2001 Chevy Blazer, What's wrong with the heater? It doesn't heat up even after 30 minutes.?

In my boyfriends blazer, the heater won't heat up. After leaving it running for 30 minutes the heater blows out semi warm air. Just warm enough to keep the windows somewhat thawed out. We can't figure out what is wrong with it. It's not the heating coils. They are fine. Any suggestions? We live in Wyoming and right now the weather is about -4 below zero. We really need the heater fixed.