How to Recharge the Expansion Tank on a Hot Water Boiler

Son watching father check hot water tank with digital tablet
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  • Working Time: 2 hrs
  • Total Time: 6 hrs
  • Skill Level: Intermediate

In many homes, heating is provided not by a forced-air furnace, but by a boiler that heats water to circulate it through piping to radiators throughout the house. A key component of a hot water boiler system is an expansion tank attached to the boiler or the nearby heating pipes. The expansion tank serves to provide space for water and air within the boiler system to expand and contract without damaging pipes or valves. If the expansion tank is missing or not operating properly, pressure in the system may cause the boiler's pressure relief valve to vent water. Or, air bubbles being released by the heating water may gather somewhere else in the system, causing a blockage that stops the flow of hot water.

Note: An expansion tank is not found in steam boiler systems, which create heat by circulating hot vapor rather than hot water through the pipes and radiators. Steam boilers are simpler systems that lack several components found in hot water boiler systems—such as the circulating pump, expansion tank, and regulators for water pressure and temperature. If your system lacks an expansion tank and circulating pump, you probably have a steam boiler system.

How the Expansion Tank Functions

Air inside a hot water boiler (hydronic) heating system is both necessary and inevitable. The presence of air assists in the flow of hot water through the piping to the radiators. Water naturally expands and contracts as it heats and cools, and air within the system provides a cushion that allows for expansion without damaging pipes or allowing the pressure relief valve (PRV) to expel water from the system. But this air must be located in the proper location within the system. If air builds up in undesired locations, a condition called hydronic airlock can develop, which essentially stops the convective flow of hot water and renders all or parts of your radiator system inoperable.

The expansion tank provides a designated location for air in the system to dwell. It is either a simple steel cylindrical tank located above the boiler or, in newer systems, a flattened round tank with a more sophisticated operation. Both types are normally fairly trouble-free devices, but they can require servicing if the tank fills with too much water or if it loses air pressure.

Two Types of Expansion Tanks

Depending on the age and style of hot water boiler you have, the expansion tank can take one of two forms.

Steel Tanks

A simple cylindrical expansion tank is found in many older installations. It is a very basic gray steel tank located somewhere above the boiler, oriented with the cylinder running horizontally. Most commonly, it will be found in the immediate vicinity of the boiler, overhead. In a few cases, it may actually be found in an attic. Wherever it is located, a single pipe runs from the bottom of the tank to the boiler or network of heating pipes; this pipe usually has an isolation valve that can be closed to shut off the tank from the system when you are servicing it. The tank usually has an additional valve to allow for draining water (this valve is located on the bottom of the tank).

In this kind of expansion tank, the air chamber inside is in direct contact with the water. To operate efficiently, these tanks should have about one-third of their internal space filled with water, with the top two-thirds filled with air. Because water can absorb air, it is possible for steel tanks to lose the proper water/air ratio, and when this happens, the tank needs to be "recharged" to restore the proper ratio. Recharging this kind of tank involves simply closing the isolation valve between the expansion tank and boiler, draining out water from the expansion tank, then opening the connection between boiler and tank to allow it to slightly refill with water.

Determining if a steel-tank needs to be drained can be difficult since you can't look inside the tank. If your boiler's pressure relief valve has been venting water, it is a good sign that the expansion tank is not functioning correctly. You may be able to slightly lift up on the tank to judge its weight. If it seems extremely heavy, it probably contains too much water. Conversely, if the bottom of the tank is not slightly warm to the touch, it indicates there is no water at all in the tank.

Diaphragm-Type Tanks

In newer installations, the expansion tank uses a diaphragm or bladder design. These tanks are usually somewhat smaller than a steel tank, with rounded ends and enameled surfaces. In these tanks, a diaphragm or bladder separates the air layer from the water layer, so you never have a situation where water absorbs the air in the tank. In most designs, the water is contained inside a flexible bladder, surrounded by a cushion of air. These tanks are designed to hold pressurized air at all times, so they are constructed in a manner designed for this. In addition to the pipe connection leading to the boiler, these tanks will have a small nipple valve that can be used to attach an air gauge or an air compressor to add more air. The tanks will also have a drain spigot for expelling water from the tank.

Problems with these tanks usually involve losing air pressure, and the "recharging" process is exactly that—pumping additional air into the tank until the optimal air pressure is restored.

