I did the plenum repair a couple weeks ago. After doing it my cab heat disappeared. I went through the whole burping sequence and got it to work a little bit (slightly warmer than outside air).
I thought that after all of this there might be trash plugging the core. Unfortunately I had to run, so I didn't get the chance to flush it. While driving I decided to flip the control to recirc to trap what little heat I was making (5 below outside). Upon switching it I had the hot air I'm use to almost instantly. At that point I thought it might just be so cold out that the heater wouldn't work, although it's never happened before.
Today it was warmer out so I decided to experiment. With the truck at operating temp, just sitting in the yard idling, I ran the heater through its normal settings. I don't have a thermometer, but I believe the heater was only about 20* warmer than outside. Messing with the temp selector (blend door) it was apparent it was working. As before, when switched to recirc it was instantly hot air. Blend door works fine in this setting also. The whole system seems to be working fine, except for the lack of heat. This is a 97 ram 1500 with 360 in case my signature isn't visible.

Thanks for reading my book and any ideas you might have.

 

Probably an air bubble in the heater core. Backflush the core, make sure it is full. Should solve the issue.

 

dude is good (funny) but good. check out the vid might help ya out!

 

Patt: nice video. Gave me a good laugh.

Heyyou: If there is an air bubble left in there, the only way it's going to come out is if I drill a vent hole in the core. I put a tee on each line, with valves at the tee, with a piece of hose about a foot long standing up in the air. I then poured water in one side until it was coming out the other. I was setting up to do a flush on it, but my garden hoses were frozen.
Any idea why it blows normal heat on recirc, but cold on fresh air settings?