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How does this pressure switch function

Noname81 Member Posts: 1
edited September 2019 in Radiant Heating
I am looking at this unit. What I am confused with is why the tub heater runs with the tube on the top right disconnect. Do pressure switches with dual ports work of differential pressures? When I disconnect the pressure hose off the unit(top right of picture the unit fires. When I attach it the unit does not fire.