Your post #477 "As I only have 600' total of pex to feed to heat a 1100 sq'"
lead me to think too little pex.

Don't ever mix DHW into heating system, only via a flat plate heat exchanger to seperate them. Due to Leigonella possibility.

Totally agree with what BBP said, + if the slab is 4" thick you can do 12 centers but 8" is more efficient. Any over pour (1.5" gypcrete, for example) should be MAX 9" spacing, 6" is better. AND.......250' max loop length. You are better off with 3 x 215 than 2 x 320. Pump must be either brass or SS, no iron body if you insist on using potable water in your loops.

The Marathon tank is an entirely plastic tank.

Thought I stated that radiant loop is closed system and that DHW will be separate. I have a Airtrap DHWHP for DHW. Do you believe a plastic tank is bad? Our elec COOP recommends the Marathon as their first choice for rebates. My first floor loops will also be 5/8" O2 barrier pex, as I have 900' I got surplus from a friends project, which will carry a higher volume of fluid than 1/2". This friend also has a bunch of solid crimps left over. Are these better/worse than the clip rings?

I don't remember or go back & read what U "Drake" posted, I was making info available to readers of this thread about Legionella, as 1 company pushes open systems.

I have read that Marathon is a great DHW, so no problem with me.

Is your pex 5/8" OD = 1/2" pex, or 5/8" ID ? The heating difference is slight but the pumping back-pressure is considerable.

I have no knowledge of solid crimps.

5/8" tubing is fine but only if it is free and then you can do 2 loops as you were planning before. Is it 5/8" "nominal" or real 5/8" OD? If it is 1/2" nominal it is .625OD.

I prefer the ring clips which is what comes with all proper manifolds. Pex crimp rings are for permanent things but can come on mini valves for the loops (please use valves on each loop) for balancing and getting the air out.

It is 5/8 ID. 5/8 is 40% more volume over 1/2 so the amount of heat it carries should be proportional and would there not be less resistance in larger tubing of shorter length? Does the pump pressure come from the added weight of the greater volume of fluid in loop? The solid copper ring crimps I can get free but I read many favor the ring clamps and there are many varying opinions out there. Free is not always the best choice if it doesn't accomplish what you want but is attractive to me if it can be adapted to work. Being creative DIY and resourceful with materials has got me far more than I could afford otherwise.

Ahhh, there is not a proportional reduction in head loss, nor a proportional increase in heat transfer capacity over a 1/2' tube.
The heat transfer is all about wall surface area (and turbulence) and not the volume of liquid in the tube. Bigger is not always better. If it is 5/8 "nominal" meaning it says 5/8" on the box, it will be roughly 3/4" OD. The surface area for a 1/2" tube is 1.57 sq in/in of tube and for the 5/8" tube it is 1.96 sq in/in but volume is considerably higher ratio than that, 0.193 cu in for a 1/2" tube and 0.307 cu in/in for the 5.8" tube.....a very different ratio.

But this is not the big part. In any tube the majority of the liquid flows in the center of the tube and does not contribute to the heat transfer much. It does, however drop the head loss quite a bit, so you can use a longer tube without needing a bigger pump. The issue here is dT over the tube, the bigger it is, the less even the floor temp will be which is one reason why we try to design to a 20F dT.

The crimp rings are great because they are fast and relatively cheap, but not removable (without destroying them).

The pump pressure in entirely based on the resistance to flow due to the friction of the tube wall and fittings, weight of the water is not an issue.

Please bare with me on what I have been seeing as the best tube size/spacing recommended in the 20+ yrs I have been following hydronic radiant flooring which I planned hopefully to use in my last major retirement home project. 3/4" pex on 12"cc was very popular, and even 7/8" is out there for in slab. I see and understand that there is great variability in details of maximizing performance in how radiant floor is being applied now. I understand also a contractors desire to stick with what they know and is proven for them, some innovate more readily than others, but almost all believe the knowledge they are working with is sound. So am I understanding that the present direction of maximizing the evenness and efficiency of the floor loops is minimizing the tube spacing? Like with insulation generally more is better. Is the tube dia. a major factor or disadvantage beyond the minimum bending ability of it? In other words in my case could I use 5/8 ID pex as if it was 1/2 in a 8-9" spacing if I can work out the return bends(which I can do as I have room to mushroom out 180's to 12" if needed) in a goal to do low temp heating in a high mass slab(which will be very well insulated)?

Drake,
" So am I understanding that the present direction of maximizing the evenness and efficiency of the floor loops is minimizing the tube spacing? " YES

"In other words in my case could I use 5/8 ID pex as if it was 1/2 in a 8-9" spacing if I can work out the return bends(which I can do as I have room to mushroom out 180's to 12" if needed) in a goal to do low temp heating in a high mass slab(which will be very well insulated)?" YES

Drake,

You are asking important and complex questions regarding maximization of efficiency, that involve multiple variables.

I don't believe that anyone on this forum, including people who have been in the trade for decades, can precisely answer these multiple-variant questions.

Why don't you use a radiant floor design computer modeling program? They are written expressly for the purpose of optimizing over multiple variables.

You can get a program that will give you exactly the information you seek.

I recommend a free program called Radiant Works.

All you have to do is download it, spend an evening or two with it, and familiarizing yourself with it.

Then you can try all the variables you want, vary the slab thickness, vary the tubing diameter, vary the spacing, vary the house insulation, vary the window R-value, vary the water temperature, vary the water flow rate, vary the floor coverings, etc.

There is nothing you can imagine that it can not handle.

Why don't you just do that?

It is made for optimizing radiant installations while juggling exactly the variables you are agonizing over.

You should do this.

-AC