We heat and cool our home with Fujitsu `ductless minisplit` air source heat pumps. But this is New England, and it’s winter. A common question is how do they do under winter conditions?
Well, today we are treated to the increasingly rare Massachusetts blizzard, a nor’easter:
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The storm producing this looks like this in EarthWinds:
It’s fed by excessively warm and moist conditions off the Northeast coast, themselves due to radiative forcing from fossil fuel emissions:
We have three minisplits. They are working fine. During a blizzard a little care needs to be taken to be sure snow does not pile up and cover the fan or the vanes which do the heat exchange, and sometimes these need to be brushed off. However, if the splits are operating at full, their built-in defrost does a pretty good job of taking care of this. They’ll continue to operate down to about -15℃ whereupon the heat pump will simply shut down. Below -12℃, efficiency drops markedly. This is a rare event, and is becoming increasingly rare. Right now, it’s about -8℃.
Should that occur, we crank up our now-off-and-orphaned oil furnace for the short time we need it, and shut it down when reasonable temperatures return.
And, should power from the grid go off — the power we’ve sent them during generation by our solar panels for the remainder of the year — our backup emergency propane generator kicks in, and can power essential elements of our house, including the oil furnace, but not the minisplits. Again that is a very rare event.
The generator (above) also needs snow kept clear from it.
By the way, here’s another shot of our beloved Rock Meadow Brook marsh, replete with Canada Geese, sheltering, and occasionally erupting in a burst of honks.
The photos above were taken just at the beginning of the blizzard. It’s good to have an after-the-fact set to compare, and to illustrate a feature of the Fujitsu minisplits. Here they are in their post-blizzard conditions.
Now, the heat exchange on the ductless minisplits happens when air passes over a fine array of thin fins at the backs of the units. The fans in the front draw air across the fins, and either heat is dumped into the air, for cooling, or extracted from the air for heating, as now. The feature of air source heat pump technology is that this heat can be extracted down to the -15℃ temperature mentioned above. All air temperature have some heat in the air, and it has nothing at all to do with differential temperature between the interior of the house and the outside.
However, in the case of inclement frosty weather, it is possible for snow to build up on these fins, interfering with their ability to exchange heat. Accordingly, because the Fujitsu units are designed to operate under such conditions, when it is present, they undergo a periodic defrost. You can see such frost on the fins of minisplit number 3:
I was fortunate enough to be outside when a defrost began on unit 3, and you can see the ice melting and puddling below the unit here:
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Minisplits 1 and 2 had already gone through such a defrost, and their fins were clear:
Naturally, this costs energy — electrical energy — but over the year it is not needed very often, and, so, the efficiency of the heat pumps, averaged over the year, remains a big win. Here’s a report from Sisler Builders in Vermont regarding their experiences.