This paper discusses the indoor formaldehyde levels in two groups of houses. With the exception of the heating and ventilation systems, the house construction, formaldehyde sources and occupancy were similar for the two groups, One group (A) used electric baseboard convective heaters for space heating and semi-ducted heat recovery ventilators (HRV) for supplying outdoor air and exhausting indoor air , The second group (B) had electric forced-air furnaces with a ducted air supply to every room, Outdoor air was drawn in via a connection to the return air ductwork. During the winter, 77% of the Group A houses had bedroom formaldehyde levels higher than the living room levels, In the Group B houses, only 20% had a bedroom level higher than the living room level. These data suggest that the semi-ducted HRV systems provide limited air circulation within the houses compared to the fully ducted forced-air furnaces. The average winter HCOH levels were similar for both house groups; however, the Group A houses had ventilation rates twice as high as the Group B houses, Since the HCOH source strengths within the house groups should be equal, superior mixing efficiency in the Group B houses may be responsible. The results also show that a continuous supply of ventilation air is necessary to prevent the build-up of indoor pollutants, Ventilation systems that are coupled to the air circulation/heating system (Group B) are not reliable during low/no heat periods. In energy-efficient houses with low auxiliary heating requirements, indoor air quality problems may develop.