24-volt relay. KSD301C. Located on furnace units that relay when to cycle heat on and off. "The trademarks, logos, and service marks (collectively the "Trademarks") displayed herein Supplying Demand are registered and unregistered trademarks of Supplying Demand, its affiliates, and others. Nothing contained herein should be construed as granting, by implication, estoppel, or otherwise, any license or right to use any trademark displayed herein without the written permission of Supplying Demand or such third party that may own the trademark displayed herein. Your misuse of the trademarks displayed herein, or any other content on this website, except as provided herein, is strictly prohibited."