I assume in home that don't have ACs, that they also don't have ducts. Do these ductless homes filter their air, control home humidity, or bring in fresh air from outside?

"Ductless homes" usually circulate hot water through radiators. If you switch to a heat pump, nothing's stopping you from circulating cold water in the summer, though obviously there is a limit on the temperature delta (you can heat water much more than you can cool it). Bringing in fresh air is a manual process (open the windows), and filtering air or controling humidity is then left to seperate appliances, if done at all. Though small fans to the outside for forced ventilation are gaining some popularity for giving you automatic air exchange and filtration.

> nothing's stopping you from circulating cold water in the summer

Condensation is - air conditioners have an integrated drip tray which drains to the outside, but if you ran chilled water through radiators they'd all make puddles of condensation on the floor. Retrofitting drains might be possible but I've never seen one and I can't imagine it'd be cost-effective vs installing mini-splits.

There are many reasons why you can't just circulate cold water though a radiator to cool. As another commenter stated you will get large amounts of condensation. But the other reason is that you do not get nearly as effective of convection currents with cool versus heating. Your radiators utilize convection currents to circulate the warm air around the room. Running chilled water though the a radiator would be like having an ice cube in the corner of the room. It isn't going to cool the whole room down and will get water everywhere.

Maybe a faulty assumption; my AC-less home has a ducted oil furnace, and we still haven't removed the ducts after installing a heat pump. Another house I lived in 12 years ago had ducts for oil furnace heating but no AC. Both were 100+ years old and in the northern US.

That matches my experience. All of our older houses around here with oil heat are forced air. Radiators are uncommon. But 'older' in this case is only 100-120 years.

My ductless house has a gas combi water heater/boiler and hot water circulation system to in-wall radiators with simple blower fans.

The house also has an exhaust fan on a timer, paired with small vents on several windows around the house, to cycle in fresh outside air. But practically for much of the year we just open the windows for fresh air since we live in the temperate PNW climate, wildfires aside :(

I looked in to installing a minisplit heat pump system last year, but the quotes I got were outrageous ($25k+) plus as a retrofit it's not the most elegant integration. Someday I'd like to do the conversion and ultimately decommission the gas boiler, but at that price tag it's not happening soon.

In the short term I bought one window AC and one portable AC for a combined ~$1000. It's not ideal but we only have a few weeks a year where cooling is really needed, and a few more where it's a nice luxury but not a necessity.

Sure, but you can get a whole home.humidifier for like $300 to mitigate that issue.

My parents installed a whole home humidifier a couple of years ago. It cost way more than $300 (closer to $2k installed—maybe they just got the wrong one?), and it also costs a lot to operate each month in additional electricity usage. Having a system that doesn't require one sounds much nicer.

I got quotes in that range as well here in boulder, so I installed one myself a week ago in about 2 hours(aprilaire 700 - $310) with no special HVAC knowledge, though I am mechanically and electronically inclined.

I haven't checked the electricity usage but I can't imagine it is much. The system is literally just a small fan and a solenoid valve. Maybe your parents opted for a steam system, whereas mine is called a flow through style.

I think you got confused with DEhumidifier. Humidifiers are cheap to operate.

Nope. It's a humidifier that uses resistive heating to vaporize water, and that takes a lot of energy.

That's like cautioning about the price of cars because you bought a Hummer instead of a Corolla. Drum humidifiers use a couple watts on any central forced-air system.

And, since the heating season is there only time you need humidifying on this context, all the "waste" heat is just heating your home.

Sorry but this is incorrect. Boilers aka hydronic heat is usually delivered through baseboard fin-tube or hot water radiators. These heat mostly through convection with some radiation. Any humidity in the house is retained. Forced air heats the air and removes humidity absent a humidifier.

Forced air doesn't remove humidity from the air either. Humidity is lost through air leakage in homes. Since the air outside had less absolute humidity in the air then humidity will drop due to air leakage. Most measurements of humidity is relative humidity though which changes with air temperature. So any system that heats air will reduce relative humidity.

Neither removes humidity, only AC does that. All types of heating lower relative humidity by increasing the temperature of the air.