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"This needs some real data."

I agree and I, admittedly, have none.

I will say, however, that a deeper dive into these efficiencies would also have to take into account the production, distribution and installation costs (embedded pollution, not dollars) of several hundred little devices vs. a single plant that is integrated into each unit during building construction.

Even considering room by room optimization (turning off AC in living room while you sleep, etc.) it's hard to imagine this penciling out vs. operating + embedded costs ...

(and I still suspect that just the operating costs are higher, but your point is well taken ...)

At least one building science expert (Joseph Lstiburek) reccomends a decentralized approach for HVAC, with tightly sealed individual units: https://www.buildingscience.com/documents/digests/bsd-110-hv...

Although I do wonder how that works with something like district heating/cooling systems, which seem to rely on centralized boilers and chillers to move thermal energy between buildings.

A common form of central heating, like burning fossils kind that delivers hot water over insulated pipes to each building, is more efficient in pretty much every way and is cheap to add during building construction than doing it individually. But it's not the case with a "central AC", it's less efficient, more expensive to add during building construction and operating costs are no better. Just think about it, you can't just deliver cold over a pipe and rely on air convection, like with heat, you need to literally move cold air into each room and make it individually adjustable. It's a ridiculously inefficient complicated expensive system requiring extra space and with installation costs higher than that of all AC units individually installed.

Could you have a split AC system with a big centralized compressor?

You could, but chilled water circulation is the more typical heat transfer medium in central, building-wide units. Water can carry a lot of heat, is cheap and readily available, non-polluting, non-toxic, easy to pump, easy to plumb, and liquid at temps convenient for heating and cooling.

The first apartment building I lived in in NY had this. The only downside was that if the board put off maintenance so long that one summer the cooling towers were busted, then they waited until may to start the project to replace them, then an early heat wave came, it could be darn uncomfortable for a week or two. (That did indeed happen, just like I told it.)

I was gonna say, the outlay of capital for a building-wide hydronic system and an evaporative tower, versus each owner be responsible for their AC, I can see why that would be possible.

Particularly if the building is older.

Also, I am curious why there are multiple single condensor units in lieu of multizone units...I guess it is financially easier to purchase one unit at a time.

I know it's only one datapoint but recently we had four indoor airco units installed in our house for the bedrooms and livingroom. I expected a single outdoor unit. However during talks with the installer he advised us to go for two outdoor units. This was cheaper in hardware, a bit more efficient and way cheaper to install by placing one outdoor unit on the front of the house and one on the back so all rooms could be reached within a meter or so from the outdoor units.

For heat pumps more outdoor units is definitely more efficient, since the outdoor unit needs to be big enough to accommodate all connected indoor units at once, and will be on even if only one is in use. Two per unit is my go to trade off on that one, with an attempt to group together units that are more likely to be used at the same time (so no bedroom / living room split, try to stick with bedroom/bedroom and living room/dining room etc).

BTW they have low profile indoor units that can be embedded flush with the ceiling or wall, so the bulkiness indoors is not even required anymore.

those ceiling mount units ... not sized for typical US construction techniques, so installing them in an existing ceiling can be quite complex. If you're doing new construction, it is possible to correctly frame up the ceiling so that installing them becomes trivial.

There are two kinds available currently for ceiling.

The cassette, that would fit into a drop ceiling, and there is also a mini-ducted unit, installed in the ceiling with small-run ducted ports.

My favorite are the ones that look like picture frames.

> > condo in Minneapolis

> This needs some real data.

Minnesota Heating Degree Days[1]: ~ 500

Minnesota Cooling Degree Days[2]: ~ 8,000-10,000

I suspect heat pumps might reach practical limits with cold temperatures.

[1] https://upload.wikimedia.org/wikipedia/commons/d/da/United_S...

[2] https://upload.wikimedia.org/wikipedia/commons/6/6c/United_S...

You reversed the numbers from your cite. Minnesota has much more heating degree days than cooling.

ugh. and now it's too late to fix the post. thank you!

I wonder if anyone has tried to commercialize a device that burns gas for combined heat and power and uses that power to directly drive a heat pump.

Alternatively, ground source heat pumps work decently well even in very cold places, but the installation costs can be high.

> commercialize a device

I think a lot of (utility scale) generators do all kinds of fancy things like waste heat recovery.

https://upload.wikimedia.org/wikipedia/commons/thumb/9/96/IG...

It would be kind of cool to see the same thing trickle down to your home.

Tesla kind of does this in reverse with the model y - it spends electricity parsimoniously, and uses a fancy heat pump to capture or dump heat.

