sounds like your heater valve is stuck or your cable that is suppose to open and close it has broken. if its a g20 with digital you have a electric motor turning the valve on and off. regardless of the heater not working on your car, sounds like you have a blown headgasket and losing compression and being sent out to your coolant passage hence blowing out air through the coolant reservoir.
1967 Datsun Roadster 1600- Weekend toy
1991 Nissan Sentra B13 SE-R- avenir powered.- Now VVL- now Avenir Powered Again.
(SOLD)1996 Nissan 200sx B14 SE-R TEAL- VVL- shell now boosted missed dearly
1996 Nissan 200sx- VVL- super sleeper