The performance of an 80,000 Btu/h gas fired furnace was measured and modeled. A seasonal utilization efficiency of 52% was noted with the furnace as delivered in the mobile home. The corresponding (installed) steady state efficiency was 68% (below the rating value of 75%) which was partly due to the duct work in the home. Measurements throughout the winter season showed that the furnace was oversized for its particular application.