This paper presents results from a recently completed 3-yr field study designed to assess the energy conservation effectiveness of retrofitting central gas-fired space heating furnaces and boilers. This study, known by the acronym SHElP (for Space Heating Efficiency Improvement Program) dealt with the development of appropriate procedures for the retrofitting, the acquisition and analysis of field-test data from over 2650 test sites monitored by 67 participating gas compa-nies from the 48 contiguous states and a province in Canada. Results for several groups of sites (similarly retrofit-modified by a single, double or quadruple retrofit) are presented in this paper and show that the range of statistically significant gas savings, obtained in this program, varied between 0.5 and 13.2% for warm-air installations and between 2.8 and 9.9% for hot-water boiler installations. For retrofit modifications that included gas input derating (with or without secondary air adjustment and with or without vent restrictor present in the vent system of the furnace), the gas savings obtained could simply but accurately be correlated with the change in steady-state efficiency, level of gas input derating and vent-pipe area reduction. The paper also presents an account of the test design, procedures for site selection, inspection, data acquisition and analysis, comparisons of quality of combustion, efficiency, and energy consumption characteristics of these installations between pre- and post-retrofit conditions.