You can recharge a window air conditioner with a refrigerant when the AC unit starts failing. If you opt for the backyard method, you will need a bullet valve, a refrigerant, an HVAC gauge, or a recharge hose. After attaching the bullet valve to a low-pressure line, connect it to the refrigerant. Then connect the bullet valve with the recharge hose before opening the valve on the recharge hose to fill the system with refrigerant.