I decided to take one more deep look at detailed correlation between my Ecobee runtimes and my Sense detection for my upstairs AC compressor (AC2) to see what things look like when Pearson correlation between the hourly usage is good (very close to 1.0) vs. bad (less than 0.7) on a weekly basis. Here’s the weekly correlation chart between hourly energy usage and Ecobee hourly runtime. The numbers on the bottom are the number of hours that the downstairs AC ran that week at about 2.15 kW / hour (3 ton unit). One of my motivations for a more detailed look is that I just replaced my evaporation coil upstairs, as well as the furnace, including the blower/air handler for my AC. I want to see if I see any differences in performance.
First, let’s look at good correlation for the week of 6/22. Here’s the week view on an aggregated hourly basis. Looks quite good except Sense seems to have missed an off-signature or 2 on June 26th.
Is there any way to get a better idea what happened there ? Maybe if we look at hourly Sense vs. Ecobee’s 5 minute runtime data below. Then again, maybe not. Given the number of actual on/off cycles, it’s hard to resolve the Sense hourly vs. the full set of transitions. Also makes me aware of how important it is to line up sampling intervals when doing visual comparisons. For the nerds out there, your simple AC system is run by pulse width modulation (PWM).
Now it’s time to look at far less good correlation the week of June 8. Very clear what the main mode of failure was here. Sense either detected an errant off-signature or decided to time out after what looks to be about 2 hours. June 10th and 11th were a fairly abnormal heat spell where the upstairs AC was pressed to run for 12 hours continuous.
Hour by hour:
Sense hourly vs. Ecobee 5 min.
A final look at one more errant week, May 25th. Looks like the same phenomenon - Sense seeing an off too early on days when the AC is running for uninterrupted for long periods, though Sense looks like it hung in there for much longer on May 25th. I’m wondering if Sense occasionally trips up on my upstairs off-signatures because it interprets an off-signature on my larger downstairs AC unit as its off ? Maybe work looking closely at one of two of these mistaken off events.
Hour by hour:
Sense hourly vs. Ecobee 5 min.
If I push down into 4 days where Sense seems to lose the AC2 (upstairs AC compressor) detection too early, when my Ecobee is trying to run the upstairs AC for many hours, I can add what my upstairs Ecobee is doing in black and my downstairs Ecobee in orange lines. What I see is that there is a lot of on and off activity on my downstairs AC when my upstairs Ecobee is calling for the the upstairs AC to remain on. Given the proximity between Sense losing the AC2 detection and my downstairs AC turning off, there is an extremely good chance that Sense misread one of the downstairs off-transitions as an upstairs off-signature and ended the detection.
