I have an indoor boiler that's piped to a forced air heat exchanger above the wood furnace but with that set up we have no back up heat. I picked up a used gas furnace and I'm wondering if the heat exchanger can be installed between the furnace filter and the furnace it's instead of in the hot air plenum? My thinking is that way if the are ever gone and the gas kicks in it won't be blowing through a cold heat exchanger? Any thoughts or recommendations? The boiler is an old Royall 6526.