The C-wire was there the whole time. When I pulled our old non-programmable thermostat off the living room wall in October, there was a blue wire folded back and tucked against the cable jacket behind it, unused, like someone in 1998 had planned for a future they weren't going to see. That one wire made installing a smart thermostat a ninety-minute job instead of a weekend.
The install, quickly
We bought an ecobee3 lite through the Consumers Energy online store, which at the time took a $50 instant rebate off the $169 price, so it cost us $119. I installed it on Saturday, October 18th, three days before the insulation crew came.
I turned off the power at the furnace's service switch and took a photo of the old wiring before touching anything. The old thermostat used four wires: heat, cooling, fan and power. Down at the furnace, the same blue wire was folded back at the other end of the cable. I connected it to the C terminal on the control board, connected the other end at the new thermostat, and that was the hardest part. Switch back on, the screen lit up and the furnace fired on the first test.
The schedule we settled on
We landed on 67 when we're home and 63 overnight and while we're away. Tessa and I both work on site Monday through Thursday, so on those days it's 67 from 5:45 to 7:15 in the morning, 63 until 4:30, 67 until 10:30 at night and then 63 overnight. On Fridays I work from home, so it stays at 67 all day. Moose has a fur coat and no opinion about any of this.
My first attempt was more aggressive: 60 overnight. On a 28-degree morning in the first week, recovering from 60 to 67 took well over an hour, and the upstairs bedroom was still cold when the alarm went off. Tessa's review: "I didn't get married to wake up in a refrigerator." We moved to 63 the next day. Now the morning recovery takes about 25 to 35 minutes, depending on the weather.
What six weeks of runtime showed us
The best part of this thermostat isn't the schedule. It's the reports. It logs how long the furnace runs every day, and I now have six weeks of that data in a new tab of the house sheet. A few things stood out:
- Before and after the insulation. I only got three days of data before the crew came, which isn't much. But Monday, October 20th averaged about 41 degrees outside and the furnace ran 4 hours and 10 minutes. Monday, November 17th averaged about 40 and it ran 3 hours and 5 minutes. That's about a quarter less, on two days with similar temperatures and nobody home during the day. Wind and sun weren't the same, so I'm treating it as a hint, not a result.
- Short cycles. On mild days the furnace runs in short bursts of six to eight minutes. Our 1998 Carrier is a 70,000 BTU single-stage unit, and after the insulation it's clearly bigger than this house needs. It heats fast, shuts off, cools down and starts again. That's something I'll keep in mind when this furnace eventually gets replaced.
- Upstairs versus downstairs. The thermostat is downstairs. With a cheap thermometer on the dresser upstairs, the bedroom used to run about three degrees cooler than the living room on cold evenings. Since the knee walls were insulated, it's within one degree.
What it can't do
A smart thermostat doesn't make an 80 percent furnace a 96 percent furnace. It only decides when the furnace runs. Whatever savings we see this winter will mostly come from the insulation and air sealing; the ecobee is just the meter that lets me watch it happen. I'm not going to say anything about the gas bills until I can line up a full January against last January, and even then, two winters are never the same winter.
The runtime tab is my favorite thing in the house sheet right now. Tessa has asked me twice to stop reading it out loud at dinner.
