Can you create a fan schedule? I’d like the fan to always run from 10pm until 6am.

Updated Answer: THANK YOU to Albert who pointed out below that the fan can be controlled through SmartThings! It was annoying to have to set up another app and service to do this, but it is a workaround to get around Sensi's frustrating limitation. SmartThings allows you to set the fan to Auto, On, or Circulate (but not the percentage of time to circulate) and automate that on any schedule you want. Unfortunately, Alexa and Google still don't allow you to control the fan at all, even though Sensi's API apparently allows it. Original Answer from June 6, 2020: No, you can't do a schedule like if you want it to run all the time during the night. Also, you can't control the fan from Alexa to schedule it that way. Very annoying and unreasonable that they wouldn't add that option since you can turn it on manually from the app. They do have the auto-circulate feature, but that's not the same.

Is it smart enough to decide whether to run the heater or cooler to keep the temperature fixed?

In spite of the snarky answer above that says that's what a thermostat does, it should be noted that a thermostat doesn't just so that unless it is a feature of the thermostat. A thermostat is for comparing existing ambient temperature to programmed temperature. On a furnace, if the temperature exceeds the programmed temp, the furnace simply will not run. THAT is what a thermostat does. UNLESS there is the ability to set said thermostat to automatically adjust the temp to the programmed temp.

Can you program just the fan mode?

As pointed out on a similar question, the Sensi app does not allow scheduling of the fan mode, but you can do it with a 3rd party integration to the SmartThings app. Unfortunately, Alexa and Google don't have the option to control the fan either, but the SmartThings workaround was good enough for me to set our fan to run automatically at night and turn off in the morning.

What if i have a C wire?

The "C-wire" is the second wire for 24VAC which is used to power advanced thermostats. Older thermostats were just a mercury switch and didn't have a need for additional power supply. Most newer systems are using 5-conductor thermostat wire.. Think of it as the plus side of a DC circuit/battery (but it is AC[thermostats have a rectifier to convert the AC to DC for their use]). Use it and the "common-wire" to supply 24VAC to the thermostat and zone valves/zone dampers. Without it, the thermostat runs on battery all the time and will quickly run down with WiFi. Add a zone controller like a Honeywell HZ311 which has full modern voltages output; i.e., c-wire with 24VAC. This control requires a 24VAC transformer and daisy chains the 24VAC to thermostats and works on older systems since it provides its own c-wire output to advanced thermostats.

Manufacturer	Emerson Thermostats
Part number	ST55
Item Weight	386 g
Product Dimensions	9.53 x 14.94 x 2.54 cm; 385.55 g
Batteries	1 AAA batteries required. (included)
Item model number	ST55
Size	No Size
Color	White
Style	DIY
Finish	Glossy
Material	Plastic
Pattern	Thermostat
Shape	Rectangular
Power source type	Corded Electric
Voltage	24 Volts (AC)
Item Package Quantity	1
Number Of Pieces	1
Display Style	digital
Plug Profile	Wall Mount
Specification met	Energy Star
Special Features	Auto Changeover, Clock Display, Daylight Savings Time Ready, Fan Control, Filter Indicator
Specific uses	Air Conditioner, Boiler, Furnace, Heat Pump
Included components	Leviton 80409-GY Decora Wall Plate, 2 Gang, Grey
Batteries Included?	Yes
Batteries Required?	Yes
Battery Cell Type	Alkaline

Emerson Sensi Wi-Fi Smart Thermostat for Smart Home, DIY, Works With Alexa, Energy Star Certified, ST55

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