Remotely Enable/Disable A Thermostat
July 20, 2009
A simple way to handle the issue of remotely enabling and disabling your thermostat can be valuable for turning the system on when you return from vacation, or turning it off when you realize you will not be home when you originally planned. The objective is to remotely control whether or not the thermostat is connected to the furnace/AC. I used the ioBridge and a DPDT Relay Board to interrupt the 24VAC that is fed to the thermostat.
Note that most of the newer HVAC systems ramp the system up and down to increase component life. I decided to use this technique on my system because it is 30 years old, and therefore not as finicky.
First I had to learn how my home thermostat is wired. Picture is below.
This is a typical 4-wire system (1 heat & 1 cool) operating at 24VAC. The furnace has a transformer that supplies the 24V which is wired to the RH (heat) and RC (cool) terminals on the above block. The thermostat decides when to activate the fan, AC compressor, or furnace gas valve by sending the 24V to the G, W, or Y terminals respectively.
All I did was just send the 24VAC power from the transformer through the relay board. Ignore the other little boards connected to the ioBridge; they are for other projects.
I decided to use the Normally Open connection on the relay board. So, in the case power is lost to the relay, it will revert to an open connection, and the thermostat will be disabled. I thought this was important because if we are traveling, I want the HVAC system to turn off when something goes wrong. Of course this means the system will turn off if we are home when something goes wrong as well.
Now, when we all go on vacation, all we need to do is decide if we might need heating or cooling when we return, and we will set the thermostat in that mode before leaving. If we had a newer auto changeover thermostat, all we would need to do is turn it on auto. Then we disable the thermostat using the ioBridge control, and leave. When we are returning, we just enable the thermostat via ioBridge (like via an iPhone on the way home from the airport.) The house will be comfortable when we get home.
Likewise, if we are out for the day and decide to not come home that night, we simply disable the thermostat via ioBridge until just before we return.