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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 | 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.
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.
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:
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:
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 honeywell.com and emersonclimate.com
Programmable Thermostat Wiring Diagrams | HVAC Control
My T-Stat only has four (4) wires and they are colored “White, Blue Red, Green”. It is a 2005 Weather King HVAC with electric heat. How would I wire up a Honeywell Home Programmable RTH7400, RTH7500 Thermostat? Thank you in advance, Les.
You need one additional wire to make it work. A digital thermostat requires a common wire or C wire. Follow the instructions in this article.
Problem: When thermostat is set for cooling, heat comes on and no 24v to contactor on condensing unit. Also heat comes on normally. What’s the problem?
Don’t know what kind of equipment you have? Is it a heat pump, AC with gas furnace? Guessing with this but it could be bad thermostat wire or a bad control board. Without looking at the problem or getting more information I cannot tell you and only guess.