In the rural area of northern China, residents often use a traditional stove-Kang system to heat their homes, which is an effective way of using locally-produced biomass to solve the energy and environment problems. However, many of these homes are not heated sufficiently during the winter months. The current system is inefficient and requires high levels of fuel consumption. This paper researches the Rang system by creating a simplified and modular system modeling program with the aim to assist in accurate real-time design and optimization of the domestic Rang heating system. This part-1 introduces the structure, principles and the underlying equations of the modular simulation system. A variety of improvement measures were modeled and tested, upon the base model, including the use of a water heat exchanger and radiator system to the existing Rang set up, solar hot water system, bathing system, and the use of phase change material at the surface of the Rang. The program demonstrates good flexibility of implementing different technical solutions as well as fast simulation capability. (C) 2015 Elsevier Ltd. All rights reserved.