How will it flush if there is nothing pushing the water out? Not only do you need the incoming water to flush the tank, when you turn the intake valve off, you create an air lock. It's kind of like a giant soda straw that is full of water. It won't drain as long as you hold you finger over the top. It is likely there is sediment at the bottom of the tank and you will need to run a stiff wire through the drain valve to help break it loose, but the intake must be on not only to supply the flush water, but to keep from having an air lock. If there is shut off valve on the hot water side, turn if off otherwise just keep the hot water faucets off in the house.