In response to the build a Nest like thermostat I feel it necessary to post a warning.
For reference: http://blog.spark.io/2014/01/17/open-source-thermostat/
Sometimes the open-source community gets crazy and needs a sanity check. This is that check.
Rule #1: If you have a gas furnace NEVER mess with your home thermostat beyond what’s outlined in the user manual & that also means never have the audacity to build your own. Let me be clear. If you have a gas furnace & something goes wrong with your thermostat things can go horribly wrong very, very quickly because gas furnaces typically pump out a lot of BTUs. Our daughter almost died because an unmodified, properly installed Honeywell programable thermostat failed after four years of use & caused our furnace to remain on for an extended time, raising the temperature in her room which was above the furnace to over 120 degrees. She didn’t die because my wife and myself managed to wake up and realize the house was way too hot. Keep in mind kids don’t wake up like adults do and I think we were likely lucky that we did.
In researching this incident I found story after story of furnaces gone awry & the built in thermal protection on the furnace not cutting it off before causing serious damage. One such story was from a couple who were out of town when their furnace went nuts and raised the temperature in the house high enough that it melted every piece of plastic in the entire home. If you make your own thermostat consider that the worst case in one of these situations is that someone dies or your house burns to the ground. The best worst case is that all your stuff melts and your insurance company refuses to pay a claim because of the home-made gadget hanging on the wall. The danger isn’t quite as high for thermal induction & heat pump systems but it’s something you need to consider depending on the size of your HVAC unit. I build a lot of projects & am entirely comfortable playing with 15,000 volts around water: http://www.flickr.com/photos/patcheudor/11616785826/ . I WILL NOT, however, mess with my HVAC unit.