Is your temperature gauge showing that it's warming up? If not, then most likely your thermostat is stuck open. You can test the thermostat by putting it in boiling water to see if it opens.
It may be that you got a bad thermostat. If the system was clogging up somewhere, you may not get heat either, but it would probably overheat.
I'd check that thermostat to make sure it's working properly. They also come with different temperature settings. Make sure that you have the right one for your vehicle. Good Luck.
〉 Answered on Dec 13th, 2007 by Lori Johnson, Owner and Instructor at Ladies Start Your Engines