I've been away a while! Here are some updates and thoughts after having my system a few years and reading recent posts:
I see a great deal of back and forth over pipe diameter, insulation/radiant barrier effectiveness, Reynolds numbers, pipe routing, required supply temp, reset to compensate for outdoor temp, etc. Don't overthink it. To keep it simple, I have learned through experience (some of this is reiterated or I have previously posted):
1: 1/2" pex is used because it is relatively cheap, stupendously durable, easy to work with and a cost effective way to get hot water to a floor with no more than 12" spacing between pipes for even heating. Go smaller and flow isn't enough, go larger and cost goes way up and installation is an absolute bitch. 1/2" works.
2: Pipes in a slab may be best, but staple up with cheap stamped heat transfer plates works fine and staple-up without plates is less than worthless. My not very efficient house envelope is heated fine with 12" on center 1/2" pex and transfer plates with 125F supply (I leave it at 130F minimum for Legionella safety since my system is open loop). I cannot imagine a climate where 8" on center with staple up plates and a minimum of envelope improvements (that should be done anyway) wouldn't be sufficient.
3: I have had no failures or problems with the hydronic system. I use regular wall mounted air thermostats telling my controller to turn each zone pump on or off, and one-way bronze valves allowing water flow in only one direction. No reset, no mixing valves, no high wattage noisy pumps, no zone valves, etc. If the house needs more heat, each zone stays on longer and off less. K.I.S.S.
4: I have gotten a few more creaks/pops as the pipes expand when a zone has fully cooled and comes back on-not obtrusive, but I would like it quiet. Instead of silicone caulk as a transfer compound between the transfer plate and the pex, I would use silicone plumbers grease. It won't get runny and saturate anything nearby and would act as a lubricant for the pex in the plates to keep it from gripping/releasing-a very tiny amount does the trick.
5: I would make each loop a max of 250 feet or so for lower pumping head and route the pipes the simplest way, including pulling a single loop through to make 2 pipes in each joist bay as I described in previous posts. I wouldn't go through extra effort trying to make the outside wall areas hottest or keeping loop lengths nearly the same. Most of the outer floor area is covered with furniture and such, it's closer to the outer wall to lose heat through, plus I REALLY like feeling the warmth of the floor with my bare feet! If the loops are less than 250 feet but more than 150 feet, there isn't much difference in head. You could always crimp in a valve to partially throttle the zippiest loop in a zone if you have a large return temp difference. If that means crossing one pipe with another, just isolate the contact point with a sheath of larger pex or pipe insulation. A cleaner install is better than one with no crossover points or undue effort to evenly heat the perimeter or keep the loops the same.
6: I wouldn't buy Grand Hall's Eternal Water Heater again. I've had numerous different nuisance tripouts cured by powering off then on, the exit mixing valve go bad (under warranty but a hassle to get them to send me a new one), and a potentially catastrophic failure of the water pressure sensor. It's made of plastic and had gotten cooked from heat. I noticed a slow water leak from the heater one day, so I shut the water off and removed the covers. I barely even touched the sensor and it broke completely off, spraying residual water pressure everywhere! I didn't want the same problem again-imagine if it broke while I was on vacation for a month and the water was turned on! It's just a simple on/off pressure switch with a metric thread very close to 1/8" NPT, so I just retapped the hole and installed a metal engine oil pressure switch that fits numerous old GM cars. Done and ok now, but I wouldn't wish these problems on someone else. Great heat exchanger inside, I may repurpose someday....
7: I have learned that copper is bacteriocidic like silver is. Having copper/brass here and there in a plumbing system is a good thing, but it isn't necessary to be all copper.
8: Having an AC system with an intermittent fan function and high efficiency filters is great for air filtration and moving heat around from one area to another. Air quality isn't a problem.
9: Realtors tell me what I have already confirmed: a warm floor is one of the best features a house can have! Aaaannndd, I get to post one of the benefits I can't believe no one else has posted on ecorenovator: Kotatsus are easy! Here's a link:
https://en.wikipedia.org/wiki/Kotatsu
A Kotatsu usually has a heat source under the table and a comforter style perimeter draped down that you put your lower body under. The warm floor is the heat source! I've draped a blanket over a coffee table or draped it over the edge of the couch to create a cocoon of extra heat coming from the floor. 100% of the homo sapiens, canines and felines I surveyed agree that it's awesome!
Lastly, it makes sense to design a system to work at DHW temps so you can combine systems and heat sources and not have to work so hard at making lower exergy sourcewater work. I've updated my plans to fire my energy companies and ask all of you for some Phase Change Material help at post 15 of
THIS THREAD
Bumping up the temp of the heat source for a GSHP improves efficiency just as much as lowering its discharge temp. The actual temps don't matter much, what matters is the amount of lift. Taking heat from a 70F source to make 140F water is essentially the same work as taking heat from a 35F source to make 105F water.
I am also insulating and stuccoing the exposed exterior parts of my concrete basement walls so they are part of the thermal mass and not losing such an ungodly amount of heat through them. 1/2" R3.8 polyiso board is $10 at Lowes. Using dollops of adhesive and some pushpins to keep the radiant barrier side of the board 1/4" of an inch from the concrete is almost as good as the recommended 1" airgap for radiant barriers. Sealing the board at the top of the wall will prevent the barrier from getting dirty and also prevents an hidden path for termites to make it to the wood a few feet above.
Best to all! AC, I hope to see your system in this thread soon....