Not enough heat or cooling

[HeadnSouth]

The unit has been this way since we moved in 12 yo. Brand new house. The master br and bath get plenty of H&C while the other rooms don't. It's a 2 or 2 1/2 ton unit that just feeds the top floor. 2x4 construction with insulation in the walls and ceiling. Double pane insulated glass. It has a central return in the hallway.
My son's room is approx. 12x12 and is fed by a 6" insulated duct. This room never gets proper H&C. I've tried throttling the MB vents to push more air to the other ducts but it never does improve. I've sealed the branches at the unit to eliminate leakage. All duct work is flexible except for the unit and that's fiber board of some sort.

Are my ducts undersized? What are my options to improve this situation?

I'm sure there is a formula to figure out what size is needed for each room but I'd be willing to bet installers just go by previous jobs. What's the rule of thumb? Thx

[Java_Bob]

Brand new home?? why wasn't a warranty call made??

any way, make sure the fan is turning the right direction this will cause low air flow

[HeadnSouth]

We have plenty of air flow in the MBR. Warranty company? They were useless.

[Hube]

These rooms that have no good air flow,do they have any RETURN air?
If they do have return and still are low in airflow then the duct system could be in need of a"tune-up". This is done by dampering off some of the airflow to the rooms that have a great air flow thus giving more air to poor ones.

Note; there is no rule of thumb when it comes to PROPERLY sizing a duct system. It is done by using heat loss/gain calculations( Manual J and D ) from the location, orientation and construction , insulation, etc, of the home.

[HeadnSouth]

Originally Posted by Hube

These rooms that have no good air flow,do they have any RETURN air?
If they do have return and still are low in airflow then the duct system could be in need of a"tune-up". This is done by dampering off some of the airflow to the rooms that have a great air flow thus giving more air to poor ones.

Note; there is no rule of thumb when it comes to PROPERLY sizing a duct system. It is done by using heat loss/gain calculations( Manual J and D ) from the location, orientation and construction , insulation, etc, of the home.

Properly being the key word here. I'll have to try dampening off the ducts again but if I remember correctly it didn't help.
I have 2 returns 1 main in the hallway about 20+" and another in the great room not sire of the size. Thx for the response.

[HeadnSouth]

I throttled down the registers like I did last time but instead of looking for more air flow I used a thermometer in the rooms I wanted to improve. I did make a difference.

[HayZee518]

long runs the furthest from the heat source may be too small and if flexible runs, the fiberglass inside the duct may be absorbing all the air flow. have you thought of replacing the long runs with galvanized trunk lines? this way with a galvanized run you can install a booster squirrel cage fan in the longest runs. cold or hot air returns don't follow no cut and dry rule, they just nail up a sheet of sheet metal on one joist space then duct it to the furnace or a/c unit.

[HeadnSouth]

So far all is working well, the MBR is not as warm as it was when getting all the heat but the other rooms that were lacking are within 1 degree of the thermostat temp. I can live with that.

[zver11]

Have similar problem on old house. Main duct very undersized and prior owner tried to patch over problem with half way fixes. While exact calculation using manual D is ideal, rough rule of thumb: need approx 200 Sq inch cross section area on supply side main duct for 2.5 ton unit. Since duct description sounds like duct board used, take width and height, subtract 1 inch; from each for thickness of ductboard and multiply two numbers. If circular main duct, same principal applies but math more complicated. Using flexible duct results in much more resistance to airflow. Main symptom of inadequate main duct is that rooms getting first tap on main backbone duct get most of the airflow and rooms at far end get little. If this is the case, should have very strong (and likely noisy) flow from first rooms fed by system. If entire backbone duct accessible, ideal choice is to replace main backbone with a proper sized metal duct wrapped in insulation. Metal better, but slightly more expensive than ductboard, but should have separate wrap of insulation applied. Insulating built into duct board and ductboard is cheaper, but has higher flow resistance and more prone to damage, varmints and bacteria/fungal growth issues.