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Comment by Liftyee | Hacker News Reader

I don't get how so many large buildings in NYC and Beijing (two cities that I've personally visited) use independent little AC units like the PTAC or minisplit types. A couple per apartment, peppered over the outer walls. Engineers intuition says surely a large central unit would be more efficient and effective.

Is it because of personal ownership reasons? Counterintuitive physics? Heat dissipation limitations on the roof?

Can someone with more knowledge clue me in?

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There are no air ducts to move air between the units. They are old brick buildings and retrofitting central air you might as well tear down and rebuild a lot of the time.

Which of course means they should use heat pumps, since the copper lines don't need nearly as much room (I suppose you'd still have to put a drywall wall in front of one brick wall potentially). Even just putting a bunch of mini-splits on the roof (instead of individual window AC units) would be an improvement, seems like.

The only difference between a heat pump and an AC is a reversal valve to change which direction you’re pumping the heat in. (Of course obviously an AC fundamentally is a heat pump, but setting that aside.) That makes it very hard to puzzle out what you mean by this comment.

Yes, mini-splits are a good retrofit option, and they are efficient as well. They are more work to install than a PTAC, though, so cheap landlords pick PTACs to reduce labor cost.

PTACs are self contained and so they don't even require an HVAC technician to install. Simply knock a hole in a wall and plug it in. They are basically a permanent version of a window AC unit.

I had the option to buy a “central” unit or several mini splits for my apartment. I chose several mini splits. The main reason is that they can fail independently, so I’m not stuck in an apartment that is hot everywhere.

But I think the main reason is that in places without central AC, people tend to want to incrementally upgrade. For example, you might get a mini split just for your bedroom, because that’s the room you require the most comfort during the hot days. Then when you start getting used to the comfort of the AC in the bedroom, you’ll want one in the living room. And so on.

I went with a multi-split central AC: despite the higher cost, it is generally a usability mess.

You can only run one mode for all rooms, and while that sounds reasonable, I have one tiny storage room that houses my old dual Xeon heater, which requires AC on even in the winter.

The main reason is that landlord only pay for the installation while the tenant pay the electricity bill. Landlord has no incentive to invest more than the minimum.

I think what also plays a role is that this way each tenant gets charged for AC use through their own power bill. No difficult central accounting to do.

Still, split units are way easier and more efficient but I guess the landlords just go for the cheapest option.

Not in US, but on top of no room for air ducts, running long copper lines (for both mini split and multi-split units) tends to be expensive (price of copper). You also need to guide condensation out, so either downward piping or (expensive) water pumps everywhere. Installation ends up being more expensive than the AC system.

They are cheaper and easier to retrofit in existing buildings. The landlord pays for the installation of these units, but typically not the cost to operate them, so they are primarily concerned with optimizing install cost and have little to no concern for efficiency.

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