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I am building a DIY thermostat similar to this blog. I want to connect it to the HVAC system of my home. How do I do that?

I found this post which is very informative but doesn't completely answer my question.

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It completely depends on the type of system, as well as the make and model of all the components. –  Tester101 Aug 23 at 18:19
    
It's best to ask the specific questions you have. But some quick concepts of a thermostat: you can think of them as automated switches. Most have a shared hot that they will then use to turn on lines to activate different parts of the HVAC system. –  diceless Aug 24 at 5:16

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Since you haven't supplied much detail, I'll have to use nonspecific examples based on typical installation methods.

Furnace and Condensing unit

If you have a setup where you have a furnace, and an outdoor condensing unit. The wiring will be similar to this (Note: This is a basic overview, not actual wiring).

Furnace and Condensing Unit

Basically, the R wire provides power to the thermostat. W, Y and G are energized depending on the state of the thermostat. For example. If the thermostat is set to the COOL mode, and the temperature in the room is above the set point. The Y and G terminals of the thermostat will be energized. This tells the furnace to turn on the blower at high speed, and the condensing unit to turn on. Alternatively, if the thermostat is set to the HEAT mode, and the temperature in the room is below the set point. The W terminal will be energized, which tells the furnace to turn on.

Heat Pump

If you have a heat pump, the wiring will be similar to this.

Heat Pump

Here the R wire again provides power to the thermostat. This type of setup uses either an O, B, or O/B terminals to change the state of the reversing valve in the heat pump. In this type of setup, the thermostat works similar to the example above, though it also has to manage an extra terminal.

Some thermostats offer both an O and a B terminal.

Thermostat O and B

When the thermostat is in COOL mode and calls for cool, the O terminal will be energized. If you have a system where the reversing valve is normally in HEAT mode, you'll use the O terminal to move the reversing valve to COOL mode. When the thermostat is in HEAT mode and calls for heat, the B terminal will be energized. If you have a system where the reversing valve is normally in COOL mode, you'll use the B terminal to move the reversing valve to HEAT mode.

Some thermostats offer and O/B terminal, and will have a jumper to select which function it provides.

Thermostat O/B

In this case, you'll set the jumper based on the type of system you have. Then when the thermostat calls for heat or cool, the O/B terminal will be energized appropriately.

For more information on the C terminal, please see this answer.

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