This paper examines the potential of many energy saving concepts to reduce natural gas consumption in residential furnaces and water heaters. Altogether, 63 concepts are considered for retrofitting existing or incorporating in new forced-air furnaces, hydronic boilers, and water heaters. Each concept is evaluated in terms of estimated steady-state efficiency, seasonal efficiency, range of expected first cost, operating power, standby power, natural gas saved, electricity saved, dollars saved, and calculated range of payback periods. In addition, several promising areas are listed where further research efforts are believed warranted.