How does a home thermostat system work?

I always thought that the thermostat display box, was wired through the wall and connected to the furnace.

I didn't know you can just buy the display thing, attach it to the wall and have it run your furnace and AC and whatnot.

So how exactly does it work?I mean, how is it able to control the furnace and AC?

Thanks.

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Answers

You can only just use a display if the house is wired to Wi-Fi.

a thermostat is a sensor with many function

It was and still is being used in the basic thermostat that is mercury switch controlled.That is all wires.
You are talking about a WiFi set up.I don't know or care as the old system still works fine....since 1975.

if it isn't wired, it's using some sort of short range radio control -- that requires installing a receiver on or near the control board and running its antenna through a small hole in the air return plenum

It is connected electronically to the heating and cooling systems.

There is a relay in the thermostat, the old style ones had a mercury bulb switch on a bimetal coil spring.
I think you are referring to the new WiFi models, they still need a WiFi receiver box to emulate contacts.
You replace the hard wire Tstat with a master and then the remotes communicate via your home WiFi.

It is simply a switch that turns the furnace (or AC) on and off when the set temperature is reached.

You're talkin about a remote control thermostat. You still have to connect one part of that thermostat to the four wires that runs the heating and air conditioning system. But you can put the control boxes pretty much anywhere you want to in the house.

the thermostat is either hard wired OR it can be wireless also.the 'stat triggers the burner in the furnace to start.in the duck work from the furnace 'plenum' is a heat sensor.when the heat from to burner gets the air hot enough, this heat sensor turns ON the furnace blower, & warm air circulates in the house ................