same old problem--no heat

[Libby]

Ok, a couple of new things that neither I nor my 72 year old mechanic didn't think of. First let me ask a dumb question---are a heater box & a plenum the same thing? (I obviously don't know cars) My mech. told me he checked the air flow thru the heater core after he flushed it, and things were good. The rad. hoses are getting fairly warm; the amount of air coming thru the vents is astounding (it just isn't warm). I can't say what shape the heater core gaskets & foam were in when it was replaced cos I have no idea; I buy the parts & my mechanic replaces them. I obviously didn't know the core had gaskets or foam, so didn't buy them. I'm assuming the next step here would be to get new ones & see what happens? As long as I've got the poor guy with his head up under the dashboard, should I go ahead & replace the heater core just to be sure that's not the problem?