I had my furnace replaced this week. The old one was 18 years old (from 2002), and had a rated efficiency of 94%. The new one is rated 96.5%. Since the efficiency is so close, I wasn’t sure I’d see much difference. This is my attempt to quantify the change.
The new furnace is more electrically efficient, although I’m not sure if there were jumper settings in the old furnace that could adjust (for example) fan speed when idle.
The percent heating duty cycle is also slightly lower for the new furnace (61% vs 67%). Meaning on a day where the exterior condition was about the same, it took 6% less heating time to maintain the same indoor temperature. But it’s also very easy to mis-analyze percent duty cycle, since a 6% swing only represents 36 minutes, and it’s easy to catch or miss a heating cycle at either end of the analysis period.
I tried to pick a time period between 10PM and 8AM because it is most likely to be a consistent comparison (not using the oven, similar misc heating sources).
My setup: I have a wifi Radio Thermostat with an open local rest API to obtain thermostat conditions. That’s how I’m getting the heating/idle duty cycle, and this data is push into InfluxDB and visualized in Grafana. I’m using CircuitSetup’s current monitoring board to read energy usage on multiple branches of my electrical circuit breaker, and this is also being pushed into Influx.
I noticed that the new furnace short cycles much more (49 times vs. 37 transitions on the old furnace). I’m not sure what that means. The difference may be due to me disabling the second stage on the old furnace (blower fan starting to go bad).