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Step 7 · Reactivate System and Purge Air

With the tank full and leak-free, you can complete the job. Return to the electrical panel and flip the water heater's circuit breaker back to the 'ON' position. For a gas heater, follow the manufacturer's instructions printed on the tank to relight the pilot light and turn the gas control knob to 'ON'. To purge any remaining air from the system, go back to the hot water faucet you used earlier and open it fully. It will likely sputter and spit air for a minute. Let it run until a smooth, steady stream of water flows out, then turn it off. The water heater will now begin heating the water. Your expansion tank replacement is complete.

Reactivate System and Purge Air - How to Replace a Water Heater Expansion Tank - Stesso