I’m now the owner of a two-headed three-ton ductless mini-split.
And lovin’ it.
Pictured above: Those boxes, on the wall, are the inside portion of my new ductless mini-split heat pump.
They blow hot air. They blow cold air. When you ask them to. One per room is adequate, unless you dwell in a house of unusual size. And one per room is required, if you want heat (and AC) in that room.
The outside, it’s about as attractive as you would expect, for a heat pump. Slightly uglier than a standard system, owing to the need to run refrigerant and electrical lines up the outside of the house, to each of those interior boxes, connecting to the compressor unit at ground level. Here, the line covers (the things that look like aluminum downspouts) blend right in with the electrical, phone, FIOS and whatnot already hanging off the vinyl siding at that end of the house.
For me, they were less expensive than my closest alternative. I paid $13K installed, $11K after the Federal tax credit. If I’d replaced my dead ground-source heat pump, it would have been $25K, $17.5K net of (a much higher) tax credit. I picked the contractor based on reputation, on a friend’s recommendation, and on the fact that we were immediately on the same wavelength, regarding a nice, simple installation.
Mine is a three-ton (36000 BTUH) compressor, hooked up to two 1.5 ton heads. Took a day and a half to install, but the HVAC guy was training a couple of guys, so a one-day install would not have been out of the question if I’d been in a hurry.
These Mitsubishi units will produce heat down to 5F. Below that, I think they automatically shut off. At 5F, their heating efficiency will have fallen to the point that they were merely twice as efficient as an electric space heater (COP 2.1), instead of 3.5 times as efficient (COP 3.5) at 47F.
The same company (Mitsubishi) that makes these makes a “hyper heat” compressor that will run at outside temperatures down to -13F, but the outside pieces (the compressors) for those run about twice the cost of a standard compressor, near as I can tell.
We got a standard unit that’ll only function down to 5 degrees F. But, thanks to global warming, that should do us, as this decade’s USDA hardiness zone maps bumped us from Zone 7A to 7B, meaning that 5 F is not a bad guess for the lowest temperature we’ll ever see here, ever again. And if not, the old gas-fired hot water baseboards still work.
On paper, they’re as efficient as any other option I could get, including ground-source heat pumps. I did an entire post on that earlier (Post #2032). Once I got up an apples-to-apples comparison, the near-equality was obvious.
For typical winter temperatures around here, these ought to run at around COP 3.5, or, as noted, three-point-five times as efficient as an electric space heater. In practice, they look like they’re going to do even better than I thought, owing at least to their “inverter” continous-speed technology.
I get the distinct impression that these heat pumps, with their variable-speed compressors and blowers, like being operated for long periods of time, at a relatively low load. So I’m rethinking my life-long habit of nighttime temperature setbacks to save energy. Don’t now what the outcome of that analysis will be.
The only thing you really need to install one is a place to put the compressor (near where the inside heads are), and some way to run a 220V electrical line there. The electrical power for the interior heads is routed through the outside compressor unit. So the electrical lines for the interior heads run right alongside the refrigerant lines.
And you need to have an empty 220V breaker in your electrical panel, of about the right size. This three-ton unit needed a 25-amp breaker. The installer just pulled the wires out of the breaker for the now-useless ground source unit, and put in the wires powering the new one. The legally-required outside electrical cutoff comes with surge suppressor built in.
The refrigerant is R410A, which seems to be the only option for any HVAC equipment that I could buy. It has greatly reduced ozone-depletion potential relative to old-school Freon (R22), but it’s still a potent greenhouse gas. Less than ideal, but that’s what commodity heat pumps are using.
Some background follows.
Ductless mini-split. A product clearly named by its competitors.
It’s just a normal heat pump.
The best way to describe it is by contrasting it with “normal” central heating/AC. For a single-family home, the typical central heat/central AC set up is:
- one hot/cold coil in the house
- sitting inside a blower housing
- typically in your basement or attic,
- blowing conditioned air into your ducts, and from there to all the rooms of your house.
A mini-split is decentralized heating and AC, as far as the inside of the house is concerned.
- one hot/cold coil in each room,
- sitting in its own stand-alone blower cabinet,
- blowing conditioned air directly into the room,
- and so cooling or heating that one room.
Outside, by contrast, a typical mini-split system is more-or-less the same as central heating systems. For reduced installation cost (and, I’d bet, better operating efficiency), you run multiple mini-split “heads” off one outdoor compressor unit. This then requires you to run multiple sets of refrigerant lines to that one compressor, as opposed to the one set that would run to the compressor in central (ducted) forced-air system.
You pick the combination of outside compressor and inside head(s) to give you what you want. My outside three-ton unit can take from two to four heads, as long as the total “ton” capacity of those heads is three tons of cooling (36000 BTUH). Available compressors and heads both span a range of tonnages.
Nothing about the name “ductless mini-split” is even the tiniest bit helpful in understanding what it is.
First, it’s not mini. It’s a full-sized HVAC unit, in terms of heat and cooling capacity. Ours is a “three-ton” unit, meaning 36,000 BTUs/hour cooling capacity. This is roughly the same as the heat pump it replaced. In this climate, that’s enough for a small house, or, in our case, the first floor of a larger house.
The only “mini” part is that these units have compact, quiet outdoor part. Two strong men can lift and carry one, and just bolt it down in place, as a unit. So they are mini, in that the outdoor (compressor) piece of this isn’t a big louvered metal box that makes a hellacious racket when the AC or heat pump comes on. Which is the tradition in the American ‘burbs.
Second, split just means “normal”. As in, there’s an inside part, and an outside part, and some refrigerant lines and wires connecting the two parts. Just like every suburban home central AC that you’ve ever seen.
