Heater only blows warm air. on 2003 Chrysler Town & Country

Heater only blows warm air. This started a week ago, seemed to get better one day then stopped blowing hot air again. Temp gauge indicates motor is operating at normal temp. I have been adding approx. a pint of coolant every 250 miles for the past year. 6 months ago I had the water pump and thermostat replaced. Van has 153,000 miles. Please help.

Asked by for the 2003 Chrysler Town & Country
Have you checked the heater core? These tend to go bad once in a while and could be the problem.