From experience and experimentation. You can try it out if you have time.
In fact was looking out for a Room AC's thermostat to fix in my zen, unfortunately, the old bellows type thermostats are not avalable easily as they are using electronic thermostat for room ACs now a days.
well mate with my experience using OBD i have seen that if you set temp. on or near 24C load on engine is not continue but wait between get increased compare to if you set it on 16C engine remain constant on load
i don't know what thing do this thing whatever it is it does control compressor function according to temp you have set
In my Zen there is no thermostat to control the temperature inside the cabin. If I move the slider to the right it only mixes warm air with the cold air, still keeps the ac compressor running. There are sensors that cut out the compressors on high/low pressures. But not thermostat to control the temperature inside.
You have said that you get better FE if you keep it in the middle and that the engine response is better. If you are getting cold in in your airvent , it means that the compressor is on. Is your engine response fluctuate? if the ac is cutting of and on you should feel that.
Attaching a page from the Owner's manual of Zen and also a Maruti leaflet explaining the function of the temperature control Knob/Slider.
What I mean to say is there is no user controlled thermostat in these cars like the one we find in our room AC/refrigerators.
These temperature control knobs (in these cars) regulates the flow of air through the air vents. Sending only cold air if the setting is in the lowest or mixing cold air with warm air as we turn the knob still keeping the compressor running.
The compressor cuts off or on depending upon the sensors that sense the high/low pressure or heat in the AC system - not on the cabin temperature.
I always had to turn the AC off when the cabin gets very cold and turn it on when it turns warm. (in my Zen)
For my Manza if I need only a slight cooling I turn the ac on and keep the air vents in closed position. The little air that escapes into the cabin is enough to keep the cabin cool. Otherwise it will be freezing inside the cabin.