Thread by @TomMoyerUT

I've got some heat pump #EnergyTwitter data to share. I monitor the electrical usage of my heat pump, but I haven't been able to measure the amount of heat it's producing, which means I haven't known how efficient it is.
It turns out that I was running it really badly. A 🧵

I've had the fan set to "auto" and a "set it and forget it" thermostat program. That's the usual advice to keep the heat pump running in the middle of its range.

The dots are 20-minute snapshots in time, with the heat pump running more or less steady state. 2/

Manufacturer's specs are adjusted for my altitude (6,700 ft, 78% of nominal).

We have three mini-split heads, of which we only ever use two for heating. "Auto" and "Max" are fan speed settings. "One" is just the living room. "Both" is running the living room and basement. 3/

Just because the manufacturer rates a heat pump at a certain coefficient of performance in the lab doesn’t mean that’s what you actually get.

It turns out that my heat pump hates operating at low output and it’s much more efficient when it’s driven hard. 4/

Because we only use two of the minisplit heads, running them both at maximum is only 72% of the compressor’s rating. Running just one of them in “auto” fan mode when it’s just maintaining temperature means the compressor is only running at about 20% of its rating.
5/

Here is what I’m using to measure the heat output. You need temperature and humidity on both the input and output side, combined with an air flow rate. The air flow is the tough part. My guess is that I’m only accurate on that to +/- 30%.
6/

Here is a more elaborate setup to accurately measure minisplit flow rate, along with daily COP measurements - from this paper.
h/t @JohnSemmelhack
www1.eere.energy.gov/buildings/publ…
7/

More good COP measurements in this paper.
Measured Performance of a Low Temperature Air Source Heat Pump
nrel.gov/docs/fy13osti/…
8/

And more evidence here that real-world heat pump performance falls short of manufacturer’s specs.
docs.google.com/document/previ…
9/

For the last week, I’ve set my heat pump to maximum fan speed, and running it mostly morning and evening, with a setback at night and during the day. Compared to last year (blue dots), that has reduced electricity usage by 30% (or increased COP by 43%).
10/

I'd love to have feedback on this from those with more heat pump data and experience than I have. @JohnSemmelhack @energysmartohio @kevinkircher @Guay_JG @krutoypotsan @xiaowang1984 @briansiana @goldenmatt @spantano @EfficiencyLast @JohnSmillie42
11/end

One thing I forgot to add - I don't think this can be generalized to all heat pumps. I think there's good reason for most people to avoid setbacks. Running at 100% output is also not good for efficiency, and lots of people have built in resistance heat that kicks in when needed.