Smart thermostats for fan coil units: 2-pipe vs 4-pipe explained
If you live in a tower in Dubai, Doha or Manama, there's a good chance your apartment isn't cooled by split units at all. Instead, chilled water comes from a central plant or a district cooling provider and runs through a fan coil unit hidden in the ceiling. The control on your wall is usually a simple panel with a temperature dial and a fan switch.
Fan coils can work with smart thermostats, but they need a different kind of adapter from split or VRF units. Here's why, and what to look for.
What a fan coil unit is
A fan coil unit is exactly what the name says: a fan that blows room air across a coil of pipe. Chilled water runs through the coil to cool the air, or hot water to heat it. A valve controls how much water flows through, and the fan moves the air.
So a fan coil thermostat has two jobs. It has to switch the fan, usually between low, medium and high, and it has to open, close or modulate the water valve. That's quite different from a split unit, where the thermostat sends a digital command and the AC handles the rest.
2-pipe fan coils
A 2-pipe fan coil has one supply pipe and one return pipe. The supply carries either chilled water or hot water, depending on the season, but never both at once. In most Gulf buildings it carries chilled water all year.
For control, a 2-pipe unit needs a fan speed output and one valve output.
4-pipe fan coils
A 4-pipe fan coil has two supply pipes and two return pipes: one pair for chilled water and one for hot water. That allows either hot or cold water into the unit at any time. Buildings use it when different rooms need heating and cooling at the same moment.
For control, a 4-pipe unit needs a fan speed output and two valve outputs, one for each circuit.
Fan speeds and valves
Most fan coils in the region use AC fan motors with three fixed speeds. Newer units sometimes use EC motors, which vary speed continuously from a 0-10V control signal.
Valves come in a few types. On/off valves are either open or closed, and can be normally open or normally closed. Three-wire valves have separate open and close signals. Modulating valves open partway, controlled by a 0-10V signal, which gives smoother temperature control.
Why a normal smart thermostat can't run a fan coil alone
A Nest or ecobee is built for North American systems. It switches 24V circuits for cooling, heating and fan. It doesn't have three fan speed outputs rated for a mains-voltage motor, and it doesn't produce a 0-10V valve signal. Most fan coils also have no 24V supply for the thermostat to run on.
How the DropAir Pro 6S solves it
The Pro 6S is DropAir's fan coil adapter. The smart thermostat tells it when to cool, and the Pro 6S does the fan coil side:
- Three-speed AC fan motor control, rated 100 to 240V, 3A
- On/off valve control, rated 100 to 240V, 1A
- A 0-10V DC output for modulating valves
- 2-pipe and 4-pipe configurations
- Normally open, normally closed and three-wire valves
- 24V power for the thermostat, so no separate transformer
It's small at 43 by 43 by 28 mm, connects to the thermostat over RJ45 and CAT6, and comes with a 2-year limited warranty. Supported thermostats include the Nest Learning Thermostat (2nd and 3rd generation), Nest Thermostat E, ecobee, Honeywell Lyric and selected Control4 and Crestron models.
Buildings with 220V thermostats
Many fan coil buildings were fitted with 220V wall thermostats. In that case the Pro 6S is paired with the DropAir iPanel. We'll tell you if you need it.
What to check in your apartment
- Find the fan coil. It's usually above a ceiling access panel near the room. The rating label shows the model.
- Look at the pipes. Two pipe connections on the coil means 2-pipe. Four means 4-pipe.
- Check the wall controller. Note the model and photograph the wiring. This tells us the voltage and whether the valve is on/off or modulating.
- Ask your building. Some buildings have rules about who can work on the fan coils. Check with your landlord or facilities team first.
A note on district cooling bills
If your building is on district cooling, your cooling is often billed separately from your electricity. A smart thermostat helps both, because the fan uses electricity and the chilled water use depends on how long and how hard the coil runs. Schedules that ease off when rooms are empty reduce both.
Summary
Fan coils need fan speed and valve control, which a smart thermostat can't provide on its own. The DropAir Pro 6S adds it, for 2-pipe and 4-pipe units, on/off and 0-10V valves. Send us a photo of your fan coil label and wall controller, and we'll confirm the setup.