Those SEER 29 units are available but in my short time once selling AC units, most went for the lowest cost unit available. Those cheaper units are not as efficient.

With split units the maximum efficiency attained is in the hands of the customer. Great for customer freedom, but not so great for the environment if the lowest of the low in AC units are available.

Semi-related question: How is fresh air turnover handled in apartments with split AC units?

Make sure if you get a mini-split that you do the following around keeping the condensate drain mechanism going. I have yet to see a mini-split product manual/installation guide that gives detailed information on these operational issues.

Many models let you run the condensate drain on your choice of right or left side. The unused side is often only supplied with a friction-fit drain plug. Over a period of many years, some of these plugs can shrink, not quite fall out, and then you get a real mess.

Make sure the drain pipe is dry. Clean it with denatured alcohol, then let dry again.

Buy a silicone sealant that carries a 20-year warranty. Buy a roll of "cork tape" [1]. Fill about 2.5 cm / 1 inch of the drain pipe with the silicone sealant. Stretch wrap the drain plug with the cork tape, so the tape thins out while sticking to the plug. Pinch/squeeze/roll the part of the tape that hangs out beyond the end of the plug so it is slightly smaller than the stem of the plug. Push the plug into the drain pipe, and try to push it all the way up to the head of the plug. The cork tape will fill out and adhere to the walls of the pipe. Finally, stretch wrap more cork tape (or self-fusing silicone waterproof tape if you don't like the stickiness on both sides of the cork tape) around the entire drain pipe and drain plug assembly, including wrapping the end, and seal up the assembly to the pipe.

Mount a very slightly tilted metal shelf underneath the mini-split air handler (the part that mounts on your wall inside your building), and install an AC-mains-powered moisture sensor that cuts off power to the mini-split when triggered, at the lowest end of the shelf. Mini-splits usually use a zero-capacity drain pan to stay as slim as possible, and if they clog up at the condensate drain pipe, they overflow rapidly (within minutes).

Learn how to take the plastic cowling off the unit before you mount it, document it by video or photographs, and stash it with your product manuals. Mini-split manuals usually don't show how to take them apart enough to get at the condensate drain, and most models I've seen have lots of plastic catches all over that are hard to see when you're standing on a ladder peering between the ceiling and the cowling. If you ever get biogrowth in it, you'll want to know how to take it apart.

Or, if you don't want to take it apart to get at the drain, you can codge together a way to run your wet/dry shop vac to mount to the condensate drain and pull stuff out. I used a shop vac mini kit [2]. I also got a venturi-based device to try to develop more vacuum [3] with the CO2 tank I normally use to make fizzy water.

[1] https://smile.amazon.com/DiversiTech-6-330-Cork-Insulation-B...

[2] https://smile.amazon.com/Wet-Dry-Accessories-VT1215-Attachme...

[3] https://smile.amazon.com/gp/product/B00CIRZGV6/

"Buy a silicone sealant that carries a 20-year warranty. Buy a roll of "cork tape" ..."

What kind of pipe is this ? If it is standard dimension copper pipe, or schedule 40 PVC, etc., why not just cap it with a sharkbite endcap ?

Bonus: cap it with a sharkbite ball valve ...

I'm not sure of the kind of plastic, it usually feels like some kind of HDPE.

I and others have had inconsistent results with SharkBite fittings for long-duration uses, for example see [1]. I suspect their O-ring is a potential weak link over a decades-span. YMMV. I like them for temporary (a few weeks to a few months) scenarios, but I haven't seen enough field experience like with PEX to know its design limits. I might use BoatLIFE Life Seal sealant in the future for the silicone if I run into another already-installed mini-split because I've seen it stand up very well to a massive amount of marine abuse in the short-term (3-5 years), but they don't warranty the sealant for longer than a year because of how much abuse it is normally put through, so I'd just be an n=1 experiment.

Personally, the next time I will install a new mini-split or similar that has such a alternate drain arrangement, I'm going to do the following before the installer arrives. Flip the unit upside down so the pipe points up, drop some JB Weld putty into the bottom of the pipe, then fill the rest of the pipe with West System Epoxy to make it truly permanent and out-of-sight-out-of-mind for any possible extreme lifecycle of the unit.

A friction-fit drain plug pointing out with the pull of gravity in a thermally-variant environment is just asking for trouble. If I had the right tools I'd probably fab up a replacement drain pipe out of stainless steel with threading to accept a silicone plug, and West System epoxy the pipe to the body of the unit, and see how that works.

[1] https://www.contractortalk.com/f9/sharkbite-fittings-do-they...