The Danfoss ECL A266 application key (087H3800) is a programming key for ECL Comfort 210, 296 and 310 controllers, providing weather-compensated control of heating and domestic hot water (DHW) preparation. It includes four subtypes (A266.1, A266.2, A266.9, A266.10) with integrated functions including return temperature monitoring, pressure monitoring and a floor construction drying programme.

 The key is inserted into the slot of the ECL Comfort controller. Once inserted, the controller displays the application name and version
  and performs automatic initialisation. The key contains preset factory values, which can be adapted to the specific system. To change the settings, the key must be inserted  (with firmware versions 1.36 and higher, settings cannot be changed without the key inserted).