Arguably, the only AC system you’ve ever seen that isn’t a “split” unit is a window air conditioner. There, the outdoor and indoor components are in the same metal box. But any home central AC you’ve ever seen is a split unit.
(And I now know why “split” is even a thing, for HVAC contractors. For ground source heat pumps, you actually do have an alternative to split units, called “package” units, where the heat pump and the air handler (that blows the heated or cooled air into your duct work) are combined in a single box. These units get a generally higher efficiency rating than split units, but I never did decide how real that was. It almost seemed as if that might be a side-effect of the way these were tested. Anyway, split versus packaged matters for ground-source heat pump options, but pretty much for nothing else having to do with home HVAC.)
Third “ductless” just means that the part that blows air around your house is a self-contained blower unit. Typically, the “head” resides in a plastic cabinet, mounted high on a wall, as above. But you can get them as console-type units. Or as units that recess into the ceiling. (And there are, in fact, ducted versions of these, where they are set up to be connected to a set of duct. But that’s not their selling point.)
Not pushing air through ducts gives these a modest energy advantage, all other things equal.
But a downside of “ductless” is that there’s no air return in this system, as there is an a traditional central forced-air system. Air does not flow out of the ducts, toward the return. It just flows out into the room its in. That limits these to heating essentially one open space per head. OTOH, these units seem to have no problem throwing warm air for a considerable distance.
It’s a different heating and cooling gestalt
In the typical central forced-air heat or AC system, you blow air out the ducts, and suck it back up in some distant central air return. This means that air flows through all the rooms, and you can count on that air flow both to mix the air, and to make the conditioned air travel the full distance from duct outlet to return inlet. Ideally, the result is a uniformly heated or cooled living space, and no apparent breezes.
A ductless mini-split is a different beast entirely, and no nearly so elegant as central forced air.
There’s no air return. This limits the “throw” of each unit to the space in which it can manage to blow its warm or cold air. The result is that a) you definitely get a warm breeze if it’s really cranking, and b) the air temperature in the room is not as uniform as it is with central forced-air systems.
Neither of which bothers me in the least. I actually like both aspects. And mine are strong enough to heat the living-dining room from one end to the other, which has to be 30-some feet.
The upshot is that if you want nice, uniform, breezeless heat throughout a room, these are not for you. As far as the user is concerned, these are more like having the best space heater you’ve ever used, hanging on the wall of your house. It works well, but not as nicely as a properly-configured central forced-air system.
(N.B., a 1.5 ton head is 18,000 BTUH, which equates to the heat you’d get from about a 5000-watt resistance space heater, where the biggest plug-in 120V space heater you can buy is 1500 watts. The astute reader will recognize that 5000 watts at 120V would be … way more electricity than the entire compressor uses, if it sits on a 25-amp breaker. Which is neither a violation of the laws of physics nor magic, but the whole point of using a heat pump, because the coefficient-of-performance (COP) is bigger than 1.0. You do, in fact, get more heat out of it, than is embodied in the energy it takes to run it. Because it’s not a heater, it’s a heat pump.)
The back story
In 2004, the previous owner of my house had a ground-source heat pump system installed. Mile of pipe buried in the back yard, hooked up to two heat pumps, retrofit to the existing duct work, using the existing hot-water gas-fired baseboards as secondary heat for the heat pumps.
(Secondary heat is the additional heat source the system turns on if it looks like it’s taking too long for the heat pumps alone to bring the house up to temperature, or maintain temperature.)
In 2007 we bought the house thinking, neat, that’s the most efficient heating we can get. Only after we lived with it for a while did we fully appreciate what a botched Frankenstein’s monster our heating system was.
This 1959 home was an energy use nightmare. (Ah, bad dream, maybe.) Rather than insulating the attic and ceiling spaces, or dealing with the huge air leaks, or the (believe-it-or-not) uninsulated walls, or the four big open fireplace chimneys, … the previous owner figured he’d just install his high-tech redneck heating system and be done with it. And botched that install in several important ways, to boot.
For example, I ask you, what sane home buyer looks at a nice new high-end kitchen remodel, stone countertops, cherry cabinets, stainless high-end appliances and says, that’s a very nice kitchen, but, does that come with heating and cooling?
Because in my case, the answer was no. They tore out the old hot-water baseboard heat and, to a very close approximation, replaced it with nothing. Which, as the owner, you don’t really figure out until winter sets in, some time later.
What sort of person would do that, in their own kitchen remodel? The same sort of person that did all the rest of the HVAC system. Near as I can tell, absolutely no aspect of it was planned or executed well. Let me omit the rest of the kvetching by saying that it has never worked well. And the kitchen is always ass-freezing cold in the winter. And that, upon closer inspection, that last problem seemed un-fixable.
So we let Frankenstein be. And did a lot of baked goods in the winter. Slow-cooker recipes. And so forth.
FF to 2024. The ground-source heat pump for the first floor is dead,and the one for the second floor is failing. And I wasn’t shedding tear over its demise, given the miserable performance.
Until, that is, I found out it was going to cost me $50K to replace them.
But there’s nothing like a pending major expenditure to sharpen your focus, if not your wits. For various reasons, my wife and I decided that it was stupid to spend $50K to repeat the previous owner’s mistakes with fresh equipment. And that maybe, just maybe, for the first time since we moved in, we could actually have a warm kitchen.
Our solution was to ignore the dead ground-source heat pump, ignore the grossly under-sized duct work, the cobbled-up secondary heat, the @#@#$ wireless thermostat that worked sometimes. Rather than try to work around the same problems that the last guy was unable to fix, we decided to start fresh with an air-source ductless mini-split that had nothing whatsoever to do with the existing Frankenstein of a system.
And that’s how we ended up here.
Winter approaches. 
