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How to Re-charge a Hot Water Boiler Expansion Tank


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Re-charging a Suspended Horizontal Steel Expansion Tank
How to Re-charge a Hot Water Boiler Expansion Tank

Older style steel expansion tanks hang high above the boiler


Steel Tank Has Excess Water and Inadequate Air

In older boiler systems the steel expansion tank may be found in the attic or suspended between joists in the basement. The expansion tank must be properly charged with air to prevent water in the system from boiling and exceeding the desired 12 psi of pressure.


  • Pressure relief valve on boiler is spurting water
  • Abnormally high pressure reading on gage (above 20 psi)
  • Expansion tank is hot to the touch from bottom to top
Possible Causes
  • Too little air and too much water in the expansion tank
Possible Repairs
  • Touch the bottom of tank. Now touch the top of the tank. The bottom must feel hotter than the top. If not it is filled with water and must be bled or drained of excess water.
  • Turn off boiler, close water shutoff valve and let tank and system cool.
  • The expansion tank combination valve will release water and let in air. Attach a hose to the combination valve and drain about 3 gallons of water.
  • If there is no combination valve on an older tank, then shut off the valve between tank and boiler and totally drain expansion tank.
  • Open water supply valves and let system fill back up.
  • Turn boiler back on (do this after water supply is back on)
  • System will recharge. Let run for 1-2 hours and recheck.

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