How To Buy a Thermostat for an Air Conditioning Unit

By | June 11, 2019

How To Buy a Thermostat for a Air Conditioning Unit

Buying the correct thermostat for an air conditioning unit depends on knowing what type of HVAC unit you have.

Thankfully, manufactures setup most thermostats to accommodate most any system but it still helps to know what type of HVAC system you have.
Types of Air Conditioning Systems
Heat Pump Unit
A heat pump can both cool and heat a home by reversing the flow of refrigerant.

If your unit can heat and cool without a furnace than you have a heat pump.

Air Conditioning only Units
Standard air conditioning units only cool a home and need furnace to heat during the winter.
Thermostat Wiring
There is a video at the bottom of the page for a step by step visual explanation.

Wiring in a thermostat is not difficult with some simple information.

First off always document the old thermostat wiring by taking good pictures or writing the color codes and what terminals they go to.

Keep in mind that while there is a color code that is supposed to be used it is not always followed.

By examining the old wiring going to the old thermostat this can be taken into account.

Often simply copying the old wires and terminals into a new thermostat can be done.
How A Thermostat Works
Thermostats control large components in a A/C unit such as the compressors and fans. These components use high voltage while a thermostat uses only 24 Volts to control them.

It does this through what are called relays that engaged when 24 volts is applied to them.

A relay will have high voltage going in on one side and a component wired in on the other side.

When 24 volts is applied to a relay a coil creates a magnetic effect pulling down a metal lever that completes the circuit for the high voltage to go through.

A thermostat is nothing more than a automatic switch that controls turning on or off the correct components depending on the temperature.
Common Wiring Color Codes and Terminals.
C———This is the common wire from one side of the transformer which is 24 Volts AC.
R———The other side of the transformer also 24 Volts AC. This is the hot side of the transformer.
G———Turns the fan on.
W——–Call to turn on heat.
Y———Call for cooling.
how to buy a Thermostat and  Wire it up
Depending on your system many of the above terminals may not be used.

The main thing to look at is the 24 volts coming in on “R” and “C” terminals.

The “Y” terminal is a call to turn on the air conditioning which will apply 24 volts from “R” at the “Y” terminal when cooling is needed.

Types of Thermostats
Standard basic thermostats work well and are commonly used since they are cheap.

Today more than ever before is a wide range of thermostats from WiFi thermostats that can control a homes temperature, to touch screen thermostats that have weather forecast.

So lets take a look at thermostats to see what types are available.

Popular Thermostats
Honeywell RTH2300B1012/E1 5-2 Day Programmable Thermostat

The Honeywell RTH2300B1012/A is a cheap and basic thermostat that will work with most any type of system.
Honeywell Smart Thermostat, Wi-Fi, Touchscreen, Works with Amazon Alexa

This is one of the new Wi-Fi Smart Thermostats that have been becoming popular. It shows local weather and forecast and can be controlled anywhere from a smartphone or computer.
Lux Products TX9600TS Universal 7-Day Programmable Touch Screen Thermostat

One of the many touchscreen thermostats. The TX9600TS has a large display and touchscreen to control the temperature.
Nest Learning Thermostat, 2nd Generation, Works with Amazon Alexa

This is a new generation of thermostats that program themselves after learning the habits of users. Can also be controlled from a smartphone or computer.

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