Homeowner has a Navien NPE-240A that has a failed heat exchanger. I contacted Navien and received a new heat exchanger under warranty. I turned off water, power and gas to the unit. I disconnected all sensors and electrical connections inside the unit. I removed the water piping inside the unit. I removed the blower motor unit from the tankless. I finally removed the failed heat exchanger, and installed the new heat exchanger. Reinstalled the blower motor unit, reinstalled water piping and reconnected all sensors and electrical connections inside. I reinstated gas and water to the unit. Tested for any leaks and found none. I then powered on the unit and allowed it to run through a boot up sequence. Once the unit was booted, I then fully powered it on. I then tested to ensure the unit was firing properly and water was getting adequately hot. The unit is now functioning as intended.