The Mitsibushi ductless minisplit successfully heated our zone 4b house over some of Minnesotas coldest weather in many years. For a well insulated and infiltration resistant house Mitsibushi Hyper-Heat minisplits can handle the Minnesota environment. If you can stand occasional temperatures in the low 60s over a couple hours a year, these minisplits can work as a sole heat source in this environment.
Our Net-zero home in Rochester MN has R-35 walls, an R-90 attic, windows with R-values around 7 and a blower door infiltration test value of 0.61 so the house was designed to handle the Minnesota winters and be heated solely by a 18000 BTU Mitsubishi ductless minisplit located on the main level. A second 18000 BTU minisplit is located on the second floor but will be used solely for cooling in the summer.
The Mitsubishi models we selected are the HyperHeat variants which function in heating mode to -15F. But how can you get heat from -15F air? Minisplits are reversible heat pumps providing heating or cooling to an internal space. They employ a reversing valve to reverse the flow of refrigerant from the compressor through the condenser and evaporation coils depending on whether or not one wants to heat or cool the interior. In the winter the refrigerant is allowed to expand into a gas (note how cold a dust cleaner can and its discharge gets) drastically dropping its temperature below the outside air temperature. The outside air is then blown over the expanded refrigerant to warm it, whereupon its re-compressed, causing the refrigerant to release its stored heat. Inside air is then blown over the refrigerant and it gives up its heat to the room.
The building inspector did not think these would be adequate for heating the whole house, especially since there would only be a single point source for heat. The rule for passing inspection is “Every dwelling must have heating facilities which are capable of safely heating all habitable rooms, bathrooms, and water closet compartments to a temperature of at least 68F at a distance three feet above the floor level and not closer than two feet from an outside wall, window or door, when the temperature outside is minus 20F.” He was wondering, what happens when somebody closes a door. Your pipes will probably freeze. We therefore had to put in wall heaters in every enclosed room as backup heat.
We thought it highly unlikely that any of our pipes would ever freeze as all of the plumbing is either located inside the house (all supply lines, some return lines) or within the space afforded by the double stud wall (some return lines). However, it does put my mind at ease that we have the wall heaters. In addition guests might want to heat their bedroom if they dont like the 3-5 degrees cooler that the bedrooms maintain over the main room.
So the real test of the system came in late January and early February where there was a polar vortex. There was three consecutive days where the night time temperature low was -15F, then a two day reprieve, then two consecutive days where the night time low was -25F. We exclusively used the ductless minisplits for the -15F span, and for the -25F spans we turned off the minisplits when the temperature reached -20 and set the wall heaters to turn on if the temperature dropped to 60F in any of the outer rooms. We monitor temperatures using homemade arduino sensors. When it was -15F I checked the fin and air temperature outputs of the mini-split. These read 120 and 100F respectively, so I believe this system could heat at temperatures less than -15F, but with coefficients of performance (COP) probably approaching or worse than straight electric resistance heat (wall heaters, COP=1).
The Mitsubishi Hyper-Heat minisplit was able to comfortably maintain a 70 degree + temperature over the -15 degree nights, and we relied mostly on a sunshiny day between the -25 degree nights to charge the home with heat. The temperatures of the main room only dropped to 63-67 in the bounding hours when the minisplits were turned off (mornings) to take advantage of solar heating.
Typically after a solar heating day, where the temperature of the main room and the concrete slab is heated to 75+ degrees, we have a 16 hour window where the temperature is ~77 degrees at 4 PM dropping to ~65 degrees at 8 AM the following morning, where hopefully the cycle can start again if the sun is not obscured or the minisplit can be re-engaged. One way to help maintain the internal temperature is to turn off the Heat-Recovery-Ventilator (HRV). When the temperature gradient is large between the inside and outside of the house or when there are large wind gusts infiltration rates increase and there is less need for a HRV. While the HRV is 75% efficient in retaining the energy of the internal air exchanged with the outside air, when its that cold out the HRV can be a major cause of energy loss. This does cause some elevation of humidity leading to some condensation on windows, even triple pane windows. This also showed us where we have thermal breaks at external door knobs and door hinges.
Electricity Use Over Polar Vortex
We currently do not have the capacity to monitor energy use by the minisplit, but we typically use 20-30 kWH a day depending on appliance usage when the minisplit is not in use. Therefore, during the polar vortex the minisplit used between 20 and 40 additional kWH. Our lowest days were 1/25 and 1/30 when the minisplit was turned off part of the day to take advantage of the solar gain of a sunshiny day.