A whole dryer model has been used to investigate the influence of the system design on the efficiency and productivity of a batch-type dehumidifier dryer. The product is an easy-to-dry timber, Pinus radiata. The model, which has been validated at both the dryer and dehumidifier levels, includes sub-models for the whole dryer energy balance, control of preheating, temperature and relative humidity, and the airflow system. The dynamic response of the system is illustrated and the influence of the dehumidifier capacity and the kiln airflow rate on the dryer performance is established. The effect of varying the airflow system losses is also determined. On the whole, drying speed and operating income increase with the dehumidifier capacity and the kiln airflow rate. The energy used by the dryer in a complete drying cycle is strongly influenced by the fan power requirements, and the airflow system losses have a significant adverse effect on the operating income. The results demonstrate the importance of balancing the dehumidifier and the airflow system losses in order to obtain an optimum combination of drying speed and energy efficiency.