I cant find my original post but I am reporting back on a low heat output issue.
Not the climate door. It was the heater core clogging up. Instead of replacing, this is what I did.
Purchased a "drill pump" and some clear hose and some CLR.
Cut both the heater hoses,taking consideration of where best to install "T" connectors.
While hoses disconnected,filled heater core with CLR(as much as I could get in it).
Hooked clear hose,drill pump configuration,on to the heater hoses.
Ran drill pump in both directions for several minutes and stopping and letting it sit for a few minutes.About 1 hour all together.
Pieces of debris came out of the heater core and I could see it through the clear hose.
Flushed the heater core out.
Installed "T" connectors and it is heating very well now.

 

Good post .I never thought of CLR . Sounds like a good idea . I have flushed , and power flushed . This sounds like something else to try .My 78 has a plugged heater core , I might try this idea .