A New Thermostat
August 11, 2018
Updated on: 2019-February-18
While we were pretty happy with our existing setback thermostat (Honeywell CT3600 … photograph below), it is an old-school programmable thermostat. That is, it has no network connectivity and all programming is done through the control panel under the cover on the thermostat.
I discovered that Radio Thermostat makes thermostats that connect to a network via WiFi and they have published an API to be able to get data from and control the thermostat. I chose the model CT80 (photo below).
The CT80 has some other advantages over my old Honeywell thermostat. Its battery is rechargeable. I had to periodically change the batteries in the Honeywell and each time I did, I lost the program. The CT80 also has a mode whereby the furnace fan (we have forced hot air) comes on for a few minutes each hour to circulate the air in the house. It also has a message area at the bottom of the screen. By default, it displays the IP address the thermostat is using on your network but can be set using the API.
One feature of the Honeywell that we have lost is what Honeywell refers to as Smart Response Technology™. Basically, this feature tries to anticipate the amount of time it will take to get to the target temperature. Without this feature, saying that you want your home to be at 68⁰ at 7 am starts up the furnace at 7 am to bring the temperature up to the target temperature. With SRT, the furnace might come on at 6:40 am so that the house is already at the target temperature at 7 am.
I knew prior to purchasing the CT80 that it would be a more complicated installation than others I’ve done in the past. Radio Thermostat was very clear that this unit required a “C” wire (or another 24V continuous power source). The builder only installed a two-wire connector, so I knew that I was going to have to run some wire.
I decided to order new 5-conductor thermostat wire and replace the two-conductor. Not only did this solve the “C” wire problem, but also allowed me to connect the green wire in order to use the circulation mode of the CT80.
In addition to the setback at the thermostat, there are several other ways to control the thermostat setting.
The CT80 has a “Save Energy” button. Pressing it sets the target temperature to the lowest setting in the current program and holds that until you press the “Save Energy” button again. This is much simpler when we leave the house than the Honeywell which required us to use the down arrow to move the target temperature to the setting we wanted and they hit the “Hold” button. Note that the “Hold” button is right next to the “Run” button. More than once we’ve lowered the target temperature and then hit “Run” which negated the setback operation.
Radio Thermostat also has an app (both iPhone and Android versions) that allows you to look at data on the thermostat and set the target temperature from wherever you are. In addition, you can use the app to set a perimeter. If you take your device outside that perimeter the app will automatically setback the thermostat. I have not, yet, tested this functionality very well.
Several months after installing the thermostat, I got an email from Radio Thermostat recommending that I sign up for a program my electric company.
If I signed up with a qualifying WiFi thermostat (my CT80 does qualify) my electric company would send me $20. At the end of the summer, they would send me $25 more. I would be agreeing to allow them to adjust my thermostat by no more than 4⁰ during times of peak electric demand. There were other details like the maximum number of times during the season they would make the adjustment, etc.
I don’t know if I would have enrolled. Although my thermostat qualified, I was not eligible as we do not have central air conditioning so there would really be nothing for them to control.
Click here if you want more information on this program.
Raspberry Pi Weather Station Control of Thermostat
The weather station I’ve built queries the thermostat every 10 minutes and logs the temperature and humidity to ThingSpeak. See: http://homeautomation.geiserweb.com/index.php/2018/08/16/a-weather-station-version-1/ for a discussion of the weather station and its interaction with the thermostat.
It seems to work pretty well. One issue is that periodically it logs a -1 value for the temperature and/or the humidity. I don’t know why and have yet to do any investigative work to figure it out.