LsFarm wrote:The fact that the water goes out at 180*, and comes back at 80*, means it is circulating, and shedding it's heat into the slab.
Maybe, maybe not. I have seen two places where tubing got plugged, or crushed, or something, and the flow was severely restricted. (one place lives with a cool zone, the other put in baseboard) I'd first visually verify flow. Disconnect the pipes, get a garden hose hooked up so you can put water in and have it come out into a five gallon bucket. Is it possible to get to that manifold & check each tube independently? If it takes more than a few minutes to fill the bucket you may have permanent flow issues. Modine time. If the flow is OK, then you probably have not enough pump not enough tubing, and , or, not enough insulation. Then we are back to adding a Modine or baseboard.
<edit> Whoa! Hold the horses! I just noticed..... Is that mixing valve in the picture the one that controls this floor? I have used that exact valve and it does not have good flow for a large zone. That in itself may be your entire problem. Also, you say the mixing valve is bad.... it's OK to use ball valves to control the temp, but if so, you should remove the guts from the mixing valve and valve the cold leg. I'll go look & come back with a better mixing valve.
<edit> Here's a mixing valve. A Hoffman 21H. About $300....but cheaper than a Modine if it is the cure.