Question 1

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

Accepted Answer

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.

Question 2

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

Accepted Answer

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.

Question 3

Can you program just the fan mode?

Accepted Answer

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.

Question 4

What if i have a C wire?

Accepted Answer

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.

Asin	B01NB1OB0I
Batteries	2 AA batteries required. (included)
Best Sellers Rank	#128 in Home Thermostats & Accessories
Date First Available	11 June 2017
Department	Unisex-Adult
Is Discontinued By Manufacturer	No
Item Model Number	ST55
Manufacturer	Emerson Thermostats
Product Dimensions	9.53 x 14.94 x 2.54 cm; 330 g

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

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