Heater problem

Just wanted to run this by you before I try changing the thermostat to see if this is the issue. I have a 2002 chevy venture with a 3.4 l engine and front and rear climate control. Anyways, it is stating to get colder here in NY and noticed over the past few days that the heater seems to be taking an awful long tie to warm up and when it does it seems to be warm but not hot like it used to be. The engine temp according to the guage is reaching temp after driving about 2-3 miles. The antifreeze level is okay and the waterpump is about a year and a half old. There is also no leaks that I can tell. Does this sound like it might be a good possibility? Thanks.

 

Doubt it's the thermostat as you said it is reaching temp. More likely I think is a problem with a blend door, possibly a vacuum leak. I confess I'm not real knowledgable about how the dual systems are set up so hopefully a pro will ring in.