the valve's diaphragm is damaged or leaky or if the valve is improperly set it may be feeding excessive water pressure to the boiler. Water hammer problems on the buiding water supply system can also foul-up the water feeder-pressure reducer.
See WATER FEEDER VALVE, HYDRONIC BOILER
Expansion tank has become heavy: as we will detail below, a waterlogged expansion tank will have gained weight.
Water leaks out at the expansion tank air charge valve on an internal-bladder Extrol-type heating system expansion tank.
How to Diagnose an Internal Bladder-Type Boiler Expansion Tank Needs an Increased Air Charge or Needs Replacement
Newer type heating system expansion tanks that use an internal bladder keep their water and air separated. These tanks should not need
frequent service but might need a small additional air charge every few years, as we explained
at WATERLOGGED EXPANSION TANK.
But there are at least three possible conditions for which to check an internal bladder type expansion tank for trouble.
Internal bladder type expansion tank air permeation loss: If the expansion tank has not been functioning properly but on removal you don't find that there is water trapped inside the tank bladder (as described above), check the tank's air pressure.
If the air pressure is below the factory-set pressure, or below the pressure that you previously set when the tank was installed, the tank may have lost its air charge over a period of several years. This tank may be perfectly serviceable if re-charged to proper setting. That's why we put this possibility first in the list.
Rapid air charge loss at the internal bladder expansion tank: When a bladder-type expansion tank has become waterlogged over a short interval it's most likely because the bladder has ruptured and the tank needs
If the bladder has burst you will probably find that a little depression of the pin in the tank's air valve will send water squirting out of the valve - a sure sign that the tank bladder is burst and the tank needs replacement.
Pinhole leak failures in an internal bladder expansion tank: But what if no water comes out of the tank. There may still be a pinhole leak of water into the air space that has begun to consume air space in the tank.
If the tank has been removed and drained through its point of connection to the heating system, and you then shake it or feel that it has gained weight over a new empty tank of the same size and model, and if you (possilbly) hear water or feel water sloshing around inside the tank bladder, then the pressure tank had a pinhole leak, water is in the bladder, and the tank needs replacement.
Additional Diagnostic Tests for a Waterlogged Heating Boiler Expansion Tank
The following tips are from Bell & Gossett's installation manual for pressure reducing valves and from their Airtrol Compression Tank [expansion tank] System Installation Manual . Our illustration at left shows the typical location of a traditional conventional, non-bladder-type heating boiler expansion tank, and is adapted from that B&G manual.
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Feel the temperature of the pressure reducer valve body: put your hand on the bell-shaped body of the pressure reducer or on the piping before and after the valve. If the metal is cold to the touch in those areas, then the valve is "passing water" into the heating boiler. This should not be happening if at the same time the heating boiler is already up to its operating temperature. Possible causes:
improper reducing valve pressure setting
a leak "downstream" from the pressure reducing valve, in the heating system piping or boiler or its controls
a faulty pressure reducing valve that is not closing when it should
If the relief valve on the heating boiler periodically leaks or discharges water, on a hydronic heating system (hot water boiler) the expansion tank may be waterlogged - losing the "air cushion" intended to accommodate thermal expansion.
A TPR valve may also leak or drip due to water hammer that occurs when circulators start or stop or some zone valve models open or close with a jolt.
See RELIEF VALVE LEAKS for a catalog of reasons why TPR valves may leak along with some important safety warnings.
Test for leaks at an internal-bladder type expansion tank: a tip for waterlogged expansion tanks that use an internal bladder is to pump a bit of air into the expansion tank, then put a dab of water (we recommend spit) on the opening of the air valve found on the bladder-type expansion tank. If the spit bubbles the valve is leaking.
Test for waterlogged conventional (bladderless) heating boiler expansion tank: shut off the boiler for about 30 minutes to let the system cool down. Write down the heating boiler pressure that you can read on its gauge. Now turn the boiler back on and watch the pressure gauge.
in 8-9 minutes of boiler operation (we mean that the burner or heat source is operating) the pressure gauge at the boiler reads a pressure within 10% of the spill pressure of the pressure relief valve on the boiler then the expansion tank is surely waterlogged and needs to be drained and re-charged with air. B&G's instruction manual for this procedure is their Airtrol Instruction Manual S10300.
See RELIEF VALVE, TP VALVE, BOILER
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 sytsem 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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