ductless units – appropriate minimum cooling output in heating climate (zone 5)
Hi, I’m designing a ductless system whose main purpose is to be the primary heating source in climate zone 5. I of course also plan to use it for cooling, but our heating season is 8-9 months long while we only really need our AC (currently, just window units) for 3 months. My decision-making has been primarily driven by what heating loads are required in which parts of the house (I did my own Manual J on Cool Calc). In some areas of the house, if I select a slightly larger ductless wall unit (ie: the 15K hyper heat Mitsubishi wall unit, or the 18K Mr Cool DIY wall unit), this will allow me to use one wall unit instead of two in that area and therefore save money on equipment costs. However, one potential problem is summer cooling: both these units only modulate down to around 6,000 btuh min output in cooling mode. So my concern here is that if I set and forget these units in the summer to whatever cooling temp (say 70F), they would basically be as energy efficient as a 6,000 btu window unit that’s been left on 24/7; in other words, leaving a 15K Mitsubishi hyper heat wall unit with a min cooling output of 6,000 btuh turned on 24/7 in the summer would be more or less comparable to leaving a 6,000 btu window unit AC on 24/7 in the summer. Right? Or am I misunderstanding something here? Thanks.
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If the load is less than what it can do at 6000 BTU/h, it will cycle on and off. That's not ideal, but it won't overcool or consume energy like it would if it just kept cranking at 6000 BTU/h.
One option for the summer is to shut off all but one unit. You'll overcool that area and undercool the others, but you'll be removing humidity, which is the most important part, and the lowered humidity will spread around well.
Most mini splits use nowhere near the electricity consumption of a window unit at low speed. My 12k Gree Sapphire only uses 80 watts at minimum (2900 btu) cooling capacity at 95 degrees with a cop of 10. An average 5k window unit would use over 500 watts.
I have a real time electricity monitor on both my splits. The Gree uses around 100 watts at its minimum cooling capacity when factoring in the fans and electronics. Even if it ran for 24 hours straight it would use like 2.4 kw's which would cost like .45 cents here in CT with some of the highest electricity prices in the US.
https://ashp.neep.org/#!/product/25406
By default they will keep blowing air, but the outside compressor will turn off once the set point is reached. And the fan is not a big energy consumer. You can have them stop blowing air as well by setting the "thermal off" setting (which varies by model but can involve clipping a resister on a circuit board).