Best Of

Typically speaking, that exchanger just gets cut into the existing return line prior to re-entering the indoor boiler with no need for closely spaced tees. The circulator on the OWB runs 24/7 so that exchanger is always hot, and when any indoor zone is running, it will essentially preheat the return water entering the oil boiler via the aforementioned heat exchanger.

Is this garage attached or detached? If the former, maybe add it as a zone to your oil boiler so you also have backup? Otherwise just make a series or parallel loop out of it and hit all 3 exchangers at all times. If the latter, you'll want to run a separate loop from the OWB to that building also with its own circulator located on the OWB and just circulate 24/7 while controlling the fans with a thermostat. As for controlling the oil boiler, I would add an aquastat or Ranco type controller on the incoming OWB supply line at the heat exchanger and interrupt your oil boiler's TT signal with it, so that any time the OWB is above say 150*, the oil boiler will not fire. Whenever the OWB temp drops, the oil boiler will fire to make up the difference. If the OWB is something you don't intend to keep hot all the time during heating season, you may wish to have some sort of thermostatic bypass to prevent keeping the water jacket at oil boiler temp due to the 24/7 circulation of the OWB circ.

Regarding leonz, please disregard everything he/she said. It's 100% false.