I don't have a heater core or heater hoses in my car but whenever I change fluids I drain my radiator and both sides of the block, then I use an 'Airlift #550000' tool to suck all the air out of the system, then I stick the hose from the kit into my bucket of new fluid and it automatically pulls all the fluid into the cooling system with no air pockets. Best tool I've ever bought.

And I also drill a couple small holes in the base of my thermostat.