I had a tech out to review my furnace that's not keeping up - a Trane XV80. They poked around, and saw that the plenum temperature is 95F. They then told me that it should be around 140F, which is clearly, substantially more. The thing that confused me is that in the same breath, he said that the temp delta between the return and the plenum should be 20-30F, which would mean a return temp of 110-120, which is impossible.
So - which is it? And what causes a low plenum temp / how can it be remedied?