An open loop system is far from best practice and is always best avoided. I am personally also not a big fan of plate systems as they are the worst performing and the most expensive heated floor option. I would recommend using something like Warmboard with 1/2” PEX which will mitigate bend radius issue. I can't imagine why anyone would spec 3/4" PEX for your application... If your structure can handle it, a 1.5” thick gypcrete slab would perform slightly better and likely could be accomplished for less cost. I would recommend looking at using something like a NextGen electric boiler to keep system simple, DIY friendly and low cost.

https://www.borstengineeringconstruction.com/NextGen_Boiler.pdf

However, the smallest NextGen would likely supply way more heat than you need, so you might want to consider expanding your heated floor area. In fact, to design this properly, you really should first do a heat loss analysis for the room(s). We have free DIY heat loss analysis and hydronic radiant floor heating design calculators on our website if you are so inclined.

https://www.borstengineeringconstruction.com/Calculators.html

Please be sure to read the associated calculator instructions before using them. If you decide to use a NextGen, please let me know and I will do a referral. Anyhow, good luck with your project!

Sailawayrb,
Thanks for your input. The 1/2 inch pex seems to make sense. I was talking to a manufactures rep yesterday before posting on this forum. He said, "Why 3/4 inch? You can get the same heat with 1/2 inch, eliminate the kinking issue and it is cheaper." I think he's right except for it being cheaper. I would most likely run two lines between the floor joists which would double the amount of PEX and I will need new heat transfer plates. If I understand the application of either Warmboard or "gypcrete," both would be installed on top of the subfloor. My project is limited to options below the subfloor, with only crawlspace access.

The primary heat source for my 2700 sq. ft. home is a very efficient cast iron wood stove that is located in my basement. Here in Missouri, we are blessed with plenty of hardwood trees. I cut, load, haul, unload, split, stack, unstack, wheelbarrow it inside, restack and finally burn it. I'm an o'l retired fart and I love it doing it year round. It's is great exercise! I most always have enough well seasoned (dry) wood stacked in the shed ready to go. sunroom is an add on and does not benefit from the wood stove like the rest of the house does.

My primary objective is to keep this project as simple as possible. But at the same time I want it to last a long time. As of now I will likely switch to 1/2 inch pex and stay with the "open" system.

Please just be advised that below-floor hydronic radiant emitters perform very poorly. You will need a very high supply temp just to drive the heat through the subfloor and a very large percentage of this heat will go into the crawl space instead of the living space even when you put a lot of insulation below the PEX and plates. Unless you have some floor to ceiling height issue, I would think Warmboard or similar product would certainly be an option. This would perform way better than your current below-floor emitter and the 1/2” PEX could be easily routed up from the crawl space and placed into the Warmboard grooves if desired.

I suspect your open system is operating at a pressure well above the typical 15 PSI closed system pressure and you may be over-pumping it besides. Than factor in the scale/sludge factor for using an open system and you are not exactly going down the long life trail... The 1/2" PEX will have more hydraulic friction than the 3/4" which may not be compatible with how you are currently pumping it. Again, you really should make some attempt at properly designing this system so as to sort out the optimum supply temp and flow rate. At typical required flow rates, your 1/2" PEX should not exceed 300 feet length...and preferably not even 250 feet length. So, at a 12" PEX spacing, you can heat at most 300 square feet with a single loop. This may be why your previous designer may have decided to specify one loop of 3/4" PEX...when they should have specified two loops of 1/2 PEX... But one shouldn't really speculate, one should properly design the system which starts with knowing the design heat loss.

The 1/2" will more than enough. If you still want to put it under the floor, staple it up with plastic staples that limit their depth. Than use two part closed cell foam spray. Wear a respirator and goggles and Tyvek suit. Have a fan blowing the whole time and a safety person on the outside watching you. Spray several one to two inch layers until you have around six inches of foam. A little pricey, but the BEST AND EASIEST installation method, IMO. Comes in two cans, with a long hose, extra gun tips, and the gun. Home Depot has it.

The 1/2" will more than enough. If you still want to put it under the floor, staple it up with plastic staples that limit their depth. Than use two part closed cell foam spray. Wear a respirator and goggles and Tyvek suit. Have a fan blowing the whole time and a safety person on the outside watching you. Spray several one to two inch layers until you have around six inches of foam. A little pricey, but the BEST AND EASIEST installation method, IMO. Comes in two cans, with a long hose, extra gun tips, and the gun. Home Depot has it.