Check your coolant level, be sure that the system is properly bled out as trapped air in the heater coil can cause this. Have you recently serviced the cooling system?
1999 540i/6 MSport- Halos, Facelift Tails, Debadge, Maint Done
2001 740il Sport- Needs maintenance.. Parts ordered..
2006 Volvo XC90- Kid Hauler