I read your posts a few times because I wanted make sure I understood it. It sounds like you have spent a lot of money on your custom home and the HVAC system is a mess. You should be very angry with your builder and his HVAC subcontractor. What you have described so far has no easy fix. I am sorry to say this, but you may have to go to a court of law to get this resolved.

Have you measured the humidity throughout the house? What are the readings? Houses become dry in the winter due to outdoor air infiltration. This is caused by leaky windows and doors. I would think your new home has tight construction. I am skeptical that the humidity is too low. The fact that you see water vapor coming out of the AC vents suggests the humidity is too high. You may be creating a second problem to solve the first problem.

I also find it curious that your house is warmer than you expect during the spring. It there a basement where you can access the radiant piping? Does it feel warm with the heat off? Something does not seem right.

I think what dovetonsils may be right about what is happening to the wood. Low humidity causes the wood to shrink. You would see gaps in the joints. If you don't see this, then low humidity is not the issue. If the floor is wrapping then it is expanding at different rates. This is worse then shrinkage in my opinion.

Don't refinish the floors until you understand what is the problem. Hire your own flooring expert if you necessary. Talk to the builder and tell them what you have in unacceptable and see if he is prepared to resolve it.

lots of anecdotal information but very few facts. Difficult to sort out on a site like this. What is your location? Tell us about your HVAC equipment, boiler, condenser, air handler, brand, model, size. What does installing dealer say about this? What if anything has he done to remedy your concerns/complaints? I probably would seek out an independent HVAC consultant to review your systems and give you a written report on the problems and possible remedies if any other than starting all over. Of course this will be on your dime but may be worthwhile in long run especially if legal action is on the horizon.

Sorry for your trouble

Good luck, you obviously will need it.

IMO

x2

"site finished hardwood" - maybe it wasn't acclimated to the inside environment before being nailed down?

Is it possible that the flooring problem is just that, and not related to the HVAC?

"large units outside that power it and it goes through vents in the house" - sounds like a minisplit to me. Are you using the radiant for heat and the wall units for A/C?

Where I work we have humidifiers in our HVAC, and "steam" can be seen exiting some registers.

Many valid performance tests have shown
that air conditioning systems 'on the average' only deliver to the
conditioned areas between 59 & 63% of their Nominal Rated
Performance.

Easy Safe testing of your A/C or heat pump
cooling performance for all the visitors to use:

First, make sure the return air filter
is clean, and then you should get a digital probe thermometer that
reads in tenths degrees, (though U can use any mercury thermometer)
and a low cost percent relative 'humidity gauge 'to check the
'indoor' humidity level.

If you have an air conditioner that was
manufactured between 1992 and 2005 it will 'probably be' a 10 or 12
SEER (though some were higher SEER even back then) R-22 refrigerant
units.

When the temperature reaches around +
80 to 95°F outdoors and the indoor temperature is 80°F and the
relative humidity indoors' is right around 50% RH the outdoor
condenser temperature split should be around 20 to 21°F above the
outdoor temperature.

If the indoor temperature is 75° and
the relative humidity is 50% then the air discharge temp-split off
the condenser should be around 17°F.

The indoor temperature split between
the return-air at the supply air grille closest to the air handler
should be a 19 to 21°F temperature drop with either an 80°F or 75°F
indoor temperature. This is also the indoor split with the higher
SEER units at 13 SEER or above.

If the indoor temperature split is too
high may have very low airflow which needs to be brought up to its
normal CFM Rate

If the outdoor condenser split is too high your air handler may be
drawing hot outside air into the return from the attic causing the high
condenser discharge air temperature; or it has dirty coils or lack of
ambient airflow through the coils, check for cottonwood if trees are
in the area.

On the new air-conditioning systems & any
systems at 13 SEER or higher, the outdoor condenser split is lower
than it is on the 10 to 12 SEER units; the indoor temp-split is the
same.

New 13-SEER units have a condenser temp-rise 'at above
conditions' of close to 17°F; a 16 or 18-SEER usually around 9 or
10°F need model numbers for more definitive numbers.

If the
temps are within plus or minus a degree or two, that air-conditioner
is performing fairly well; if not, call an HVAC service contractor
(in your case the installing contractor) and show them your test
results; tell them you want it to pass their Delivered Performance
Test.

If they don't know how to perform those tests call to
find someone in your area hopefully that can do those tests; with
modern test instruments they are not that difficult to do.

Wow! Thank you so much for all of your responses! I'll gather up all of this information and post it tomorrow. I'm eager to get more educated on the topic so I can figure out the next step! Again, I can't believe how helpful you all are!