XR3i Escort Heater blows cold, why?

I have a 1988 Escort XR3i. The problem I have is that even when the engine is at max temp (before the fan cuts in) the heater only blows luke warm air. I have had all the heater out, checked the matrix and all cables and everything is fine. The matrix gets very hot under normal circumstances but the air from all the vents including the floor vents (which have no ducting) only blows out luke warm air. Anyone know why and how to fix it?