Yes, only the EventSentry agent is required to monitor temperature, humidity, water and/or smoke. The management console is needed to configure these sensors however.
To configure the sensor(s), copy eventsentry_gui_x64.exe (or eventsentry_gui.exe for 32-bit agents) from the "C:\Program Files (x86)\EventSentry" folder on the EventSentry server, and paste the exe to the remote computer. Log onto the remote computer, launch the pasted exe, and configure the sensor using the "Environment" option in the tree view.
USB-only Environment Sensors
Since the USB-only sensors require a virtual COM port driver to be installed, please see the following article regarding the driver: https://www.eventsentry.com/kb/213.