A couple of follow-up points:
I have never built a system that needed an air scoop/ burping device of any kind. A simple air vent and a flushing tee with shutoff valve is all that is ever needed. These are placed back at the boiler and not at the manifold. Remember that these systems are closed loop. Once the air is out, all that special air handling equipment is just sitting there doing nothing, forever. The only thing you need to do is get it burped enough to start running. Then it will clear itself completely over time by placing a single air vent in a good location, or an open expansion tank up high. Bubbles are the first problem, and they are relatively easy to deal with. Entrained air is the next problem and air vents do not do anything about that. Oxygen in the system, absorbed into the water will get used up in a slight bit of corrosion initially and then the corrosion stops and you have a perfect closed loop system that will run for many years without rust. Oxygen barrier tubing almost completely prevents oxygen from permeating into the system and a good additive is best to prevent minor corrosion issues.
In my house, I put the expansion tank up in the attic area so it would accumulate the last of the free air in the system and provide a place to fill the whole system. It is an open tank with no bladder. In fact, it is a sandblasting tank of about 6 gallons. I did not want, or have any reason to fill my system with a fill regulator.
As far as pressure testing, I may not have been clear. ALL systems must be placed under pressure before the pour and held there during the pour. Code says a100 PSI hydrostatic test. That isn't necessary for function though. A 60 PSI air test is fine. Sitting overnight is my general rule. 99% of all leaks are at the manifold, above grade, but be sure you find any that occur and fix them before the pour. Do not leave the tubing exposed outside for any longer than is necessary. I've had squirrels eat holes in it overnight.
As far as tube spacing goes, 1' is a good general number because it heats fairly well. And when tied to rebar is fairly easy to walk around in during the pour. A 9" spacing heats better, but gets more abuse during the pour. The rebar spacing should always the same as the tube spacing so the tube can be tied alongside the bar. Never run the tube along by itself in a concrete pour. Do not worry about the position of the the as far as being close to the surface or deep down. Consistency is better and it all heats well anyway.
I have drilled many holes in my radiant slab to bolt things down, to glue in vertical rebars to build a glass block wall, to add more plate hold down bolts and to install thresholds. You just have to study photos and draw a heat map on the floor to match the photos. It's scary, but completely doable.
Back to thermostats. A conventional setback thermostat is as good as you can get for comfort, timing and efficiency, with a few minor exceptions. A NEST thermostat is generally a waste of money and causes a loss of comfort, in most applications. For slab sensing a setback thermostat with remote sensor ability is the way to go.
Being in the business of installing these systems means sometimes work must be done in a more conventional and consistent way. That's fine. But on your own system you can afford to be more creative and fine tune the design more to your own needs and the house design.
With dry gravel under the slab you get excellent support for the slab. There is no heat loss straight down under the middle of the house. There is conduction, to a certain degree that increases the mass being heated. That is not a disadvantage or an energy loss. Remember, I made my slab 8" thick and it works very well with all that mass and no insulation under the slab. I do recommend insulation around the perimeter, say, two feet in from the edge under the slab and slab edge insulation And, of course in the walls. I also alway hold the heat back from the edge of the slab by about 1' from the inside of the walls. This reduces slab edge losses too.
When positioning a slab sensor tube in the slab, the exact location is not important as long as it's between some heating tubes and not off by itself, but it is only detecting an average temp at it's particular location. There is no "proper" reading that represents true comfort as slabs are different temperatures all over their surface. What we want is a reading that we can measure for a relative and consistent number. That's all. So the exact temperature is not important. Consistency is. I have a thermostat lying on the floor in one location that I use to monitor a representative spot. I know where it should read to be pretty comfortable in the house. Comfort is a fleeting and different situation for everybody and every situation in every room. Radiant rooms can and are very comfortable with quite cool air temps, for instance. So looking at a wall thermometer often has limited meaning. Reading the temperature of a radiant return line in the slab, with a floor sensor, is pretty much meaningless. A representative area that has had a chance to settle in over hours and is similar to the area you want to spend some time in, is much more accurate as a measure of whether you'll feel comfortable or not. And that spot is mainly valuable because you can set your floor thermostat to watch it for you.
I overwhelmingly use 1/2" PEX (5/8" OD) in new installations. But in my own home with an 8" slab I used 3/4" PEX (7/8" OD) throughout. I used number three rebar (3/8") on 12" centers in both directions and I added extra bars wherever needed. All tied with 6" or 8' rebar ties. Using bulk tie wire leaves a lot of sharp ends that can puncture. The 3/4" PEX is extremely rugged and allows a lot longer loop if needed. Longer loops are not always helpful, but sometimes. A stronger tube is less likely to get damaged during the pour and a thick slab gets a lot of stomping around during the pour.
So much of this endeavor is simply common sense, and trial and error. It takes into account mass, various temperatures, energy, the nature of water, lifestyle and time. Often, as in using thermostats, the thing you expect, is not what happens. An example of this is setting a thermostat to a certain temperature and expecting to have the house arrive there and stay there. Not gonna happen. Why not? That is where the fun begins. Another one is "average" floor temp. Slab vs room sensing and why. Insulation where and why. High vs low mass. Predicted and proper recovery rates (this is a big one). What the best energy source is. Weeding out inaccurate or poor advice. Understanding that there are too many variables to just expect a one-size-fits-all approach will be best for you.
