How to Drain a Heating Boiler Expansion Tank
How to service the expansion tank (compression tank) on hot water heating systems
EXPANSION TANK DRAIN & AIR RE-CHARGE - CONTENTS: how to replace the air lost from a steel, bladderless heating boiler compression / expansion tank. This procedure addresses expansion tank waterlogging & fixes relief valve leaks traced to that problem.
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Hot water heating system expansion tank / compression tank drain procedure to re-charge lost air:
This article describes the steps in draining of a steel compression or expansion tank used on hot water system heating boilers - hydronic heat. Draining water out of a waterlogged expansion tank is a common heating boiler service procedure used to replace a lost air charge. If this step is not taken when needed the waterlogged compression / expansion tank will cause boiler leaks and improper, even unsafe boiler operation.
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.
How to Drain & Re-Charge the Air Needed in a Heating Boiler Expansion Tank, step by step
Periodic draining of non-bladder boiler expansion tanks:
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.
Newer internal-bladder type expansion tanks don't need draining but might need an additional air charge, as we explain at EXPANSION TANK DIAGNOSIS
As we detail below in another photo, the green arrow indicates the expansion tank drain valve and the blue arrow marks the isolating valve that gets shut off first.
So if no one has ever drained the expansion tank, or not for years, chances are it's ready and waiting for that attention. Or as we've explained above, it's past due for draining.
How to determine when the expansion tank needs drainage
A traditional rule of thumb practiced by old heating service technicians (including me - DF) was simply to always drain all of the water out of the bladderless-expansion tank at every annual service.
If a call-back customer complaint led to a need to drain the expansion / compression tank more often than once a year we figured something else was wrong and needed to be diagnosed and fixed. At that point we'd check to see if the expansion tank were waterlogged. If not we'd look for a different problem such as LEAKS into the BOILER CAUSE RELIEF VALVE LEAKS
If the heating boiler expansion 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.
Watch out: don't go pushing the expansion tank all over the place. Just a tiny test push is sufficient. If you wiggle the tank around too much you're asking for a pipe leak. Also some expansion tanks are strapped tightly against the ceiling so you can't move them anyway.
Watch out: modern internal-bladder type expansion tanks such as AMTROL's EXTROLl®, the FILL-TROL® Diaphragm-Type Expansion Tank do not need periodic draining - those tanks use an internal bladder. If your Amtrol expansion tank is waterlogged there is a problem to correct. It might be a slow loss of the tank's air charge over several years, fixed simply by adding air. IF the tank has a pinhole leak or a ruptured bladder, the tank probably needs replacement.
See EXPANSION TANK DIAGNOSIS if you missed that step
see EXPANSION TANK PRESSURE ADJUSTMENT if you need to adjust the pressure in a bladder-type expansion tank.
So Now, Finally, Let's Drain that Heating Boiler Expansion Tank
Many expansion tanks use a special drain valve that permits
air to flow into the tank as water is drained out.
Turn off the heating boiler at its electrical service switch.
Identify the expansion tank feed pipe. There will usually
be just one pipe leading to the expansion tank.
That pipe will be found coming from the boiler or perhaps a nearby heating
water pipe to the tank to permit the tank 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. In our image at left, the red arrow points to the feed pipe.
Turn off the isolating valve that should be located on that hot water feed pipe between the expansion tank and the boiler or heating line.
This temporarily isolates the expansion tank from the rest of the hot water heating system. If you don't do this and you start draining the expansion tank you may end up draining water from your heating piping too - not what you want. We only want to drain the water that is inside the expansion tank. In our photo at left the blue arrow points to the isolating valve.
Technical note: an isolating valve often is omitted if the expansion tank is an internal bladder Extrol type tank since those don't need to be drained.
Connect a garden hose to the expansion tank drain valve and position the hose so that it drains into a floor drain or convenient slop sink. In our photo the green arrow points to the expansion tank drain valve. [Click any image to see an enlarged, detailed version.]
Open the expansion tank drain valve to allow
water from the expansion tank to drain out
air from the room to enter the expansion tank
Watch out: this hose idea is convenient and is recommended by some folks who lack experience with expansion tanks. But the hose only works if a special air-admittance valve is installed on your expansion tank. If your tank just has a regular boiler drain valve installed, the hose thing will be very difficult to work - in that case plan some trips with a bucket held under the opened tank drain valve instead.
The expansion tanks we show with our arrows in the photos above had a simple boiler drain. We open the drain and let it spill into a bucket. That lets air bubble back up into the expansion tank - something that would take forever if we had a hose hooked up here. This can be a tedious process. Some expansion tank drain valves include an extra feature, an air inlet that allows air to enter the expansion tank through an air inlet on the drain valve at the same time that water is draining out of the expansion tank.
Watch out, the expansion tank water could be hot, and is usually a bit smelly and dirty.
Let all of the water out of the expansion tank. You could get away with a bit less than all, but all is good. This lets us start with an expansion tank full of air. Don't worry, water from the heating system will be back in there in a jiffy.
Close the expansion tank drain valve.
Open the isolating valve between the expansion tank and the heating boiler or heating boiler piping (photo below left). You will doubtless hear some water entering the expansion tank.
That's because the water in the heating system is sitting there somewhere between 12 and 28 psi and your expansion tank was full of air at "0" psi. When water stops running into the expansion tank, don't worry, it won't be waterlogged again. Rather it will contain some water, and some air, both at whatever pressure is that of your heating system at the moment.
Our photo above shows an expansion tank isolating valve. Below it and to its left you can see the top of a B&G water feed valve that sends water into the heating boiler.
Check for leaks. Make sure that the isolating valve (that no one had touched for a few years until you got to it) and the expansion tank drain valve (that no one has touched for a few years before now) are not dripping.
If you see a tiny drip around the valve stem of either valve, you can usually stop that by gently tightening that large packing nut that surrounds the valve stem. GENTLY. Don't over-tighten nor break anything.
Turn the boiler back on.
Check for leaks one last time when the boiler has heated up and the system is at full operating pressure. You should be ok.
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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.)
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