Heater does not blow hot enough to defrost windows

My 1994 cavalier rs, the heater blows very slow on hot I cant even defrost my windows, but if j switch it to cold it blows faster. I have replaced the blower motor and an in the process of taking the dash apart.
The heat works but blows very faintly, but it works on cold air, also all the valves switch from floor to defrost to front vents fine but makes a small hissing or pssss noist while switching. It suck I have to use a Porto heater for 20 mins to drive everyday in the winter. Please help me figure this out,

David