Hot water expansion tanks are designed to protect the home's hot water boiler and plumbing system. When water is heated, or the hot water is shut off quickly, the tanks, filled with both air and water, will equalise the resulting water pressure. Unequal water-to-air ratios caused by a built-in barrier failure, pressure valve leakage or low air pressure in the expansion tank cause problems with the system's efficiency, so troubleshooting may be necessary.
Tap the side of the expansion tank slightly above the halfway point, listening for a hollow sound, or check if the lower half of the tank feels warm and the upper half feels cool. Check the water pressure relief valve on the side of the tank for leaks.
Turn off power to the boiler if the tank sounds solid, the whole tank is warm, or the relief valve is leaking.
Close the water shutoff valve on the side of the tank and let the water cool.
Attach the garden hose to the drain valve on the side of the tank and drain up to three gallons of water into the bucket. If no valve is present, shut off the valve between the tank and the boiler. Drain the expansion tank.
Turn on the water supply. Turn on power to the boiler. The expansion tank will fill as part of the system's normal operation.
Check air pressure with the gauge. Pressure should measure 12 psi.
Attempt to raise pressure with the air pump if necessary and observe regularly to see if the tank holds the pressure.
Even if air pressure holds to 12 psi after pumping, check regularly, as pressure correction may be temporary.
If a flooded expansion tank is not drained, water pressure build-up may cause pressure valves to leak, which could lead to mineral deposits forming at the valve opening. This build-up can block the valve, which may cause an explosion. Excessive water pressure build-up can cause non-leaking valves to pop, leading to heavy water spillage. When draining the tank, be aware that water temperatures may be above boiling.