bad check valve or vac leak
check the black check valve under the hood , the one at the rear of the driverside valvecover by the firewall , it has three ports , one is vac signal (1/4") the other two at the top , one goes to the heater-a/c system and the other to the reservior ball (and cruise servo if you have cruise) , the ball is under the front driverside fenderwell . the check valve is the connection point for the reservior and the heater system so there is vacuum for the heater system even when the engine doesnt have vac in the manifold , the heater and reservior vac lines should have vaccum in it even with the car off . with engine off pull one of the hoses (heater or reservior) off at the check and if you dont hear a hiss then you have a leak in the hoses or a bad checkvalve .
if theres a leak in the vac lines or the check valve is bad you loose vacuum as you accellerate and the doors in the heater system will allow hot air in from the cowl , they need good low vacuum that the reservior ball stores up on decell and idle.
as mentioned if your heater valve is stuck then you could get heat from the engine cooling system but more than likely your feeling external engine heat blowing into the vent system