In slow cooker, mix potatoes, onion, celery, broth and water.
Cover; cook on low heat setting 6 to 8 hours.
After the 6-8 hours, in a small bowl, mix flour into milk; stir into potato mixture. Increase heat setting to High. Cover; cook 20 to 30 minutes or until mixture thickens.
Remove about 5 cups of soup and place in a bowl. Add 1 bag of cheddar cheese (2 cups) and puree with an immersion blender until the cheese is incorporated. Return the pureed soup to the crockpot.
Add 2nd bag of cheese (2 cups) to all the soup in the crockpot and stir until melted. Garnish individual servings with bacon and green onions. Sprinkle with pepper if desired.