Barnes & Jones cage unit for Mepco (formerly Dunham-Bush) #1 and #1A traps. This cage unit requires a new cover when first installed. Cage units replace the existing thermostat and seat and are heavy duty thermostatic element and seating units surrounded by a protective stainless steel cage that are precisely calibrated to ensure solid opening and closing. They are held in place by a cage protrusion at the bottom of the unit that protrudes into the orifice of the trap. A high tension spring at the top of the unit will apply pressure to the cover, eliminating any movement. When the thermostatic element senses steam, it expands causing the ball plunger at the bottom of the element to close against its internal seat, thus keeping the steam contained until the steam cools to a liquid state. The element then senses the cooler liquid condensate and contracts, which raises the plunger from the seat and allows the condensate to drain.