Define heat exchanger, that a broad term in HVAC

z1700
I am looking at an outside air heat exchanger to replace my existing oil fired furnace. How much do I have to modify the furnace system to make this happen, and what exactly HVAC stand for??

Thanks,

Heating ventilation air conditioning add an r and and s and it would include refrigeration and solar HVACRS

still not enough info as to what your talking about
outdoor wood boiler??