[dom96]

Yeah, I wasn't planning to run the cold loops through the existing radiator piping. I just want the ability to cool at least 1 room by reusing the air-to-water system, even if that means the FPU requires its own piping from the outside.

In that scenario, are FPUs a good option? Are there other drawbacks?

[thyristan]

Yes, they are a good option, provided you get one that has a condensate drain. Always use a piped drain, never just a tray that you have to empty, make the hose wide enough and clean it every few months, because algae will clog it if you don't, and then you'll have your own privat lake. But that's basically the same as for any air conditioning.

Insulate all piping properly, the outdoor part will freeze in winter, and the indoor part will drip in summer if you don't. The best products for this are foamed rubber based. Make the insulation seamless with the right glue or tape, every opening will drip.

Add manual or automatic valves to shut off the heating loops in summer, so you don't accidentally get cold water in there. Your cold water loop might also need an extra circulation pump, depending on how you run your piping, but that's hard to say without a lot more detail.

FCUs are available in the same form factors as indoor units for refrigerant-based systems, including a thermostat remote control, e.g. https://www.sinclair-solutions.com/files/user-manual-en/fan-... . But get one from the manufacturer of your outdoor unit, so the communication lines work, e.g. so the indoor unit can instruct the outdoor unit to start/stop/regulate.

[spockz]

Most Francoils I know don’t have a way to get rid of any condensation that happens in the unit like internal air conditioning units do. So even if you run the piping outside of the house you have condensation in the unit and the floors, not to mention the walls.

Even then, there is a fair limit in cooling and efficiency you can get from them because of the lack of phase changes occurring.

If you have a balanced mechanical ventilation (both inlet and outlet) that would be one of the best options to condition the whole house without putting air conditioning units everywhere. Having said that, we turn on the air conditioning in the attic during hot days which removes the biggest heat and moisture from the house. Next up is installing more sun blinds to prevent the heat from entering in the first place.