Abstract: A supplementary thermal system controller for assisting a main system controller in retaining a desired temperature rise by monitoring the temperature in the return duct, wherein the supplemental controller comprises a microcontroller for reading and processing return duct temperature measurements, a signal conditioner circuit for preparing temperature related signals for processing by the microcontroller, processor readable medium of program code for determining the rate of temperature change in the return duct and providing control signals based on the rate of temperature change, and a supplemental thermal source driver for controlling a supplemental thermal source, such as electrical resistance heating elements, to cause the assisting temperature source to increase or decrease its thermal output.