no heat

i have a 2004 impala ss the heater won't get hot can u please tell me what to do to fix it and i have already changed the thermastat and the lines are getting hot but still no heat

I can only think of a couple of things here: 1. You got a bad new thermostat or put in upside down. (this happens more than you might think) 2. Radiator pressure cap faulty (not holding pressure) but then you would most likely overheat. 3. Clogged lines to/from the heater core (not likely) heater controls not functioning...and last aircondition button turned on. Hows your fluid look, is it low or getting gummy? Mileage? And dash work done like new radio etc?

I have heard this can also be caused by air in the coolant system. Maybe you need to have it bled.
Can someone who knows more confirm this?

i found out the problem the heater lines was stopped up that run to the heater coil so i flushed them out with a water hose i took both of them off and put the water hose in and all type of black stuff came out then i ran the water until it was clear then hooked it back up then i got heat so thanks so much 4 the help

Thanks for the re-post on the fix...and yes air in the system could also be a cause but most likely to cause overheating. Heater (core) proper term here. fyi only...