Have your water valves ever turned on? If not, then coolant is bypassing the heater core...which could be why the air is always cool.

The "water valves" are often referred to as "heater valves"...and there should be a lot of info about them. They can fail electronically...or fail physically by crud building up, and corrosion rotting away the valves & plungers inside the valve. There's lots of pics posted of failed heater valve/water valve (search using both terms).

Good luck.

Thanks!
I'll try searching up.

I only saw it ON when the set temperature is below 26'C.