for the record, both of those options involve “heat exchangers”…tube and shells work great, but throwing the coil you have in an insulated vessel and running a chiller to it will work just fine.
to size your chiller you want to do some math. either match your heater, or figure out how many Btu you need to remove to get your desired recovery rate.
see: The ULTIMATE chiller calculator thread
do you have a flammable gas/LEL meter?
Get one for O2 content as well.