In a 3 quart saucepan over medium heat, cook bacon until lightly brown; add onion and cook until tender, about 5 minutes. Add water to clam juice to make 2 cups, if needed. Stir 1/4 cup flour into the onion mixture until blended. Gradually stir in the clam juice and cook, stirring until mixture is slightly thickened. Add potatoes, celery salt and pepper to saucepan; cover. Cook about 10 minutes or until potatoes are tender. Mix the rest of the flour with a small amount of half and half; set aside. Add clams, half and half, and butter to pan; Add rest of half and half mixture; cover. Cook until heated through and thickened, about 5 minutes, stirring often.