The perhaps non-intuitive answer is - everything that reduces the need for HVAC. I'd be looking at things like passive solar design so windows are shaded in summer but let sunlight in for heat in winter. Very good insulation and building sealing. Double glazing (possibly triple if you're in a very cold climate). Thermal mass in the right places for temperature stablity.

On the HVAC side, you want a system with a high CoP and I'd probably be looking at some form of energy recovery on the ventilation side.

Yup, very much so: https://www.treehugger.com/praise-dumb-box-4853131

I live in a relatively new building in NL, where many of these configurations are applied and on its own it won’t go lower than 21C in winter and 25C in summer. Thick walls, thick anhydrous cement floor layer for insulation (although I think it’s also meant to give the option to implement floor heating), _external_ blinds on the South side.

I simply can’t justify a smart thermostat/radiator valve build, despite my inside nerd constantly whispering I should.

Only downside is the heat exchanger air recirculator: it dries the air terribly in winter. You need a humidifier to avoid getting parched skin and mucosa.

> everything that reduces the need for HVAC

This is the answer! Even more important than insulation is air sealing, so you keep the conditioned air inside.

I've learned an enormous amount about building efficiency from

https://www.greenbuildingadvisor.com/ and https://www.energyvanguard.com/

What if you wanted good ventilation/co2 levels?

With modern construction, active ventilation is a requirement. So sizing that for the intended turnover would probably be all you would need to do.

If you are in a temperate climate, like The Bay Area, looking into hydronic heating.

I installed it, and would never go back to forced air. For some reason, it's not popular on the west coast.

It's so simple, a DIY'er could do the instal.

My energy bill is down, and allergies are better.

(I've noticed every store I have been in has forced HVAC, and I have a weird feeling forced air spread viruses. Plus--most retail stores have old systems. They might have slapped in some high mir filters, but I doubt they help with anything on old leaky systems.

(To those interested in hydronic heating, buy Modern Hydronics, by Stegenthal? I'm too lazy to look up his last name, but his book is used as text books.

TIL:

- The term hydronic heating.

- That most Americans don't have hydronic heating.

That's so weird, here in France radiators are just ubiquitous.

This is the detachable showerhead thing all over again. How do you people survive like that? You're supposed to be the highest GDP country in the world!

You'll find radiators in many older buildings (before about 1930 or so, at least in my area). Some are steam and some are hot water. The problem with them is that they tend to have higher maintenance costs plus if you wanted AC, you'd need to have a separate system, including ductwork. AC is relatively cheap to add to a house with forced air heating and quite expensive to add to a house with radiators. Mini Split AC systems are changing that somewhat, but they haven't caught on too well here.

You can also find baseboard heating, but that tends to be electric and have quite expensive operating costs. It's mostly only used in additions and places where you would only heat the space if you were currently in the room.

> This is the detachable showerhead thing all over again.

Uhhh... Everywhere I've lived in the US has had a detachable showerhead, from places built in the 1890s to places built in the 2000s. It's a normal thing for apartment dwellers to buy a nicer showerhead to replace the likely cheap and crappy one that landlord installed and then put the crappy one back before moving out.

It’s because most AC systems require forced air, so hydronic is a second system.

You’ll find hydronics in expensive houses that can justify the double expense or in colder climates where the heating benefits are more important than AC.

Also everyplace I’ve lived in the last 25 years has had detachable showerheads.

For anyone else wondering what hydronic heating is, it's using warm water as your main heat source. Either using radiators, or tubing in the floor to heat a room.

This contrasts against the more common American method of using hot air to heat a home. I believe air heating is more common in the US because AC systems already require the relevant air ducting, so it's easier to install.

Meanwhile here in NL airconditioning is very rare, and 95% of people use a gas fired water heater and metal radidators in rooms for heat distribution.

i would go as far as to say that nobody in continental europe knows the term 'hydronic', it's just heating, maybe central heating.

I looked up the last name: Siegenthaler