Question 1

Will this work with Mitsubishi split-type A/C units?

Accepted Answer

It will not work. Mitsubishi uses a thermistor on the indoor unit. That sense the air temperature

Question 2

I have red, green, white, and yellow wires.  No black.  I am assuming black is "common" wire and thus I can not use this thermostat, correct?

Accepted Answer

Each system is different. I have the same colors as you, green, white and red for my A/C and white and red for my heater. I didn't have a black wire. I think your right, black is common.

Question 3

Since the system auto-connects after a power outage, what is the point of the return air sensor? I don't know how or whether to install it.

Accepted Answer

If the battery powered thermostat fails and the system is stuck in the "on" position the system could continue heating for an extended period of time and potentially cause a dangerous situation.  To avoid this, the return sensor is a temperature sensor placed in the air intake duct (drill a hole and push it through).  The main control unit (with the red label on it) is usually located in your utility closet and wired with a short 6' HVAC wire directly to the HVAC system (no need to go through any walls - very easy to wire).  The main intake duct usually enters the bottom of the HVAC system in the utilities closet - this is where you drill the hole for the sensor and run a short wire over to the control unit.  If the thermostat does fail, and the system is stuck in an on position when the temperature goes outside the normally acceptable range per the return sensor the system will shut off.

Question 4

If i just need a replacement part (the programmable thermostat), do i need to buy the whole kit?  already have the eim, installed.

Accepted Answer

No, you don't need the whole system. The thermostat is a Honeywell TH6320R1004. As long as the thermostat has "Red Link" you should be able to use it. Normally thermostats don't go bad, unless water, bugs, or people do something to it.

ASIN	B001O4F8Y4
Additional Features	Time - Wireless
Best Sellers Rank	#102,461 in Tools & Home Improvement ( See Top 100 in Tools & Home Improvement ) #123 in Home Programmable Thermostats
Brand Name	Honeywell
Color	White
Connectivity Protocol	Wi-Fi
Connectivity Technology	RedLINK, WiFi
Control Method	Remote
Controller Type	Push Button
Customer Reviews	4.5 4.5 out of 5 stars (486)
Display Type	Digital
Finish Types	Glossy
Global Trade Identification Number	00085267329408
Included Components	Equipment Interface Module, Return Air Sensor, Wireless Thermostat
Item Dimensions D x W x H	4.38"D x 5.56"W x 1.25"H
Item Type Name	Wireless Focuspro Thermostat Kit
Item Weight	454 g
Manufacturer	Honeywell
Manufacturer Part Number	YTH6320R1001/U
Model Name	Honeywell YTH6320R1001 Wireless Focuspro Thermostat Kit, Programmable Redlink Enabled
Model Number	YTH6320R1001
Mounting Type	Wall Mount
Number of Batteries	2 AA batteries required.
Number of Items	1
Power Source	Battery Powered
Product Style	Wireless
Shape	Rectangular
Specific Uses For Product	Cooling, Heating
Specification Met	certified frustration-free
Temperature Control Type	Cooling, Heating
UPC	085267329408
Unit Count	1 Count

Honeywell 115369 24V WIRELESS FOCUSPRO THERMOSTAT KI

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