I'm having the same problem with my 2001 Sebring. I was only managing to get warm air out of my heater. I did a cooling system flush, replaced the water pump (at the same time as my timing chain and oil pump) and also replaced the radiator cap. The first time I drove it after all the work the heater blew hot air and I thought all was well but this morning on the way to work (it was 25 degrees outside) it was back to blowing only warm air. I've noticed that the car never gets much hotter that just above the cold marks on the temp gauge. A while back I replaced the temp sending unit housing because it was leaking and I noticed that ever since then the temp gauge shows it running cooler. Could that be the problem? Maybe it's not reading properly and it's not telling the blend door to open?

 

If you have searched for this problem here on the forum you might have seen reports from people who have found that aftermarket temperature sending units seem to read lower than the OEM ones. That has nothing to do with heater performance, merely where the needle sits on the gauge.
Poor heater performance is first of all from low coolant level. After filling, the system is still likely to have air trapped. DON'T try to bleed it out with the bleed screw. You will only cause the metal insert that the bleed screw threads into to spin in the plastic housing, resulting in leaks.
The best method for removing air is to let the system bleed itself through several warm-up and cool-down cycles, then top off the coolant.
Another possibility is that the blend door in the plenum is not operating fully. That's not an easy fix.
As you can see from the above posts in this thread, a plugged heater core is another possibility. Replacing the heater core is a huge job involving removal of the dash.

 

Thanks for the reply dcotter, I'll admit I didn't do any searches because I read this one and just jumped on. Thanks for taking the time to reply however! I was thinking about doing the heater core when I had everything apart for the timing chain until I read the procedure and decided not to. You are right, it's not easy! I'll have to look into the blend door.. not sure I really want to mess with it, it's at least blowing warm enough air to make it comfortable inside.