I was asked this recently on a web forum:
I am doing a house addition and replacing old HVAC system with a new system (Heat & Cool). The new master bedroom in the addition is 3-4 degrees colder than my existingin the old part of the house. The HVAC installer put in 3 vents in the new MBR (plus one in the closet and one in the bathroom for a total of 5 vents). For the old MBR, they ran only a single duct and connected it to the only existing register. The old MBR is only slightly smaller than the new one and gets sun for at least half of the day. I don’t think they installed any dampers on the ducts in the attic. My contractor tells me that I can close as many register dampers as I need to in order to balance out the temperatures, but I am hesitant to close too many. I tried closing 1 or partially closing 2, but that wasn’t enough. I am thinking of asking him to remove a register from the new MBR and add it to the Old MBR. Does this make sense or is he right? It is a single return system. In Norther NJ.
Closing the registers will only shift the airflow by mere fractions. You will not really notice it. The reason is that the air has already traveled down the duct losing its pressure due to friction and duct length. Adding a duct will only reduce the total airflow handled by each vent. Properly designed duct systems are engineered using factors such as duct material, fittings, length, size, blower capacity, and designed available static pressure to obtain the proper airflow in Cubic Feet Per Minute (CFM) as designed to meet the heat load of the spaces they serve. The best way to adjust airflow (since the ducts are never perfect) is to have dampers at the beginning of each branch. Thereby building up the pressure in the TRUNK duct so that the other branches have more airflow. Once the air is down the branch, it will not return back with the same force and flow down another branch. So your system will lose the pressure and become less efficient by shutting down the registers.
By adding another room you have added more heat load to the conditioned space. Things to consider would be is the blower not rated high enough in btu/h to handle the additional load? If you move the branch duct will it be enough in the new space and will the other two branches be enough for the existing space? If you add another branch are you using the correct size and fitting for the new branch? Will the existing trunk be large enough to handle the additional volume of the new branch?
I am quoting a similar project right now and it is requiring a new and larger system, larger trunk ducts, and three additional registers due to the additional heat load. One room is being added while another room was added 10 years ago and the homeowner complains the older addition never is cool enough because the duct to that room was improperly sized and incorrect fittings used and did not account for a larger trunk needed.