My car is olny blowing cold air when the heat is on

Hello am having a
problem just blowing
cold air when the heat
is on. Replaced
thomastat, blender
motors, flushed
system.My head
gasket is not good. ,
but am using this bars
leak head gasket
sealer& & you need
the heat on high so
this bars leak head
gasket will seal. The
mechanic put in the
sealer. I think it's
working because I
drove on route 17
doing fifty mph, for a
half an hour,,but no
heat. Thank you.
Robert Butler