Hydraulic soft temperature sensors

Good day,
I, as a layman, have a question for better understanding.

A hydraulic separator of the brand MAGRA is installed.
Gas condensing boiler and underfloor heating throughout the entire house.

On the secondary side of the separator, a pump is installed in the supply line.

Now my question is, where must the supply temperature sensor be connected? At the separator or at the boiler?

Do the supply and return not mix in a separator in the secondary circuit so that the desired supply value must be compared with the actual value at the separator?

I would not position the sensor on the boiler and maybe not on the manifold yet either. Depending on the direction the water flows in the manifold (depending on the ratio of the current pump performances to each other), the measurement result might be distorted. If, then the sensor must in any case be placed at the very top.

In my opinion, you are certainly safe if the sensor is mounted on the outgoing pipe to the underfloor heating, at least a few cm away from the manifold. The temperature actually flowing into the underfloor heating is what you want to measure.

The hydraulic separator even has a socket for a temperature sensor. However, my heating engineer thinks it is sufficient to measure the flow temperature at the boiler.

But if the boiler pump is running at high performance, but there is only a low volume flow in the heating circuit, then you might possibly have a small measurement error. Maybe it's doable, but if I want to measure the "flow temperature in the heating circuit," then I would place the sensor exactly there. I strongly assume that the boiler has its own measurement!

If the circulating pump of the underfloor heating should fail, the correctly displayed flow temperature would be close to "zero" because the water almost only circulates between the mixing valve and the boiler. However, the display would correspond to the boiler temperature, so it would be much too high. Conversely, if the heating circuit pump is pumping full throttle because the heating demand is so high, then cold water from the return flow is mixed into the flow through the mixing valve. If the sensor is located at the boiler, it might then report 75 °C – all fine. But in the flow of the underfloor heating, due to the mixing in of the return flow, it might only be 40 °C, the rooms don’t reach the target temperature, the pump might switch to a higher speed, and the problem intensifies.

I don’t want to make you crazy; the system probably works as intended by your heating engineer. Definitely if the pumps run at fixed speed. But if I had the choice, I wouldn’t do it that way.