The ECON button on the climate control system ensures that neither the air conditioning nor the Webasto auxiliary heater operate, as both consume fuel. By pressing ECON, you're limiting the climate control system to using only outside air and waste heat from the engine.
Air conditioning is the active removal of heat from air. Air conditioning turns your A2's cabin into a fridge. As such, whether or not you have ECON pressed, the air conditioning system isn't going to be activated during winter, because the climate control system can cool the cabin simply by using the cold air from outside.
If you're driving in 28 degree heat during the summer, and you request for the cabin to be at 19 degrees, the climate control system cannot cool down the cabin by using the outside air, so it has to switch on air conditioning. This burns additional fuel. If fuel economy is your priority, you can press the ECON button to ensure that the air conditioning system remains off, but you're then limiting the climate control system to using outside air, meaning you can't have the cabin at the 19 degrees that you requested.
Auxiliary heating is the same scenario in reverse. If it's 2 degrees outside, and you request to have the cabin at 23 degrees, the auxiliary heating will switch on because there isn't enough waste heat from the engine to maintain such a warm cabin when it's so cold outside. The auxiliary heater burns fuel. If fuel economy is your priority, you can press the ECON button to ensure that the auxiliary heating system remains off, but you're then limiting the climate control system to using waste heat from the engine, meaning you can't have the cabin at the 23 degrees that you requested.
The climate control system will always use outside air and waste heat from the engine as its priority, whether or not ECON is pressed. As such, pressing ECON saves no fuel unless the climate control system needs the air conditioning or auxiliary heating in order to maintain the driver's desired cabin temperature.
Cheers,
Tom