Been compiling ideas for my first house for many years and we would build in the next few years after several years living in multiple houses throughout the South.  I have never lived in a house that was insulated correctly/completely and I lived in one that looked like the local village idiot had designed the duct system.  

In KY we have good and bad winters, hot humid summers and generally windows-open springs and falls.

Some basic thoughts currently being tossed around:
2x6 walls with foam or ICF construction or earth sheltered construction (including attached garage)
Heavily insulated with foam/cellulose everywhere to run a smaller ton unit
Unvented spray-foamed attic or conditioned attic (standing seam metal roof)
Conditioned insulated deep crawlspace or walk out basement with poured walls/external rigid foam
Passive solar/south facing/no north facing windows or doors
Possibly more than one unit (had a house with a upstairs and downstairs unit which was great when one had a problem)
Serious doors and windows (best I have seen to date in someone's house have been Pella)


Interested in small things that only HVAC guys notice and your general thoughts.  Thoughts on modern insulation practices, various manufacturers, whole house fans, duct construction/layout, etc would be welcome.

Thanks for the comments

Access and ease of maintenance.

heat-pump

Most houses are so tight now days you need an air exchanger to help not get "sick house" syndrome.

worst

maggot infested filled with dirty diapers and rotten food house

Worst, condensate drain tied into DWV pipe to septic tank. Got called after tank backed up, toilet paper and shit water filled the air handler up about three inches.

LOL that takes me back to my landscaping days.

Why would anyone possibly tie those two together??

Last house had an attic that was basically an unused 3rd floor complete with several double hung windows with a huge live oak shading most of the house.  My HVAC guy said I had best attic in the area.

My current house has a crawl space you are either crouched or standing in the whole thing.

Personally I believe in access to everything - mechanical, electrical, plumbing etc

I have dozens of absurdly piped boiler pics. Later I'm glad to share!

Best things are architecture pics. I'll get some from a few buildings.

I've seen turtles drying out, dogs leashed to a bathroom doorknob all day so it had to sit in one spot all day, all types of paraphernalia, sex toys sitting out in plain view with lube next to it, dirty sheets with clothes everywhere, and so on.

One time I told the upstairs tenant I was cutting out a rotted section of waste pipe that was letting real waste splat inside a kitchen wall behind a cabinet. Cut open wall from bathroom side in downstairs unit, 'stuff' everywhere in the wall. Downstairs tenant goes into her bathroom to get makeup and things and upstairs tenant uses toilet and flushes. It goes all over downstairs tenant bathroom and tenant. Says "sorry, I forgot". I heard the yelling from the parking lot where I was cutting new ABS.

I refused to go back in until cleaned up. Took all day, city got involved, health dept had to sign off.

One homeowner answered the door fully naked. Big fat dude. Told him I would wait until he was clothed and he says no, it's cool. Come on in. Told him ummm, no way. Find some clothes dude. Was not going in.

Is that "zone AC" ?
How much price does it add?

Then there was the washing machine that drained into a pipe, which led to a big dug out spot in the dirt under the house. Just filled and drained. Really stank bad.

Holy duck.  

This is timely as I'm thinking of adding insulation and possibly replacing a traditional system that uses Propane for heat. Dual-Fuel would be the way I'd go there but again not sure if insulation alone would be wise as opposed to dropping 5K+ on a new heat-pump to work with my furnace.

Worst - Long Beach, California, installing a complete system in a 100+ year old house.
The crew before us had removed the old insulation, I was still in a moon suite sweating my ass off.
I was in the attic cutting the lath and plaster to lay-in the new register enclosures and to set the ducting and run the compressor lines.

I cut open the kitchen where the family had a big pot of fish heads on a slow boil on the stove.
I got hit with that smell and puked from the hole in the ceiling onto their floor.
The guy I was with thought it was funny, my boss was not amused.
Even to this day.....

Best was new home construction in San Dimas, we were hopping fences to place the concrete pads, set the compressors and then to wire the unit and solder the lines.
A couple of the homes were already occupied, one house I hopped over the fence and found a hot little 18 year old naked tanning by the pool.
She was so happy to see me since I was going to get the AC running.
Nothing happened, although after she put her bikini on she got me and the guy I was working with some ice tea.
The journeyman walked into the back yard and gave us a lecture about customers and work for about 2 seconds until he saw her.
We set her unit up first.

Fortunately, I don't do residential...except for the occasional side job, and I've pretty much gotten away from those altogether, except friends and family. I work on commercial equipment for my company's buildings only, so I always know what to expect and it's never bad. All Carrier, Trane and Bard and all set up the same. Good work if you can get it. I hope to God I never have to go to work for a residential HVAC company.

Look into American Standard or Trane equipment.  Two stage or modulating/variable speed with zoning.

Noted.

Up to the price of the original install.  Depends on the type of zone system.

 

I need new AC.  I have 10k saved for it.  Think I will need more.

This deserves it's own thread buddy

I'm in Ohio and I have 2 hybrid systems in my house. I live in a rural area but luckily I had natural gas line on my road. I ran the line up to the house and paid the hook in fee and I'm really glad I did now with LP prices through the roof. I have one system in the attic and one in the basement. 3200 sq ft house. 2 ton upstairs, 3 ton down. 17 SEER 2 stage heat pumps and 95+ modulating furnaces. I installed them myself so I did some things that most companies wouldn't do, like install returns in every room.

Guy in my shop tells this one as well.  Pretty funny, but really nasty.

Do commercial A/C but if I were building new I would go with a VRF system (Mitsubishi). Or a Trane variable speed.  I say Mitsubishi because I have installed many of these and have very few issues.  Mitsubishi has a different line up for residential but still effective.  
Go with hard duct externally wrapped with a minimum 2" foil backed duct wrap.  
Get a proper heat load calculation for your geographic location done before by a competent a/c company, not the guy that guesses or uses rule of thumb for everything.  
Worst thing I have seen in a residential unit, looked at some ductwork for wife's friend in house she was purchasing and thought it was internally lined at first then upon closer inspection discovered it was actually a layer of mold.  She didn't buy the house.
Worst on a commercial system was at a bakery where we used to repair a lot of the equipment.  Not going to go into detail(pretty gross) but I really don't eat bread anymore.

You guys are cracking me up with a serious post then a puking/naked girl post back to a serious post.

Taking note of all the comments.  Glad to see multiple posts to get hard ducts as I prefer them but so many houses are just the rolls any more.