Heating Boiler Expansion Tank Repair

Troubleshoot, drain, or service & repair the expansion tank (compression tank)
on hot water heating systems
on water heater tanks

What is a Heating Boiler Expansion Tank & How Do Expansion Tanks Work

Heating boiler expansion tanks are metal tanks of varying sizes that are installed to absorb the initial pressure increase that occurs when the heating boiler system warms up.

Air molecules entrained in water inside the heating boiler itself as well as in the heating system piping, baseboards, or radiators, expand and thus cause an initial pressure increase in the heating system.

That pressure is increase is absorbed by allowing water from the heating system to squash or compress a reservoir of air inside the expansion tank - which is why some heating experts call our expansion tank a "compression tank".

Photo below: a closer look at a ceiling mounted hot water heating boiler expansion tank.

This gray expansion tank is in a Two Harbors Minnesota home and served for more than half a century (when it was drained periodically at boiler service time) until it was finally replaced by a much smaller modern expansion vessel.

Inside of a working expansion tank is a reserve space of air. In fact, when the heating boiler and system are cool, the traditional bladderless boiler expansion tank will contain mostly air.

Or in newer smaller internal bladder type expansion/compression tanks an internal rubber bladder keeps air separated from the heating system's water - avoiding rapid loss of air by absorption into the heating systems' water.

As the heating system warms up and as air entrained in the water raises system pressure, the increased pressure forces some of the heating system water into the expansion tank, squashing its air-charge into a smaller volume, thus permitting the tank to absorb the initial increase in system pressure.

Technical note: the water and air inside the expansion tank will always share the same pressure, but since water is not (very) compressible and air is, the increase in pressure will squeeze the air down to a smaller volume.

That's why some heating system water can pass into the expansion tank.

If heating system pressures and/or temperatures exceed normal (for a residential heating boiler that would be a pressure above 30 psi or a temperature much above 200 °F) then the expansion tank is dismissed from duty and the pressure/temperature relief valve on the boiler will open to provide emergency relief.

What Happens if the Expansion / Compression Tank is Not Working?

The loss of air cushion in the expansion / compression tank means that at each heating-on cycle the system pressure can be too high, causing the temperature/pressure relief valve to spill.

See RELIEF VALVE LEAKS for a catalog of causes of TPR valve leaks.

Watch out: if you see water dripping or squirting out of the temperature/pressure relief valve on any vessel, such as a hydronic heating boiler, the situation is unsafe.

The valve may be doing its job of relieving excessive pressure or temperature, but there is a risk that the valve becomes clogged by mineral deposits or crud left by the escaping water. A clogged TPR valve is no longer protecting the system from an explosion

See details at BLEVE EXPLOSIONS

A second problem with older expansion/compression tanks - those that do not use an internal bladder to keep water and air separate, is the loss of air out of the compression tank and into the heating system water. We won't like the result.

Why do we care about keeping air out of the heating system water?

Air in the hot water heating system can cause loss of heat: air in baseboards radiators keeps hot water out - they don't get hot. And air in hot water heating system piping can simply keep hot water from ever reaching the radiators or baseboards.

Details are at AIR-BOUND HEATING SYSTEMS

See HOT WATER PRESSURE EXPANSION RATE if you're feeling curious about the physics involved in the pressure increase as we heat up water,

How & Why do Expansion Tanks Get Waterlogged?

Loss of Air Charge in a Bladderless Steel Traditional Heating System Expansion Tank

In older hot water heating boiler expansion tanks that do not have an internal bladder to keep air and water separate, over time air in the expansion tank may become absorbed into the heating water.

Air may also find its way via the heating water to other air bleeders or vents in the system.

As air is lost and water just stays in the expansion tank, the space and pressure cushion provided by air in the expansion tank gets too small, or is lost entirely.

There is no more air cushion to absorb initial pressure increases in the heating system during each heating boiler on-cycle.

