"Recovery of the sensible heat of high temperature stack gas from combustion systems (boilers, furnaces) by  retrofitting a heat exchanger seems to be a standard but nevertheless controversial recommendation. Implicit  complications are increased pressure drop, fouling of heat exchanger and occasionally the need for an ID fan to  overcome the pressure drop. We are looking for information about best and as well worst cases and why these work  or don't work. Furthermore documentation should be given with regard to financial attractiveness because recovery of  waste heat is not always a cost effective option."