Heat Pumps’ COP > 4
Could this strategy improve efficiency?
Heat pumps are more efficient at the lower end of their output range (and less efficient at their max output), but if you size them too large, their minimum output would exceed the required load of a space and you’d get cycling (they start and stop) – which is bad.
So why not have two heat pumps for a single zone? One can be sized to produce the needed heating load for mild temperatures (say 47F) and the other sized for colder temperatures (5 or below). You only use one at any given time and each would operate at the low end of their range, which would be at very high efficiencies. I estimate COP’s in excess of 4 across an entire cooling season if you combined two or more in this way.
Aside from the setup costs, why couldn’t this work?
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Replies
depends on how expensive it is to install 2 heat pumps (in an ersatz staging setup)
and depends on your energy costs.
also illustrative is some temperature bin analysis. if memory serves, even in Montana roughly 80% of heating hours are closer to the 47 deg mark than the 17 deg. your comparison is between a heat pump and some other supplemental heating source (which could be another heat pump or strip heat or a combustion source like gas or wood or whatever)
adding another heat pump may be more efficient than electric strip heat, but is it worthwhile from a lifecycle cost? maybe it is... maybe it isn't.
or maybe that's not your primary motivation.
I haven't analyzed it rigously for my house, but I have a wood stove for my backup heat source when the heat pump can't muster enough btus on its own.
Aside from the setup costs...
To complete Malcolm's sentence: Aside from the setup costs, there are the upfront carbon releases associated with manufacturing twice as many minisplits as needed.
It's hard to imagine the energy savings, even over say, a 15-year heat pump life, would ever offset the initial cost of installing 2 units instead of 1.
I could imagine it...
The energy transition will weird all of our intuitions. This should be easy enough to do a quick spreadsheet analysis, but I don't have time to do that right now.
To give two examples, a Mitsubishi FH12 ductless has COP of 4.31 cooling at 95 degrees at min capacity, and COP 3.47 at max capacity. In heating mode at 17 F, COP is 3.08 at min capacity, and 2.1 at max capacity. (ALL DATA FROM neep.org)
A Trane XV20 ducted is COP 4.06 min capacity cooling @ 95, and COP 3.96 at max capacity. COP 2.38 heating min capacity at 17 degrees, COP 2.27 at max capacity.
For most units, the difference between min capacity and max capacity is not that massive. To take just the Trane example, a second unit would probably add ~$12,000 in costs over a single unit. Say it lasts 15 years. That's $800 per year or $67 per month. Not to mention, probably a few hundred per year in extra maintenance costs.
I don't think a COP difference of even 1-ish at the minimum capacity versus the maximum capacity is going to account for anywhere near $67 per month for the vast, vast majority of people.
In many climates with decently built homes and reasonably efficient systems, folks may not be spending $67 per month TOTAL on their heating/cooling energy use. (In my ~3000 sq ft average-built house, I spend about $50 per month on cooling in the hottest month, and maybe $100 per month on heating in the coldest month). I suppose if you are in Maine or Phoenix the math shifts a bit. Perhaps in a really fringe scenario the two-system setup would ultimate be cheaper but at least for me I still find it hard to imagine that being the case for the vast majority of houses/climates.
All of that said, for two story houses, one system for each level makes a massive comfort difference assuming the systems are sized properly and, in my opinion, is well worth the extra cost. But OP is talking about two systems for a single zone. The only meaningful benefit I see for that scenario is redundancy.
The problem with this plan is when you see the super high COPs in the cooling mode the unit is removing almost zero latent heat (moisture) from the air as the coil temp is far above the dew point. Since you are not removing much moisture from the air the relative humidity will be uncomfortably high and the occupants will lower the set point of the thermostat until they are comfortable.
Walta
Indeed walta. A strong consideration.
since we don't have a climate zone in the OP, it's hard to evaluate whether there is a latent load to consider. And, this could be readily managed outside of the heat pumps themselves