OWB used with forced air with AC?

I installed my boiler coil directly above the AC coil. Works like a charm. Plenty of folks have both coils in their plenum. Quite normal.

Very common to do what you're suggesting. I mean, that's what I did at least. I also installed my heat exchanger directly above the AC coil. There was plenty of room; it was just a bit difficult minimizing how much I cut up the plenum to slide it in there. Let's just say that the end result was a lot of foil tape used!

If you do pursue this, what you'll have is an aquastatic and a relay. If you do it like I did, you'll have the boiler water pumping all the time. Let me try and explain it as concise as possible, then if you have specific questions just let us know:

When your thermostat (you don't need another termostat for the house) sends a call for heat, it will identify if the water is hot enough to justify just running the fan instead of firing the propane burners (you set the temp on the aquastat that you learn is acceptable). If the water is hot enough, it will send that message to the relay, which will tell the furnace control board (pst...just turn on the fan). If the water is not hot enough, it will fire up the propane burners.

I know someone around these parts that gave me everything I needed to know...and actually, he probably lives pretty close to you. Great guy and a wealth of knowledge on this subject; I'm sure he'd be happy to help and maybe he'll chime in here too.

Good luck