I just had my ancient HVAC unit replaced with a new Trane system (92% gas upflow/13 seer XR series AC). The new system is WAY better than the old (of course), BUT there does seem to be a problem.

One branch is not giving out air that's as warm as the other branches in the house. At first I thought it was due to running through an unheated crawlspace (about 17' length), yet another, shorter branch (about 8')in the crawl was blowing air 10 higher at the register. Even this warmer of the two crawlspace branches isn't as warm as the branches that run through the basement, but the difference there is only about 3-4. That seems ok to me, but the one being 10-14 less than the rest of the house doesn't.

The crawlspace branches are the only ones using flex ducting (6" R4.2), with the rest of the branches using the original sheet metal ducting in the basement except for the new takeoffs and such to connect new system to old ducts.

Upon further investigation I noticed that the takeoff for the coolest, problem branch is substantially cooler to the touch than the other takeoffs right at the furnace plenum. The other two takeoffs are rectangular, probably 8x12, but the cooler one is 8" round.

I did a temp measurement at the point that this same branch changes from the sheet metal duct to the flex, and it substantiates the fact that the air in that branch simply is cooler to begin with. (I stuck a thermometer up into the takeoff for the flex duct, into the main part of the branch.)

It's obvious that if the air starts out cooler at the point it leaves the furnace, there's no way it can be the same temp (same being within 2-3) when it leaves the register vent. But how can the air leaving the plenum into the takeoffs be different if they are all coming out of the same plenum?

I'm going to call the contractor back about this, but I wondered if someone could give me some idea why this may be happening? I don't want to have paid all this money and end up with one room of the house that is always colder than the rest.