Water heater may need to have air purged to fill it up. If you start using the heater afterwards, without purging the air first, then anywhere from 6 to 10 gallons of water will travel into the water heater tank lowering your fresh water level. You also run the risk of damaging the water heater by running the heater empty.
When I fill my freshwater tank, after about half way full, I turn on the water pump and open all my hot water faucets and they sputter a lot of air. Once each faucet runs steady water I turn it off. After all faucets run steady water, the water heater tank is full.
