1995 Chevy Cavalier with no heat.?

[loveisblind]

My Cavalier has no working heat. My fiance changed the heater core because it was leaking coolant on the passenger side. Since he did that, there has been no heat. The heat worked just fine before he changed the heater core. Now it has no pressure when it blows out, and it doesn't get hot. I was told that it might be the thermostat. My fiance says its because he didn't seal the heater core properly, but i don't want to believe him because he seems like he doesn't know what hes doing.

 

[kashew]

It might be because there is air in the line, there should be a little valve on the coolant pipe under the hood. Its a silver pipe and should have a small screw looking thing on the pipe