I just had the same issue before I tore mine down, little or no heat at idle. I am wondering if you can hear any venting out of the degas bottle. I had a low pitched whistle on and off throttle from the degas bottle from the increased pressure that released through the cap. Just installed head gaskets and studs and the venting went away and now my heat will chap your face.. lol
I can only guess that bad head gaskets are related to the heat issue due to the coolant flow and pressure. dunno.
I'm sure someone with alot more experience than me could explain it.