When a Xenta 121-FC changes from heating to cooling the fan output turns off for 5-10 seconds.
The firmware version has been enhanced and this was a enhancement that has been implemented.
Upgrade your 121-FC to version 1.25 (or above).
Download the upgrade files from The Exchange Download Center: Xenta 121-FC Firmware Files (NXE/APB/XIF)
See Downloading new NXE or APB files using Lonmaker for instructions on how to download the application to the controller using LonMaker.