When I start car, gauge moves up to just above C and never goes up, even after warm up period( if its warming up).
Checked reservoir, plenty of green ( no discolouration)- maybe too much coolant could be blocking thermostat? Or is my engine actually just running too cold?
Also my HVAC system makes gas hissing noises for ages when I use Cold/ Hot control. I'm thinking these may be links.
Did search but only found relating to overheating, not this (The opposite).

Change your thermostat it's most likely stuck in the open position

what he said. ^ ^

you could also change the coolant temperature sensor, this caused me problems with my fairmont, when the ecu gets the wrong reading it sends everything spaco in accordance, like the thermal fans ect.