Does the water heater fill when you fill the fresh water tank?

NO; After filling your tank, turn on the water pump an a hot water faucet to bleed the air out of the water lines and the water heater. Run the water until no air is coming out of the faucet. If you have a city water hook-up then the water heater will fill when the city water is turned on. You still must bleed the system as above. If your water heater also can be run on AC current, do not turn on the water heater to AC unless you have bled the system and the water heater is full of water or you will blow out the heating element.

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Jim L & Faye
2805C
2003 Silvarado diesel crew cab
2017 Silvarado, crew cab 1500 6.2L gas w/ 8 speed tranny
central VA

A faster way to purge the air from the hot water heater is to pressurize the system (with the pump or city water) and open the water heater relief valve by pulling the spring loaded handle. You will still need to purge some air using the faucets, but you won't get as much splashing.

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Raul

2408T
2010 Nissan Frontier, 4x4, Crew Cab

To allow for expansion of water when it gets hot and prevent over pressuring the water lines there needs to be some air in the water heater. After filling the water lines and the water heater shut the water pump power off or turn off the city supply. Then hold the pressure relief valve open until water stops flowing out. This will then provide a small pocket of air. I only recently learned about that by reading the instructions.

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My Great Wife Joyce
2018 Jayco Jay Flight 21QB
Formerly owned 1705T and 2310H
2012 F150 4X4 SuperCrew EcoBoost w/Leer Cap
Reese WD Strait-Line Hitch
Amateur Radio K3EXU