its a 3 way shunt valve.
I have a circulation pump Yes.
The valve is on top of the furnace or boiler and mixes the boiler water with the returning cold water from an array of heating elements. the valve is design exactly for this purpose and are commonly used in sweden to mix the water without restricting the return feed wich could damage the pump and make the flow digital and impossible to regulate.
It's a fully operational system which came with the house but since then i have radically altered the control system, so an array of digital temp regulators are controlling the entire furnace from electrical heating cartridge to the incinerator unit. This part is working perfectly because i bult the systems myself with cheap pid controller and ssr relays
To buy a new furnace with shunt system would cost 15000 usd.
the shunt controller itself costs around 800 usd but will not really suit the furnace.
We (Swedes) never measure the flow on existing systems, but they are controlled via the output temperature, and outdoor temperature.
The system is very slow due to the heating is hose imbedded in concrete floor, and that's why the cheap controller i have now doesnt work. it was build for heating element on the wall which are much faster and this causes the systyem to selfocillate
So i aim to use only 2 ds18b20: one outoors and one on outgoing pipe on the 3 way shunt valve.
Here is the result of the sketch i patched up the get the ideas going /Markus