Well BillS by the picture very clearly from the water temperature electrical sensor in the side of that engine outlet housing as DonM said there has to be a mechanical engine coolant flow control thermostat in there.
Your 1/2" hose out the front side is a coolant by-pass flow house for coolant to be able to flow with the thermostat closed. Other wise you'd make steam bubbles in the cylinder head.
The 1/2" pipe plug comming out the brass sensor side of the outlet housing would be for an auxiliary cab heater hot-side flow source.
Decision time for you.
IF you tee into the big flow hose from the outlet housing to the radiator and you may never get any slab heat flow on really cold winter days. Thermostat may never need to open.
IF you directed ALL engine coolant flow then you may engine overheat on some days with the slab unable to take out enough heat. Thermostat open all of the time.
I'd go with the 1/2' side outlet flow with a manual control valve opened j-u-s-t enough.
One setting for Cold conditions.
Another for COLD, COLD conditions.
Shut off when slab heat not needed.
Just like a vehicle cab heating system.
You are gong to find warming up time benefits from air flow blocking the front of the radiator in COLD, COLD.
You could benefit from taking off the engine driven cooling fan and puting on a thermal switched controlled auto radiator air puller/pusher fan.
See . . . the more complexity "automation" you add, the more to set up, maintain, wear/fail, then diagnose, and repair.
The more manual steps you add, the more to forget when distracted, tired, cold and wet.