The system has to have an air vent somewhere. I'll bet there's something where ever the expansion tank is. I have a radiant floor system served by an instantaneous hot water heater. The heater, pump, expansion tank, and other such gizmos are in a downstairs room at the low point of the system. The system has one bleeder and it is downstairs. Works fine. As spdrun says, if the system doesn't have any sharp, local high points like you would have with old-fashioned radiators, one bleeder at the expansion tank fitting should be enough.