Interesting Experiement - Furnace Fans and AC

When I reset my Sense near the start of Aug, I decided to try an interesting experiement from two perspectives. My thermostats allow the option of running the furnace fan for a minimum amount of time each hour so I set both units to run for a minimum of 5 minutes each hour. I wanted to to see two things:

  1. Did this help with energy consumption (mostly AC this time of year) ?
  2. Does Sense learn to separate the furnace fans from the AC compressor, or will Sense detect the blended AC/furnace fan combination as a single device ?

So far, here’s what I have learned.

  1. I do use less energy when the furnace fan is turned on regularly every hour, even with the same thermostat settings. Our situation may be somewhat unique, but we typically have cool nights (60s) and warm (75-80) days during summer, but the upstairs heats up late in the day due to stucco and roof heating from the sun (even with heavy insulation). We can open some windows / doors during the day when the outside temp is below the set point (74), but it’s painful to open windows in some of the rooms due to blinds/furniture, etc. The fans do a fair amount of mixing throughout the house, plus between the cooler downstairs and warmer upstairs, postponing the onset of AC.
  2. Sense has learned both furnace fans and has learned the upstairs AC so far. It meticulously detects the standalone fan every time, plus picks a few stray AC activations as “Furnace Fan”. But most of the time it detects the AC compresser plus furnace fan together.

Hi @kevin1,
My Nest Thermostat has the option (they call it airwave) to run the AC fan for a few minutes after the compressor is shutdown. This is supposed to use up some of the left over cooling in the evaporator(cooling unit). Since 90+% of U.S. ACs are over sized, it could help with them not running long enough(Short cycling).

I think the Next has the option to run every hour during a set period during the day. The only issue for me is that the shortest time is 15 minutes. I could see doing 5 minutes every 30–60 minutes but not 15 minutes per hour.


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Hi @jonhawkes,

I used to have Nests in my house, but for various reasons I eventually swapped out for Ecobees and moved the Nests to our vacation place. The main motivation was that the Ecobee let us add additional sensors in the bedrooms and smart vents that open/close in coordination, depending on individual room temperatures. I liked the Airwave option - Ecobee has something similar called Fan Dissipation.

You can see the hourly circulation cooling down the aggregated upstairs temperature, without any compressor/cooling needed, in this graph from Ecobee’s web app (the little drops in the white line)
. The white bar at the top shows that the compressor/cooling is only needed when my wife manually turned down the thermostat.


Hi Kevin,
Is your AC a large one (i.e. 5 or 6 ton)?
The power usage is similar to my crappy 15 year old unit.

The Ecobee looks nice. I would like to see my Nest usage on the same day.
I think I saw something like it on “This Old House” (also where I saw the sense first).


Hi @jonhawkes
One of the main reasons for running a fan after the Compressor has shut down is actually a method of drying out the evaporator & pushing the rest of the tempered air out of the ducts. Unfortunately mold growth is a common issue on evaporators because of the dehumidification process. I personally think it’s a good idea to run the fan for five minutes longer after the AC has shut down for this reason alone.

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I have two ACs, one for each floor. Downstairs is around 2500 sq ft and has a 4 ton unit. Upstairs is about 1500 sq ft and uses a 3 ton unit, but of course both floors are connected by a 2 story entryway that is close to both thermostats. Both units are 20 years old, but outside of a few runstart capacitors, haven’t required any major repairs. We live a place with moderate temperature swings with only a few 90+ days per year and relatively low humidity. The downstairs unit is seldom on, except on the hottest days. The upstairs, far more often.

Interesting discovery today… I had a mystery Motor 1 that Sense would occasionally detect as a 1-2 second power spike in the 3500W range, maybe 10 times a day. After turning on “ON notifications”, I figured out that the spike correlated with my upstairs AC turning on, a device that Sense had already “found”. I quickly did a merge of the AC Spike and AC devices, and now have a composite that looks like this…


I have a similar issue as well. My Condenser Fan Motor is picked up by Sense at its initial start for about the first 1-2 seconds and then appears as off. I’m certain of what it is, but it’s only being registered when it starts. So I’m not sure if I should merge it or delete it. I’ve notified Them of the issue, I guess I’ll just wait and see or maybe merge as well and cross my fingers.

I have an EcoBee 4 with 3 remote sensors; but I’ve started to do the same thing; in fact, if you run them longer at night, I’ve even seen it help more during the day. Having a thermostat (or at least a senser on each floor) all tied to a single controller also makes huge difference.

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