Making sense of the first month of minisplit heating data
I have 4 Fujitsu mini splits (non xlth) hooked up to (2) 24k condensers. They were primarily for AC but I’m trying to reduce usage of my aging 199k/152k steam boiler while the temps are relatively mild (Boston). Right now I’m using a 12K head per condenser, per floor, so 2 total. Dana had some good advice as the min modulation range was about 6200 BTU/hr per unit, and to run both units, so this weather seems to be the sweet spot for efficiency and load.
11/6-12/5 I used 1107 kwh of electricity. Subtracting out .85kw/hr of household electrical usage, I used .68kw/hr for heating in a 26 HDD period. Both units have been modulating nicely, running almost nonstop. One day i watched the condenser and it ran 13/16 minutes or 80%.
I do feel either the first floor unit can’t keep up with the ~800sq, or just isn’t measuring the temp accurate enough to keep it at a reasonable set point. It’s positioned in the best spot for the layout, however logistically could only be 2″ from the ceiling above a window. The second floor unit does a good job keeping the floor consistent, but heat also rises so it’s load could be less.
I keep the remote at 80 for it keep a setpoint around 69. With the colder nights I’ve kept my boiler on as a backup at 68, and it ran for 40 hours in the same period adding in 8500 BTU/hr @ 82%, not including pickup losses.
Do these numbers seem reasonable for the HDD period for a roughly 2000sq ft home that is well insulated? I thought the splits would be able to shoulder more of the load, or maybe I’m just picky and can’t handle a swing of more than a degree or two. Even at a COP of 4 at temps of 40, that’s only 9200 BTU/hr split over the 2 units? It can’t even go that low, can it?
I’m going to try bumping the thermostat up a couple more degrees to see if I can get my boiler usage down even more. Last resort would be to put in a wired thermostat, but I’ve heard those are pretty pricey installs. The ecobee is averaging a couple of sensors on the first floor within the split zone area, so I know it’s accurate to how we feel.
I’d appreciate any advice or insight.