Plenty of coolant - are both heater hoses hot?
If so, then probably looking at a temperature blending door problem - physically stuck, actuator failed, or if you have an automatic temperature control system - perhaps a problem with it.
1989 F150 XLT standard-cab shortbed 4x4, 5.0L, AOD, 3.55 gears, manual hubs, 31x10.50 tires, 218K miles. Repainted Oct09. SOLD - Jan2015
2014 F150 XLT standard-cab shortbed 4x4, 5.0L, 6-speed auto, 3.55 gears, auto hubs, locking diff, trailer pkg, 5K miles.