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Comment by vladvasiliu | Hacker News Reader

I agree with your other points, but does actually each unit need a dedicated AC?

I live in a building with district heating, we all get the same hot water. If I don't want to live in an oven during the winter, I'll turn down the heaters' thermostats, which lets less hot water in.

Couldn't we have a similar thing for cold air in the summer? Send the same temperature to everybody, and allow people to choose how much of that air they allow in their units. Shouldn't this also be more efficient than a dedicated AC unit per home?

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I lived in NYC. Problem with non-individual pipes is that you get Marijuana smoke and cigarette smoke from neighboring apartments to yours. Also if there are multiple units, there is always plausible deniability on who the culprit is.

This is real and very stinky.

Wouldn’t the air pressure keep whatever smoke is in an apartment outside the pipe ? So that would prevent it from traveling from one to the other.

I used to live in a high rise which had a common pipe for extraction, I never had any odor coming in, and I know my neighbors were smoking since I would get smoke if the windows were open.

Yes, depends on how much air pressure there is. The downside to too much air pressure out is that you also suck out valuable cold air (in summer) or hot air (winter.)

If the air pressure isnt sufficient, you get some smoke coming thru, esp if the pipes are horizontal. Also depends on how much people spoke and depends on how many units the exhaust travels past before being sucked up into the building-top exhaust.

This is called an FCU (fan coil unit) and is common in Asia. It consists of a high efficiency central chiller run by building management, and pipes to bring coolant to each unit. Then in each unit there is a heat exchange coil with a fan to blow the room air through the coil. Hence fan coil unit.

The advantages of this system are that the only sound in the unit is the fan, and air is not circulated between units. The disadvantage is that building management can turn off A/C centrally if they want to save money.

For some reason Americans are slow to pick up HVAC innovations that are common elsewhere: heat pumps, split-system air conditioners, FCU, etc. I guess it is because energy is cheap to them and they don't mind noise.

> For some reason Americans are slow to pick up HVAC innovations that are common elsewhere

Few want to spend the money to convert older buildings. That includes homeowners, building owners, and condo/co-op boards.

Someone a few weeks ago posted a long essay about how much heat pumps make sense from an investment and environmental standpoint. It glossed over the fact that most American middle income households, when presented with the choice of dropping $25,000 on a heat pump/mini split or sticking with window mounted ACs and that cost a fraction in terms of up front costs, will go for the cheaper option ... or spend the money on some other home improvement or accessory like a car.

Not saying it's right, but that's the way it is.

But why would this be any different in America than the rest of the world? What you've written seems to me like a universal.

It's more expensive in America - larger houses requiring larger units and more ductwork/labor, and probably the manufacturers pricing for the world's richest market.

One of the big questions that I haven't seen a compelling answer for re: heat pumps in the US is why heat pumps are so expensive compared to AC exchanges. The amount of equipment differences between an AC and a heat pump are largely a valve to reverse refrigerant flow and the small bit of electronics to control said valve. Yet heat pump units in the US are significantly more expensive for effectively the same COP and operating efficiency ranges as their cooling-only brethren.

We do have ductless AC in the US. It's technically a bit different but accomplishes the same thing using the same principles.

Of course, most of those systems are made by Japanese companies like Mitsubishi...

How does ventilation (the V in HVAC) work for FCUs? If the pipes only bring coolant to each unit where does fresh air come from? Conventional HVAC systems have Heat Recovery Ventilation so what's the equivalent here?

I don't know. I don't think that most Asian apartments have central fresh air. Usually the unit owners open windows regularly.

But opening the windows would negate most of the benefits of the higher efficiency of the FCU, would it not?

Heat pumps (for heating) and split air conditioners are fairly common in the US.

Lots of cities have building codes for multifamily residential that requires independent ventilation for each unit to reduce cross contamination, or requires zones and duct work that prevents such mixture of air.

This generally isn't true in commercial office buildings, which are more along the lines of what you're talking about. But often tenants themselves don't even directly control their temperature at will; it is managed by property management.

You can consolidate the outdoor units e.g. one outdoor unit can supply several apartments worth of air handlers (heads). There is an edge case wherein during the transition between heating and cooling season, all the heads associated with one power unit can either heat or cool collectively, not some combination of both.

Mini splits also allow a lot more flexibility of layout as you're only running linesets from the outdoor units/branch box to the the heads, i.e. you have to plumb a couple of small copper lines to each head instead of running a huge ducting tree.

That "edge case" is simply unacceptable to many people. Agree with your neighbors in terms of cooling/heating? These days most apartment dwellers don't even talk to their neighbors in the first place.

The only kind of consolidation I can imagine is consolidating the outdoor units of different rooms in the same apartment.

They only need to agree on whether it's a heating period or a cooling period. I know many buildings are different, but there's a sizeable number of them in my city providing some form of central heating. I do live in a very dense city, where I bet most people don't know their neighbors.

The building management and HOA usually set up a fixed scheme, such as "heating is on from start_date to end_date. These dates tend to make sense according to the usual local climate. If I'm not mistaken, in my building it's something like late October to early May.

Of course, when the actual temperature is outside expected ranges around those dates, it's not optimal. Luckily, it's never more than a few days.

The same approach could be applied for providing cooling.

If you're aiming for efficiency and not individualized comfort you will have less interested parties.

Some people run hot, some cold.

Is there any risk of no interest in NYC?

I'm sure I could sell a used sausage to someone in NYC.

>Couldn't we have a similar thing for cold air in the summer? Send the same temperature to everybody, and allow people to choose how much of that air they allow in their units.

No. AC does not work like this. In fact, you're not supposed to change vents in individual rooms to control temperature or you risk causing problems for the AC unit.

It also seems thermodynamically inefficient to mix a small volume of supercooled air with a large volume of warm air!

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