What is OpenTherm (OT) January 23, 2018 Open Therm (OT) is a standard communications protocol used in central heating systems for the communication between a central heating boiler and a thermostatic controller.As a standard, open therm is independent of any single manufacturer. A controller from manufacturer A can in principle be used to control a boiler from manufacturer B. However, OpenTherm controllers and boilers do not in fact always work properly together. The openTherm standard comprises a number of optional features and some devices may include manufacturer-specific features. The presence or absence of such features may impair compatibility with other OpenTherm devices.