In a large skillet, cook the bacon over moderate heat until golden brown, about 4 minutes. Add 2 tablespoons of the butter and when it melts, add the celery, onion and green pepper and cook over moderately high heat, stirring occasionally, until softened, about 4 minutes. Add the clam broth and bring to a boil. Transfer the mixture to a slow cooker. Add the water, bay leaves, thyme sprigs and clams. Cover and cook on high for 4 hours, stirring a few times.