Combine, potatoes, onion, celery and water in an electric pressure cooker, and cook according to the manufacturer's instructions (ours called for 5 minutes on high pressure using the quick release valve).
Add celery soup, clam chowder, salt, thyme and celery seed; cook on the simmer setting until heated through. Once warmed, add 2 cups milk.
In a small saucepan, melt butter; mix in flour and stir until smooth. Stir in 1 cup of milk gradually, and cook until thickened, stirring constantly.
Add flour mixture into hot soup.
Cook until soup is thickened, stirring constantly.