A little help, please.
I have two ductless mini-split heat pumps:
Daikin FTXS12LVJU (about 8 years old, now) Location: Bedroom of about 280 sqft.
Daikin FTX15NMVJUA (about 4 years old). Location: Living Room, kitchen area.
The entire single floor house is about 1250 sqft., so a modest size, ranch style with average insulation. The house was built in the 1960s.
I live in the area of Bangor, Maine where the average Jan-Feb temps are in the 20s F.
Typically we (wife and I) will turn off the HPs when the temp is < 20F and turn them back on when above. That 20F threshold was just a pretty arbitrary number, not supported by any real data.
So I was trying to be a bit more deliberative about how and when we use the HPs vs Oil furnace. From this Reddit community I discovered the Efficiency Vermont spread sheet and entered our oil price, (we have a Biasi 127000btu oil furnace), of $3.10/gal and a couple heatpumps. I can't select two different models so I just used 2x the newer FTX15NMVJUA.
We average around 820kwh/month. We do have 2 Electric vehicles, an EV with a 123kw battery and a PHEV with an 18kw battery.
Ok, so that's the background. I'm trying to find that temperature where it's simply more cost effective to burn oil. The Efficiency Vt tool implies that point is about 29F. This surprised me a bit, becaause the newer Daikin (FTX15NMVJUA) is supposed to be more efficient at lower temps, (or so I thought)
We do have criminally high electricity costs, approaching $.30/kwh. I entered $.024 because we do participate in a community solar farm and it does save us a bit.
So, in short I have two questions:
Does this 29F value seem reasonable? It seems high to me. Oddly if I enter two of the older heatpumps into the tool, the value point drops to about 2F. That seems wierd.
Secondly, based upon some poking around this community I am thinking our whole approach to using the heat pumps may be wrong. Maybe its best to turn them on and leave them at a certain temperature setting, rather than turning them on and off as the temperature flucuates. Is there a best approach?
Posted Jan 7, 2026, 3:11 PM UTC