Loss of Air Charge in an Internal-Bladder-Type Heating System Expansion Tank

In a modern internal-bladder type expansion tank use on heating systems it is not usually necessary to add air nor to change the tank pressure. There are however several sources of loss of the air charge and causes of water logging in an internal bladder-type expansion tank:

• Expansion tank bladder rupture:
  
  Occasionally the internal bladder of these tanks an rupture, leading rather quickly, perhaps in less than a single heating season, to a water-logged expansion tank tank. In that case the symptom may be observing that heating system pressure relief valve may spill or water may be found leaking out at the expansion tank's air pressure adjustment valve.
  
  See this field report: RELIEF VALVE LEAK DUE TO EXPANSION TANK that traces a leaky relief valve to a failed expansion vessel on a Cosmogas boiler.
• Expansion tank pinhole leak:
  
  Occasionally a pinhole leak develops in the internal bladder of an expansion tank. We first received reports of this subtle failure from readers reporting on waterlogged water supply system pressure tanks.
  
  The pinhole leak is almost impossible to observe even if you cut apart the tank to remove and inspect the rubber tank bladder or diaphragm.
  
  Rather the symptom shows up as a slow loss of tank volume that can result in cyclic leaking at the heating systems TPR valve.
  
  Since other heating system problems such as water hammer can also cause sporadic TPR valve leaks or drips this defect can be hard to track down.
  
  On water supply system tanks we may observe an increase in the air pressure of the tank above its initial charge, measured at its pressure valve, even when there is no water pressure on the system.
  
  That's because water has leaked into the air space, reducing the available volume for air and thus permanently increasing the pressure therein. Remember from your gas laws that P1V1 = P2V2? If we reduce the air volume the pressure must increase.
• Expansion tank membrane permeability: 
  
  Reader NJT, Holohan, and several other sources cited atReferences or Citations note that a bladder-type hydronic heating expansion tank may lose air pressure at about 1 psi per year as air passes through the tank membrane and into the heating system's hot water.
  
  Considering that heating system pressures and temperatures are cycling constantly during the heating season, one may speculate that the unidirectional movement of air from the tank through its bladder and into the heating water would have to be occurring when system pressures are highest.

Boiler Expansion Tank Installation & Operation Manuals, Guides, Standards

Illustration: an example guide showing the location of two types of expansion tanks, excerpted from Amtrol's Extrol instructions from 2014, cited in detail below.

• AAtanks, the company's NLA Series ASME expansion tanks, - original source http://www.aatanks.com/expansion-tanks.php

• Amtrol EXTROL® hydro pneumatic expansion tanks, (2004), Website: www.amtrol.com, - retrieved 20 March 2015
  
  AMTROL Corporate Headquarters 1400 Division Road, West Warwick, RI USA 02893 Telephone: 401-884-6300
  
  AMTROL Canada, Ltd. 275 Shoemaker Street, Kitchener, Ontario N2E 3B3 Telephone: 519-478-1138
  
