Preheat oven to 400 F. Spray a large baking sheet with nonstick cooking spray. Place turkey carcass & other turkey bones, onion, carrots & garlic on it & bake, uncovered for 1 hour, turning once. Transfer the carcass, bones & vegetables to a large soup kettle. Add 18 cups cold water; set aside. Pour remaining 2 cups cold water into baking pan, stirring to loosen browned bits. Add to kettle. Bring to a boil. Reduce heat; cover & simmer for 3-4 hours.