Visualizing mini-split performance data
I’ve been digging through Mitsubishi M-Series performance data for a project I’m working on. I made some graphs of the capacity and efficiency of these ductless mini-split heat pumps. I figured I’d share the graphs here, since many GBA community members are interested in mini-splits.
1. Capacity and efficiency depend on the indoor and outdoor air temperatures. (See graphs #1 and #2.)
2. Efficiency also depends on how hard the heat pump is working. Efficiency degrades at very low load, when the heat pump starts cycling off and on rather than modulating continuously, and at very high load. (See graph #3.)
3. Maximum capacity is significantly higher than rated capacity. For heating, maximum capacity is typically 150% to 185% of rated capacity; for cooling, 110% to 150%. (See graph #4.)
4. Minimum capacity varies by model. For heating, minimum capacity is typically 15% to 30% of rated capacity; for cooling, 20% to 40%. (See graph #5.)
By capacity, I mean output thermal power (in kBTU/h). By coefficient of performance (COP), I mean the ratio of output thermal power (converted from kBTU/h to kW) to input electrical power (in kW). COP is a dimensionless quantity.
By nameplate capacity and COP, I mean the capacity and COP that the manufacturer reports at the specific test conditions of 60 F indoor/47 F outdoor (for heating) and 80 F indoor/95 F outdoor (for cooling). By rated capacity, I mean the capacity measured at given indoor/outdoor temperatures. Rated capacity is less than maximum capacity. By rated COP, I mean the COP measured at rated capacity at given temperatures. Rated COP is usually the highest COP the heat pump achieves at given temperatures. In other words, rated capacity is an efficiency ‘sweet spot’.
The primary source is Mitsubishi’s reported performance data, pulled from their 2019 submittals, their longer technical specifications, and/or the NEEP “Cold-climate air-source heat pump specification product listing” spreadsheet. This data is for the 6, 9, 12 and 15 kBTU/h Hyper-Heating models. I focused on this manufacturer and product line mainly because the M-Series is popular, efficient, and known to heat well in cold climates. (Also to keep the project size manageable.) I sanity-checked against Jon Winkler’s 2011 report, “Laboratory test report for Fujitsu 12RLS and Mitsubishi FE12NA mini-split heat pumps”, and various real-world monitoring studies (Roth et al. 2013, Faesy et al. 2014, Ueno et al. 2015, Williamson et al. 2015, Korn et al. 2016, Sutherland et al. 2016). I can share data or link to these studies on request.
There isn’t much empirical data on COP while cycling at very low load. The best information I could find was from Winkler 2011, who found in two heating experiments that the 12 kBTU/h model’s COP degraded by 10 to 37% when it started cycling. The lines below 20% capacity in graph #3 are not precise. I’d love to see more data on this.