Mathematical modeling of a helical coil thermal oil heater is difficult due to the complex heat transfer interaction between thermal oil and flue gas. The present publication reports the development of a transient model for the study of transient and steady-state behavior of fired helical coil thermal oil heaters. The mathematical model aims to predict the distribution of the flue gas temperature, thermal oil temperature, and tube wall temperature. A model is used for the accurate prediction of flame temperature and tube wall temperature. Prediction and control of the tube wall temperature are critical for the design and operation of a fired thermal oil heater. © 2022, Begell House Inc. All rights reserved.