Two-Pipe Fan-Coil Unit Controls
Looks can be deceiving when it comes to these unassuming systems.
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.
Then came the new fan-coil units which came with 3-speed fans and a thermostat designed to automatically adjust the fan speed based on space temperature deviation from set point. The new thermostats were single set point/auto-switchover controllers. The new thermostats had five push-buttons and a screen display/user interface for selecting between optional operating modes.
There were a number of very unhappy residents. The most common complaint was the automatic fan speed adjustment and the resultant variety of noise levels generated in people’s homes. In addition, the default thermostat setting was for continuous fan operation, regardless of the call for heating or cooling. This drove many people crazy because they had no control over their fans, and the fans appeared to have minds of their own (which they did).
These deceptively simple thermostats could be changed to an auto fan mode of operation (i.e., operate only upon a call for heating or cooling), but it took a Ph.D. in reading owner’s manuals and simultaneous button-pushing to make that change through the screen user interface. There were so many combinations of features and button-pushes that it took the installing contractor three trips to the first thermostat before they figured this out. Compare that to the simple ON-AUTO switch of the trusty old Honeywell round.
The default thermostat setting was for continuous fan operation, regardless of the call for heating or cooling. This drove many people crazy because they had no control over their fans and the fans appeared to have minds of their own (which they did).
The 2-pipe thermostat also automatically switched between heating and cooling as needed to achieve a single space temperature set point. This meant that even when there was no chilled water available the thermostat controlled the fan as if there was (while holding the control valve closed because the pipe temperature sensor indicated hot water instead of chilled water). Therefore, whenever the space was above set point by at least 2°F in the winter (due to solar gain, cooking, high-density occupancy, or simply the owner’s attempt to save energy by lowering the set point), the fan would start and then increase speed if the space temperature continued to rise.
This was very unpopular, partly due to the homeowner’s loss of control and partly due to the perception of increased energy use by the fan. It was possible to manually override the fan off with a button, but this would prevent the fan from activating automatically when truly needed — a very bad idea in a cold winter climate. It was also possible to manually override the fan speed to low, but that would result in a continuously-running fan on low speed — again not desirable for many homeowners. For those who simply could not stand it, the installing contractor offered to replace the auto-switchover thermostat with a manual-switchover model for an additional charge.
I took away the following key lessons from this “pretty easy assignment.”
Never again assume fan-coil unit controls will be simple.
Communicate clearly with homeowners about thermostat functions and options before ordering new thermostats in our next condominium fan-coil unit replacement project.
Treat fan-coil unit start-ups with the same mindset as chiller or boiler start-ups when the thermostat has so many hard-to-access options and modes that should be selected correctly the first time. Figure it out on a prototype before replicating it across an entire building.