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How to Re-charge a Hot Water Boiler Expansion Tank


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Re-charging a Diaphragm Style Expansion Tank
diaphragm boiler expansion tank

Diaphragm expansion tank for a hot water boiler

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Diaphragm Expansion Tank Has Excess Water and Inadequate Air

In newer boiler systems you will find a smaller expansion tank located near the boiler. The diaphragm type expansion tank is easy to maintain. In fact, if it is a pre-charged tank, it is maintenance free. This tutorial assumes it is not a pre-charged unit but has an air recharge valve at the bottom. The diaphragm tank has a diaphragm separating the upper half from the lower half of the tank.

Unlike the old style horizontal steel tanks, the diaphragm tank holds hot water in the upper half and pressurized air in the lower half of the expansion tank. It is not to be installed in the floor joist space.

The expansion tank must be installed on the supply side of the system and 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
  • Expansion tank is hot to the touch lower than the middle of the tank
Possible Causes
  • Too little air and too much water in the expansion tank
Possible Repairs
  • Turn off boiler, close water shutoff valve and let tank and system cool.
  • Water-side pressure must be at 0 psi in order to get an accurate read of the air pressure on the diaphragm.
  • Check the pressure in the expansion tank by attaching a tire gage to the air recharge valve (Schrader valve) at the bottom of the tank;
  • If air is needed, add more by using a bicycle tire pump to fill the valve to 12 psi or as otherwise indicated by manufacturer's instructions.
  • 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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