# Heating circuit - help me understand control scheme of return temperature

I'm looking at a P&ID of a heating circuit. The burner/boiler is non-modulating. Red is flow/hot, blue is return/cold. AFAIK the pump is fixed speed (no FI).

As far as I understand the control logic, the temperature sensor on the return line (or rather the control circuit it's a part of) will open the three way valve and mix hot water into the return, and close when the return flow temp is warm enough. The boiler delivers a constant $\Delta T$, controlling for return temperature ensures that the flow temperature is correct.

• Is this control scheme likely?
• Why not control for flow temp. directly?
• Please do spell out technical abbreviations, to help others to find this question: people won't find this question by searching for "FI" or "TC" as they'll get too many irrelevant hits - they'll use the full terms. – 410 gone Feb 3 '16 at 7:14