I’m not necessarily familiar with subfloor heating systems but I’ll give it a shot. I found this video that helped sort some of it out.

https://youtu.be/OSJkjcTHVLE?si=aGyvO6ue3Ng0FT9I

From what I can tell it looks like the hot water comes in from the top line and heads into the manifold but also down to the mixing/regulating valve on the bottom line. After the water runs through the system it comes out to the return line and back out to the heating system.

My first thought is to try closing the blue valve on the return a bit and give it a minute. My thought process is that reducing the cold/out flow will help the circulating pump pull the water from the mixing valve. You could also try messing with the regulator to see if the temperature changes when you adjust it.

In the last year or so I heard about how the water level is dropping due to drought which is concentrating all the pollutants in the lake. It’s also becoming l so saline that the few species living in the lake are dying and washing up on shore. Then the high winds are blowing around extra salty sand combined with dead carcass particles so it’s actually a breathing hazard to be around. This is also combined with the runoff of pesticides from the farms to the north that also polite the water. So it sounds like a great place to hang out!

I was literally just thinking today how much fun this would be to play once it’s out and now I’m sad.