Installment 3 - On and Off History for Furnaces
When I started looking at the energy histograms for my furnaces, especially my upstairs furnace, I noticed something a little funky. There seemed to be a few spikes on the low end of the graph, indicating multiple energy modes that all deserved attention.
But before I took the histogram at face value, I realized I would have to reckon with another challenge in defining energy modes - resolution mismatch. I remembered that my furnace run cycles were short, never longer than 10 minutes, most only about 5. And they come in two flavors, 1) blower-only, for AC and fan modes, and 2) full furnace mode. I have my Ecobees programmed to run the blower for at least 5 minutes every hour to mix air in the house, unless a heating or AC cycle has already done 5 minutes worth of circulation. Here’s a view of typical operation through both heating and hourly blowing cycles.
So in reality, I have a complex mix of roughly 5 minute cycles being energy-summed on an hourly basis. Plus I’m missing the 6-7 Wh (0.007kWh) idle level that both furnaces use in between blower cycles, because I’m sampling at far too large a time interval.
Once again, selecting the best sampling time resolution is critical to “seeing” the best results. And I’m not able to see the real story of what’s actually happening with my furnaces at the one hour export resolution
I initially premised that one spike might be the minimum, a single blower cycle per hour, while the next spike represented maybe a double. But I was wrong.
When I tried to validate this hypothesis in the hourly data for the upstairs furnace, I encountered a different pattern. The baseline hourly energy usage (1 5-min fan cycle per hour) for the furnace ran at 0.021kWh until mid-day on Nov 12th, then it jumped to 0.038kWh. This seemed very strange until I was reminded that I only installed the smart plug on that furnace on the 12th. I had thought export was only producing the smartplug data, but in reality, export was presenting all the pre-existing data for identified devices, in addition to the subsequent smartplug data.
Plotting the number of “smartplug” devices exporting data each hour, over time, gave me a better appreciation for what I was seeing. This graph shows my current situation with 14 HS110s installed, with one turned off at the smartplug most of the time. 4 of the smartplugs are installed on previously identified devices, including both of the furnaces, which were discovered via the signature of the blower fan alone. The 4 previously identified devices show up in time before any of the smartplugs were installed (pre-existing data). The smartplugs also contribute to the uncertainty in this graph, since identified devices only deliver data during hours when the Sense sees them as active. Sense cannot see the continuous, little-changing “idle” power of these devices. The graph only settles (barring a few dropouts) when the last smartplug gets installed in late Dec., and all the devices provide continuous data stream via the smartplug for every hour.
I also spotted a little surprise on Nov 4th - my smartplug count doubled for exactly one hour. Turns out that is the extra hour from Daylight Savings Time, when we fall back. Real data, but under corner-case conditions.
4) Another analysis pointer - know your data and the changing conditions under which it was collected. More specifically, mixing data collected under two different conditions in a histogram gives misleading results.
So it appears that I should strip off all the data collected prior to the smartplug install for all four of the devices that we detected earlier. If I was really methodical, I might briefly “unmerge” all four smartplug devices from their pre-existing devices, then export to separate them. But instead, I’m going to take the fast and dirty route and strip the preexisting furnace data based on time.
Here’s the original upstairs furnace histogram (again), including legacy data. And below it, the new, smartplug-only, histogram. Notice the disappearing spike at 0.021kWh and the movement of the red triangle local minimums.
And the same for the downstairs furnace. But not much of a difference since there was actually very little legacy data: Sense had lost track of my downstairs furnace blower as we entered the heating season.
So in final analysis, if I was only looking at the export data, without some careful filtering and examination of the higher resolution waveforms, I would have added a “false mode” to my upstairs furnace, and missed the true 7W “idle” state that remained hidden below my hourly blower usage, for both furnaces.