  AMTROL Asia Pacific Ltd. 89 Owen Road, Singapore 218902 Telephone: 65-6294 4611
• Amtrol ASME Commercial Expansion Tanks, Amtrol, 1400 Division Road, West Warwick RI, 02893, USA, TEl: 800-426-8765, ASME-rated expansion tanks, - retrieved 19 March 2015, original source: http://www.amtrol.com/media/documents/extrol_asme/
  MC2682_01_14_EXTROL_ASME_Brochure.pdf
• Amtrol Corporation, INSTALLATION & OPERATION INSTRUCTIONS, EXTROL (R) HYDRONIC DIAPHRAGM EXPANSION TANKS IN-LINE & SX-SERIES [PDF] (06/15), Amtrol Corporation, 1400 Division Road, West Warwick RI 02983, Tel: 401-884-6300, Website: www.amtrol.com, retrieved 2016/05/10, original source http://www.amtrol.com/media/documents/extrol/9015183_06_15_EXTROL_IO.pdf
• Amtrol Corporation, "Extrol® Hydronic Expansion Tanks for Non-Potable Water Installation & Operation Instructions, Models EX-15 through EX90 Models SX-30V through SX-160V", Amtrol corporation
• Amtrol EXTROL PRO EXPANSTION TANK INSTALLATION & OPERATION INSTRUCTIONS [PDF] (at Amtrol's website) for Models EX-15PRO, EX-30PRO, EX-60PRO, local backup copy saved as Amtrol-Extrol-Pro-Instruction-Manual.pdf
• AMTROL's EXTROL®, the FILL-TROL® REPAIR GUIDE - [Web article, in-depth w/ more citations & documents for Amtrol Extrol expansion tanks used on heating systems]
• AMTROL, EXTROL® EXPANSION TANKS INSTALLATION & OPERATION INSTRUCTIONS EX-SX [PDF] (2016) Models EX-15 - EX-90, Models SX-30V - SC-160V, Extrol Corporation, 1400 Division Road, West Warwick RI 02983, Tel: 401-884-6300, Website: www.amtrol.com, retrieved 2018/08/03, original source: https://www.amtrol.com/wp-content/uploads/2017/04/9015-941-10_16-Extrol-IO-w_InSight-Cap.pdf
• AMTROL EXTROL® EXPANSION TANKS IO INSTRUCTIONS EX-15-90, SX 30V-160VSeries [PDF] (2014) Models EX-15 - EX-90, Models SX-30V - SC-160V,
• Amtrol EXTROL EX-SERIES EXPANSION TANK INSTALLATION INSTRUCTIONS [PDF]
• AMTROL EXTROL® EXPANSION TANKS IO MANUAL SX-Series [PDF] (2003) In-Line & SX-Series EXTROL Expansion Tank instructions, retrieved 2018/08/03, original source: https://www.sunsourceproducts.com/library/documents/Amtrol-Extrol-InstallOperation.pdf
• AMTROL FILL-TROL COMBINATION EXPANSION TANK & AUTO FILL VALVE INSTALLATION & OPERATION INSTRUCTIONS [PDF] (at Amtrol's website) - local backup copy saved as Amtrol-Fill-Trol-Manual.pdf
• AMTROL FILL-TROL® EXPANSTION TANK IO MANUAL FT-109 - FT111 [PDF] (2014) Models FT-109 - FT-111, Amtrol, Inc., retrieved 2018/08/03, original source: http://www.amtrol.com/wp-content/uploads/2017/05/9015026_06_14_FillTrol_IO-1.pdf
  
  Excerpts from Amtrol's Fill-Trol Instructions: For Use In Closed Hydronic Systems Only
  
  FILL-TROL® is a combination of an EXTROL® expansion tank and a specially designed automatic pressure reducing valve. Together, they manage water expansion to provide satisfactory heating system performance.
  
  The EXTROL is pre-charged at the factory to 12 PSIG and is ready to install in most residential and light commercial systems without adjustment.
  
  The valve is designed to fill the system with water and automatically shut off the flow when it reaches the precharge pressure in the tank. It will also restore the system to 12 PSIG if the pressure drops below that point.
  
  The precharge may be changed to set the desired fill pressure. Do not install on domestic water systems, or in open heating systems. Corrosion and tank failure may result.
  
  Use a Therm-X-Trol® or Therm-X-Span® for domestic water systems.
  
  Use a Radiant EXTROL® for radiant heating systems where air elimination equipment or barrier tubing is not used.
• Amtrol EXTROL SIZING GUIDE For Pressurization and Expansion Control of Low Temperature Water Systems [PDF] (2019) Amtrol, Worthington Industries, 1400 Division Road, West Warwick, RI USA 02893 T: 800.426.8765 www.amtrol.com - retrieved 2023/08/10, original source: amtrol [dot] com/wp-content/uploads/2019/06/9003-036-03_19-EXTROL-Sizing [dot] pdf
  
  Excerpt:
  The sizing methods, such as manufacturers’ selection tables and short cuts nomographs, which system designers may have used in the past, are at best, rule of thumb approximations only.
  
