A solar flat-plate collector is tested under two types of day: a clear day and a cloudy day. Its thermal characteristics are described. Results of experimental tests and thermal simulations are compared. It was clarified that the experimental results agree with the calculation results. The theoretical model was validated and then used to dimension a heating system using flat-plate collectors to improve the internal greenhouse microclimate. (C) 2000 Elsevier Science Ltd. All rights reserved.