Guide to Heating Boiler Expansion Tanks
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Here we discuss Heating System Boiler Expansion Tanks, Waterlogged tanks, How to Drain an Expansion Tank and we provide a heating system expsansion tan k Troubleshooting & Repair Guide.

This website answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs. The photo at page top shows a modern Extrol(R) bladder-type heating system expansion tank. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

Because some controls are used in common on hot water heat, hot air heat, and steam boilers, readers should see these other articles: see BOILER CONTROLS & SWITCHES, and also see BOILER COMPONENTS & PARTS for a detailed list of heating boiler controls, other heating system components, parts such as circulator pumps & draft regulators. If your building uses warm air heat, see  FURNACE CONTROLS & SWITCHES. If your building uses steam heat see STEAM HEATING SYSTEMS.

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Heating Boiler Expansion Tanks

Heating boiler expansion tanks are installed to absorb the initial pressure that occurs when the heating system warms up. Air molecules entrained in water inside the heatin boiler itself as well as in the heating system piping, baseboards, or radiators, expands and thus cause an initial pressure increase in the heating system.

If we do not provide some way 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 the boiler.

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.

When the heating boiler and system are cool, the expansion tank will contain mostly air. As the heating system warms up and air entrained in the water raises system pressure, the increased pressure forces some of the heating system water into the expansion tank, thus permitting the tank to absorb the initial increase in system pressure.

Naturally if the heating system is not operting properly and the boiler temperature or pressure continue to rise above acceptable levels, and since the expansion tank is a fixed volume which cannot absorb an unlimited heating system pressure increase, eventually the relief valve should also open to spill excess pressure.

Signs of Trouble with a Heating Boiler Expansion Tank

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.

Expansion tanks on hot water heating systems can be divided roughly into two groups: older type bladderless heating system expansion tanks and newer type bladder-type expansion tanks such as those sold by Extrol(R).

Some heating system expansion tanks need periodic draining

Older heating system expansion tanks such as the one shown in this photo need periodic service: because air in the expansion tank can become absorbed into the heating water over time, eventually the expansion tank can become waterlogged. A waterlogged heating system expansion tank will be unable to absorb the initial heating system pressure increase - it stops working. This can lead to spillage at the boiler relief valve each time the heating system warms up. Spillage at the 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. We can determine when the expansion tank needs drainage: if the tank is heavy (try pushing it up or tapping on it) or if the relief valve is leaking, we probably need to drain the tank and let air return to it. Many expansion tanks use a special drain valve that permits air to flow into the tank as water is drained out. We drain the expansion tank simply using a garden hose connected to its drain valve. But watch out, the draining water could be hot.

Newer expansion tanks using an internal bladder should not need service

Newer type heating system expansion tanks that use an internal bladder keep their water and air separated. These tanks should not need service. If a bladder-type expansion tank has become waterlogged it's because the bladder has ruptured and the tank needs repair or replacement.

Attic Expansion Tanks for Heating Boilers

Attic expansion tanks and pressure relief systems for boilers: Don't confuse an old heating system attic-mounted expansion tank like the one shown here for a water tank storage tank. The heating system expansion tank will be connected to the heating system radiators or basement boiler and may have a simple overflow pipe to permit excessive water (or system pressure) to spill outside. This attic expansion tank may have been connected to a drain that spilled outside of the building in case the tank became over-full. You can also see a sight glass on the front of this tank, allowing the service person to see its conation. Don't confuse this little heating system expansion tank with a water storage tank, nor with the larger range boiler tanks discussed here. Heating systems with this equipment installed may not have a modern pressure and temperature relief valve. Certainly in the original design the tank was put in the attic so that heating water could rise to the upper floors of the building by pressure within the heating system, but if pressure got dangerously high, heating boiler water would just spill up into the attic and thence to outside. Systems which rely on a remote attic-mounted expansion tank are less safe since than a boiler that has a pressure and temperature relief valve mounted right on or at the boiler. It's an easy fix, usually, to just add a relief valve on or at the boiler.

Cisterns or open overlfow tanks in basements or attics are an open-type water storage reservoir found indoors, and are discussed further at CISTERNS. But at left, this little "box" found in an attic was more likely a home-made expansion tank, or a toilet flush tank. We'd need to look further to know just what it was. This system was in the attic of the Justin Morrill Smith historic home in Vermont.

Basement Expansion Tanks for Heating Boilers

Our client is pointing to a do-it-yourself insulating job on a water heater. But that reddish-brown horizontal tank over his head is an expansion tank for the hydronic heating boiler in this building. This is not a water storage tank, it's not a range boiler, it's an expansion tank. There will usually be just one pipe leading to this tank, coming from the boiler or perhaps a nearby heating water pipe to the tank to permit it to accept the expansion of water in the heating system (or expanding air entrained in the heating system's water) as the boiler gets hot. Otherwise the boiler relief valve might open and spill every time the boiler went through a heating cycle. You'll also find a drain on the bottom of this boiler, used by the heating service technician. So unlike the attic expansion tank, this basement expansion tank is expected to be on a heating boiler that also has a pressure and temperature relief valve.

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• 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 Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
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• Installation Guide for Residential Hydronic Heating Systems
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• 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.)
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