He would need to do A manual j and manual d for the whole house including addition... Not just adding duct and some dampers.. The whole dust system will more than likely need to be redesigned or you may need to upgrade system size. This is what manual j will tell you. Then you will have a option to use manual dampers and balance system properly or you do a complete zoning system... You can't do both.. But as long as its designed properly then system should thrive..
manual J is a method of determining a structures heat loss and gain. manual D is a method of determining the total equivalent length of a duct system to determine what friction rate loss to use on a duculator.
A couple of books from the bible of the HVAC world.
When doing the Manual J heat-gain/heat loss, look at fixing those areas that allow too much gain or loss; as if you can reduce the equipment size that will help the existing duct system to work better.
Though you'll still need to run a Manual D so you know the system is going to work properly. Far too many never check airflow at all...
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