Building my own thermostat

Community Projects
1

Hi, I’m thinking about building my own thermostat because none of the thermostat on the market do what I need them to do. (NEST is closed standard, radio thermostat is full of bugs). I have a forced air HVAC with 5 wires comming to my tstat right now.

Basically, I’d like to know if it’s just a matter of using 3 relays: 1 for cool, 1 for heat and 1 for fan.

I understand my software needs to implement logic like starting he fan if I start heat, and avoid cycling the unit too often etc… My question is more about the hardware side. Is it just a matter of controlling a relay that would close the circuit for heating, cooling and fan? Just a normal 24vac relay would do?

I would appreciate getting some guidance on a basic design.

Thanks.

2

After the purchase of Nest by Google, an open-source thermostat project is starting up. You might find some useful information and code here: http://blog.spark.io/2014/01/17/open-source-thermostat/

3

Thanks. But I would prefer to build my own.

I guess my question really is, can I just use a relay to close circuit with “w” with “rh” or “y” with “rc”? A relay? A triac?

Do I have to activate the fan or this will be done automatically when the heat is turned on?

which wire will give me power for my device?

I know this post is pretty specific to HVAC, so I understand I would need to be lucky enough to have an HVAC expert to view this thread to get my answer.

Thanks.

4

Yes, you can just use a relay. The signals should be low current (under 1A), and are usually 24VAC.