When you have it in the summer mode (if applicable) and you have either the house tank full and the pump running, or you are on city water. Open the hot water faucet on the bath and galley sinks. Let the water run until no air comes out, just water. You can also check by opening the pressure relief valve on the water heater momentarily. Water should come out. Don't leave it open too long or you will lose the air head.
The air head makes room for the water to expand as it heats up.
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