I recently commissioned a project that involved replacing eighty 2-pipe, dual-temperature fan-coil units in a nearly 40-year-old high-rise condominium. This sounded like a pretty easy assignment. How hard could it be to connect a few pipes and wires and control a fan and 2-position valve for heating or cooling?
The original fan-coil units were mostly controlled by old-fashioned Honeywell round line-voltage thermostats. A pipe surface temperature sensor was used to determine whether there was hot water or chilled water in the pipes. The single-speed fan would start or stop based on a thermostat command (either ON or AUTO), and the control valve would open or close based on the thermostat set point (either HEATING or COOLING) and default deadband. Life was good. Life was simple. Everyone knew what to expect.