the heater is supplied with (HEAT) from the engine coolant, the heat is transfered to the cars cabin by pushing air thru the heat exchanger thats basically a small radiator under the dash with ducts for routing air from an electric fan, if your getting good air flow blowing out of the vents Id be looking for restrictions to the coolant flow rates thru the heat exchanger under the dash/firewall area.
OR ISSUES with the correct routing or restriction of the air flow thru the heat exchanger, theres usually a vacume or electrically controlled solinoid that moves a gate that pushes the air flow to the floor or dash, a loose wire , blown fuse or leaking vacuum hose can cause problems
WIKI SAYS
"car heater works like the reverse of the normal cooling radiator - in fact the heater is like a small version of the rad thats on the frnt of the car - the hot water from the engine block goes into this small rad and the heater fan blows air thru it into the car - the hot heater rad loses heat into the car and - bingo - you get warm inside "
http://www.misterfixit.com/heater.htm
http://auto.howstuffworks.com/cooling-system10.htm
http://www.2carpros.com/first_things/heater_failure.htm
http://www.allpar.com/fix/heat.html
http://autos.yahoo.com/maintain/repairqa/fluids_heat_air_conditioning/ques020_3.html
OR ISSUES with the correct routing or restriction of the air flow thru the heat exchanger, theres usually a vacume or electrically controlled solinoid that moves a gate that pushes the air flow to the floor or dash, a loose wire , blown fuse or leaking vacuum hose can cause problems
WIKI SAYS
"car heater works like the reverse of the normal cooling radiator - in fact the heater is like a small version of the rad thats on the frnt of the car - the hot water from the engine block goes into this small rad and the heater fan blows air thru it into the car - the hot heater rad loses heat into the car and - bingo - you get warm inside "
http://www.misterfixit.com/heater.htm
http://auto.howstuffworks.com/cooling-system10.htm
http://www.2carpros.com/first_things/heater_failure.htm
http://www.allpar.com/fix/heat.html
http://autos.yahoo.com/maintain/repairqa/fluids_heat_air_conditioning/ques020_3.html