  In many cases, the designer has used these methods to arrive at a general size range and then added their own safety margin to select a tank of larger size than originally calculated. While this practice has resulted in tanks sufficiently over-sized to include an adequate safety margin, it does not meet the critical sizing requirements that must be used in modern system design.
  
  This data has been excerpted from the
  
  AMTROL ENGINEERING HANBDBOOK [PDF] (1977, reprinted 2011) Chapter Two, Section B, “Hydro-pneumatics in Hot Water Heating Systems”. The complete handbook covers the application of hydro-pneumatics in heating, plumbing, cooling, water supply and commercial water heating systems.
• Amtrol, Extrol® HYDRONIC EXPANSION TANKS FOR NON-POTABLE WATER INSTALLATION & OPERATION INSTRUCTIONS [PDF] Amtrol Corporation 1400 Division Road, West Warwick, RI USA 02893 T: 800.426.8765 F: 800.293.1519 www.amtrol.com , retrieved 2016/06/06, original source http://www.amtrol.com/media/documents/extrol/9015183_06_15_EXTROL_IO.pdf
  

• ARMSTRONG L-SERIES BLADDER EXPANSION TANK OPERATING INSTRUCTIONS [PDF] (2003), Armstrong Pumps Inc., 93 East Ave., North Notawanda NY USA, 14120, Tel: 716-693-8813, Website: www.armstrongpumps.com
  
  or S.A. Armstrong Ltd., 23 Bertrand Ave. Toronto Ontario Canada M1L 2P3, Tel: 416-755-2291
  
  or Armstrong Pumps Ltd., Peartree Road, Stanway, Colchester, Essex United Kingdom CO3 0LP Tel: +44 (0) 1206 579491
  
  This tank uses a replacable internal tank bladder.
• ARMSTRONG L-SERIES BLADDER EXPANSION TANK INSTALLATION MANUAL [PDF] (2000) top outlet, installation & operation instructions, includes instructions for changing the expansion tank's internal bladder. Armstrong fluid technology.com Op. Cit.
  

• 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 main.
  
  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.
• Bell & Gossett AIRTROL® SYSTEM INSTALLATION, OPERATING AND SERVICE INSTRUCTIONS [PDF] Bell & Gossett, 8200 N. Austin Ave., Morton Grove IL 60053, USA - Tel 847 966-3700 Fax 847 965-8379. Retrieved 8/9/2012, original source B&G, [copy on file as Airtrol_Installation_Manual.pdf linked in this reference]
• Bell & Gossett, BELL & GOSSETT PRESSURIZED EXPANSION TANKS (ASME) INSTALLATION, OPERATION, & SERVIVE INSTRUCTIONS [PDF] Instruction Manual A01500, [PDF] retrieved 2016/06/06, original source: http://www.azurecorp.com.tw/download/ITT/Expansion%20Tank-IOM.pdf
• Bell & Gossett B&G PRESSURIZED EXPANSION TANKS (ASME) Series B, D, B-LA INSTRUCTION MANUAL [PDF] (2012) Bell & Gossett, Xylem Inc. 8200 N. Austin Avenue Morton Grove, Illinois 60053 USA Tel: (847) 966-3700 Website: www.xyleminc.com/brands/bellgossett

• Extrol Corporation, INSTALLATION & OPERATION INSTRUCTIONS, EXTROL (R) HYDRONIC DIAPHRAGM EXPANSION TANKS IN-LINE & SX-SERIES [PDF] (2011) Extrol Corporation, 1400 Division Road, West Warwick RI 02983, Tel: 401-884-6300, Website: www.amtrol.com, retrieved 3/3/2014, original source http://www.amtrol.com/media/documents/extrol/9015183_3_11_EXTROL_IO.pdf
• ExpanFlex, by Calefactio, 1120 Blvd Michele-Bohec Blainville, Quebec Canada J7C 5N5 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/

• HONEYWELL EXPANSION TANK TK300-XPS MANUAL [PDF] (2006), Honeywell International, Inc., 1985 Douglas Drive North, Golden Valley, MN 55422 USA
  or Honeywell Limited-Honeywell LimitÈe, 35 Dynamic Drive, Toronto, Ontario Canada M1V 4Z9
  Website: customer.honeywell.com

