Really think this sounds like a mess and how would you protect the piping if it is exposed . I think you need to either go for it or not. I use solar to heat my pool in the summer . Where I live there is not enough sun in the winter to make solar practical without back up. But many people have gas fired hydronic in floor heating and love it . Where do you live ? We have always had forced air heat until this house where we have hot water baseboard heat and I would never go back to forced air but to do in floor heating woulf have been out of our budget. I guess the minus is the fact that we need mini split air conditioning since of coure neithe hot water ot hydronic supports AC so I guees it is really important ot assess what you need where you live.

I think the pipe needs to be embedded in concrete, gypcrete or similar to work. The hot water in the pipe heats the mass of the concrete and THAT is was heats the room.

That solar collector only raises the temp 6-10°f (and I assume that's only with full sun). FWIW, the boilers in the hydronic systems we have raise the temp about 40-70°f or more (they maintain 150-180°f).

I've lived with solar hot water like that in Tanzania. Not cold is a better description than hot. :-)

Your experiment will not simulate having a boiler. Simply having pex exposed to air does not transfer the heat as well as if it’s connected to a metal surface (normal hydronic heat uses copper pipe with fins; radiant floor puts the pipe in a metal plate that attaches to the floor)

We live in northern NY state and have hydronic heat, I love it.

If you go hydronic in the floor, you have to have it in the slab or in gypcrete, you need the massing the concrete will supply.

Have you thought about a heat pump ( mini-splits). They're very efficient, (COP of 2.5-3.0...they produce 2.5-3.0 times more BTU than the energy they use to produce it. Obviously solar is perfect and no cost to heat anything other than the cost of te equipment) and you can get heating and cooling from the same device. They can be mounted as wall heads, or flush mounted into the ceilings. Check out Mishubishi

Radiant floor hydronic heat does not have to be installed in a slab. It can be installed in subfloor.

https://www.warmboard.com/warmboard-s

Or use a product like Warmboard We have hydronic heat in Colorado on all three floors with warmboard on the main level and upstairs and love it. Only the walkout is in slab.

ETA. i see Sasha and I were posting at the same time. Warmboard is a bit more expensive than other installations but well worth it.

Hydronic heating is a well understood, proven technology and the economics are well known. If you're interested in a hydronic heating system for your home, I suggest you speak with folks who design and sell residential hydronic heating systems instead of pursuing what's likely to be an extensive and expensive series of trials and errors.

@Seabornman great suggestion to use foil as inexpensive fins. I am thinking of using copper instead of pex. 1/4" coils of copper piping should give lot of surface area for heat exchange and using a small fan to move air around should help speed up heat exchange.

What you are talking about doing with the coil and fan does NOT simulate hydronic heat. You can buy a section of fins.

https://www.homedepot.com/p/FabTek-Logic-Fin-Clamp-Baseboard-Element-for-3-4-in-Copper-Baseboard-Piping-1-ft-Section-FCL-75CX1FT/300641178

https://www.lowes.com/pd/Hydrotherm-8-ft-330-BTU-Hydronic-Baseboard-Heater/3134801