What You'll Need

Equipment / Tools

For Steel Expansion Tanks

  • Bucket to catch water
  • Garden hose (optional)

For Diaphragm-Style Tanks

  • Pressure gauge
  • Air compressor or bicycle tire pump
  • Bucket
  • Garden hose


How to Recharge a Traditional Steel Expansion Tank

  1. Shut off the Boiler

    It is best if the heating system is shut off while you work on the expansion tank. This can be done by simply turning the thermostat down to a very low setting, which will prevent the boiler burners from firing while you perform this maintenance. Or, your boiler may have a shut-off switch mounted on it to turn off the burners. It takes only a few minutes to service a traditional steel expansion tank, so your boiler will not need to be shut off for very long.

    With the boiler turned off, close the isolation valve that separates the expansion tank from the boiler piping itself. Sometimes this will be a pipe running directly to the boiler, or it may be a pipe connected to the network of heating pipes running to the radiators. Either way, it will be the only pipe running directly to the expansion tank.

  2. Drain the Expansion Tank

    Hold a bucket under the water drain valve on the expansion tank; or attach a garden hose to the threaded fitting and extend the other end of the hose to a bucket or floor drain.

    Open the drain valve and allow all water to drain out of the expansion tank. Once the tank is empty, tightly close the drain valve and detach the hose.

  3. Open the Isolation Valve

    Now open the valve separating the boiler from the expansion tank. Some water should naturally rise up into the expansion tank from the boiler system. Your expansion tank is now restored to proper function.


    Standard steel tanks should be inspected every few months and drained whenever they waterlogged.

How to Recharge a Diaphragm-Style Expansion Tank

Diaphragm-type expansion tanks are increasingly taking the place of traditional plain steel tanks. They are less likely to require recharging, but servicing them is a bit more complicated. They also have a limited lifespan; ten years or so is about average, but some have been known to last 25 years or more.

  1. Shut off the Boiler

    It is best if the heating system is shut off while you work on the expansion tank. This can be done by simply turning the thermostat down to a very low setting, which will prevent the boiler burners from firing while you perform this maintenance. Some boilers may have a switch mounted on the side that turns off the boiler's burner.

    With the boiler turned off, close the isolation valve that separates the expansion tank from the boiler or water pipes.

    Allow the water in the expansion tank to cool completely. Don't continue until the tank is quite cool to the touch.

  2. Drain the Tank

    Place a bucket below the tank's drain valve, or attach a garden hose to the valve and run the other end of the hose to a floor drain or bucket.

    Open the drain valve fully and allow the water to run out of the expansion tank completely. The bladder or diaphragm inside the tank should now be empty of water, leaving only the cushion of air around it.

    Close the drain valve fully once no more water drains out of the tank.

  3. Check the Pressure

    Attach tire air-pressure gauge to the air-fitting connection on the expansion tank. The location of this fitting may vary from manufacturer to manufacturer; you may need to check the tank manual to identify it.

    Read the pressure setting. Most expansion tanks require a PSI reading of about 12 PSI; check your manual for the precise rating for your expansion tank. If the pressure is less than 12 PSI, you will need to add some air.

  4. Add Air

    If the pressure setting reads less than 12 PSI (it probably will since you drained water from the tank), attach an air compressor or hand air pump to the air-fitting connection, and pump air into the expansion tank.

    If your pump has a pressure gauge, add air until the pressure is 12 PSI. If your pump has no gauge, you will need to periodically disconnect the pump and check using an air gauge. Usually, about 10 seconds of compressor pumping will suffice to pressurize the tank adequately. If you overfill the tank, you can bleed off a small amount of air until reaching the optimal 12 PSI.

  5. Open the Isolation Valve and Turn On the Boiler

    After establishing the proper air pressure, open the isolation valve on the pipe leading to the expansion tank. Water from the system will now flow into the diaphragm or bladder inside the tank, further pressurizing the air surrounding it.

    Turn the boiler back on and allow the system to run normally for several hours. Check the pressure at the expansion tank again, and make sure it is within the recommended range.


A diaphragm-type expansion tank that refuses to hold its pressure or which fills with excess water may have a bad diaphragm or bladder. Such a tank will need to be replaced. A diaphragm expansion tank that is more than 10 years old should be regarded as nearing the end of its life.