• TACO EXPANSION TANK MANUAL CA-SERIES [PDF] (2014), Taco, Inc., 1160 Cranston St., Cranston RI 02920 USA, Tel: 401-942-2800,
  or TACO (Canada), Ltd., 6180 Ordan Drive, Mississauga, Ontario Canada L5T 2B3 Tel: 905-564-9422
• TACO EXPANSION TANK BAG REPLACEMENT INSTRUCTIONS CA-EXP Series [PDF[ (2015) Op. Cit. , retrieved 2018/08/03, original source: http://www.taco-hvac.com/uploads/FileLibrary/402-013.pdf
  
  Most Taco Expansion Tanks are available with either bladders or diaphragms so that water and air are permanently separated.
• TACO EXPANSION TANK MANUAL CBX [PDF] (2013), Op. Cit., retrieved 2018/08/03, original source: http://www.taco-hvac.com/uploads/FileLibrary/402-020.pdf
• TACO EXPANAION TANK CBX BLADDER REPLACEMENT INSTRUCTIONS [PDF] (2004), Op. Cit., retrieved 2018/08/03, original source: http://www.taco-hvac.com/uploads/FileLibrary/402-090CBXExTankRep.pdf
  
  How to repalce the internal bladder in a Taco CBX expansion tank.
• TACO EXPANSION TANK CW INSTRUCTION SHEET [PDF] (2010), Op. Cit., retrieved 2018/08/03, original source: http://www.taco-hvac.com/uploads/FileLibrary/402-093.pdf
• TACO EXPANSION TANK CW EXP BLADDER REPLACEMENT INSTRUCTIONS [PDF] (2001), Op. Cit., retrieved 2018/08/03, original source: http://www.taco-hvac.com/uploads/FileLibrary/402-025.pdf
  
  How to repace the bladder in a Taco CW EXP bladder type expansion tank
  
• TACO EXPANSION TANK CX INSTRUCTIONS [PDF] (2013), Op. Cit., retrieved 2018/08/03, original source: http://www.taco-hvac.com/uploads/FileLibrary/402-015.pdf
• TACO PS-Series EXPANSION TANKS are also available in plain-steel construction in a range of sizes.

• WATER WORKER EXPANSTION TANK INSTALLATION INSTRUCTIONS [PDF], Water Worker, Tel: 401-884-6300, Website: www.waterworkerdiy.com, sold at Lowes building supply stores, retrieved 2018/08/03, original source: http://pdf.lowes.com/installationguides/642031654595_install.pdf
  
• WATTS ET SERIES EXPANSION TANK INSTALLATION INSTRUCTIONS [PDF] (2008) Series ET, Models ET-15 - ET-160 (1/2" NPT for use with an air separator) & Model ET-90 (3/4" NPT) Watts Water Safety & Flow Control Products, 815 Chestnut St., No. Andover, MA 01845-6098 USA, Website: www.watts.com or Watts, 5435 North Service Rd., Burlington, ONT. L7L 5H7 Canada, Website: www.wattscanada.ca
  
  Excerpt:
  Watts ET Series may be installed in a tee or any other suitable tapping in the heating system and can be installed in a vertical or horizontal position.
  
  CAUTION: The tank must be properly supported in horizontal applications. Max. temperature 210°F (99ºC),Max. working pressure 60 psi
• WATTS ET ETX EXPANSION TANK INSTALLATION INSTRUCTIONS [PDF] (2008) Series ETX, ETSX hot water heating system expansion tanks, Op. Cit.
  Excerpt:
  Series ETX may be installed in a tee or any other suitable tapping in the heating system and can be installed in a vertical or horizontal position. Series ETSX are floor models.
• See additional citations atReferences or Citations
• Also see MANUALS & PARTS GUIDES - HVAC - home for manuals for HVAC equipment, all brands.

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.