The problem of measuring area-averaged heat flows in room-sized spaces in the presence of solar radiation is discussed in the context of a room-sized calorimeter facility (MoWiTT). Development of a new type of heat-flow sensor suitable for making such measurements is described, and test results for moderate-sized prototypes 1 ft (0.3 m) square are reviewed. Construction of full-sized units - up to 2 ft x 4 ft (1.2 m x 0.6 m) - is described, and problems encountered during development are discussed. It is concluded that this approach represents a viable method for determining net energy balances in rooms.