Usually used with heat pumps so tstat knows when to shut unit down outside and turn on heat strips or emergency heat.

Big al

Two major reasons come to mind.

1. If you have thermostat that has capability of displaying outside temperature at the thermostat.

2. Main reason though is for dual fuel application like heat pump with backup furnace. With the correct thermostat and outdoor sensor, homeowner can be in control of changeover point between HP and furnace.

Disagree with previous post. You don't want to disable a HP for electric heat strips unless it is a very low outside temperature. Most people run their HPs just fine without an outdoor sensor.

IMO

Some hydronic heat systems are set up to modulate the boiler set point with outdoor temp. OP wrote "furnace", but often that is used for boilers.

whaaat?

There are some thermostats which use the outdoor temperature to determine which stage of the furnace or AC/heat pump to use as part of the decision algorithm. You also may have options to lock out a low or high stage depending on the outside temperature.

tigerdunes,

Not disagreeing, just don't understand.

"Disagree with previous post. You don't want to disable a HP for electric heat strips unless it is a very low outside temperature."

OK Salti

You don't need an outdoor sensor on a traditional all electric HP system with air handler and backup heat strip for both supplemental heat, to temper air on a defrost all, and emergency heat when outside HP condenser as malfunctioned for whatever reason.

The only exception that I can think of is at extremely low outside temp where HP heat produced is less than a COP of 1. This means it is costing you more to run the HP condenser than the heat(BTUs) produced.

Hope this helps.

I generally agree with Mike's post although this gets into high end systems with high end controls that can do every thing but dance.

IMO

tigerdunes,

Got it! Tnx.

The outdoor unit is not a complete thermostat, but just a sensor.