Programmable Thermostat Wiring Diagrams - Installing a programmable thermostat is not unlike installing any other thermostat for your HVAC system. The key is to get the right thermostat for your HVAC system. And to make sure it is wired properly.

The subbase of a programmable thermostat showing the termination points of the thermostat

Programmable Thermostat Wiring Diagrams

Programmable Thermostat Wiring Diagrams | 2 wires - 3 wires - 5 wires

Many thermostat manufacturers provide detailed installation instructions with their thermostats to make sure you have all the information you need to install the thermostat properly. Sometimes the instructions that come with the new thermostat can be vague or even confusing and that is what we want to help you with here at High Performance HVAC.

Please see our other articles for installing thermostats on our other pages to help you with your thermostat installation. Below are some good tips and detailed instructions for installing your new programmable thermostat. And don’t forget after the new tstat is installed you still have to program it.

heat pump thermostat wiring chart

Programmable Thermostat Wiring Diagrams | 2 wires - 3 wires - 5 wires

The above chart will help you understand the wiring terminals and color designations for a heat pump thermostat. If you are installing a programmable thermostat for a boiler it should only require two wires. It is a two-wire thermostat installation. This is the battery-powered thermostat and the two wires are only for switching on and off of the boiler for heating. It is better with 3 wires but with the battery-powered thermostat, two wires will work. You will simply have to keep the batteries fresh in the thermostat.

two-wire thermostat control

Wiring your programmable thermostat for boiler or furnace control with only two wires will work just fine except you will need to keep replacing the batteries on a regular basis. If you want to have the thermostat powered by the transformer the wiring needs to be as in the following graphic:

3 wire thermostat control

Whereas the common wire from the transformer goes into the thermostat to provide a complete 24-volt circuit for thermostat power. For an air conditioner the wiring should be as follows:

Air Conditioner

Programmable Thermostat Wiring Diagrams | HVAC Control - The Basics

These are the basic wiring diagrams for installing a programmable thermostat. There are some other combinations for wiring a thermostat but the information here will cover 90% of the combinations most people need.

If you have something different or unique email us using the contact form and describe to use what you have. We will respond to you with some follow-up questions and ask you to send us some photos of what you have. Then we will respond to you with our advice on how to wire your thermostat.

If you have two transformers the wiring will be slightly different than shown. Refer to our article “How Can I Tell 1 or 2 Transformers for Heating/Cooling System” for more information.

Additional information for HVAC thermostats can be at and

High Performance HVAC Air Conditioning and Heating Systems

Programmable Thermostat Wiring Diagrams | HVAC Control