The Taco T563-2 is a temperature aquastat. Used on 00 Series Circulator Pump. The 00 Timer/Aquastat combination is designed to cycle the circulator during peak demand periods. Use the two devices together or separately. When using only the aquastat, the circulator cycles continuously to maintain a temperature between 95º F and 115º F. For maximum convenience and flexibility, you can retrofit both to any Taco 00® Series Cartridge Circulator.