I own an old house. The furnace/blower was located in the hall w/ the return air directly underneath & supply located in the centers of MOST of the rooms. I replaced the unit and put the furnace/blower in the attic. 3 new, 12" round returns were added, 1 in the den, 1 in the living room & 1 in the master bedroom. The house is 2500sf, ranch style, 5 ton unit.

I need to replace the sheet rock on the ceiling in the master bedroom, & I thought it would be a good time to move the suppy to a more efficient location. The master is 13.5'x17.25'. The return is located near the center, along one edge of the 17.25' walls. There is a 35"x70" window on each of the 13.5' walls (near the centers) and the head of our bed will be near the center of the other 17.25' wall(opposite the return). The supply is from a trunk line that continues to a hall and large walkin closet. I want to run a new line to just the bedroom, but what size duct and where should the suppy/supplys be located? The master is approx. 18' from the main supply plenum.

Also, there is NO supply in the dining room! The dining is 13'x12' and located almost directly below the main supply plenum. The thermostat is on one 12' wall. What size supply duct in this room and the location?

Much too large a task and much too little information. You need a good HVAC contractor or a balancing contractor to start from scratch, and design your system so that it will function as a unit. It isn't as simple as you make it sound.

Diffuser locations are usually based on manufacturers' printed airflow diagrams and data. There are one-way, two-way, three-way and four-way diffusers, and circular diffusers, and even linear diffusers, and each will perform differently and will need to be placed differently.

But, my system functions just fine! Running new supply ducts to 2 different rooms is not that complicated, and not that large of a task. I understand the differences in the diffusers, but diffusers can easily be changed. The locations of the supply can not so easily be changed! There is NO supply in the dining room and only a center supply in the master bedroom and there are not that many different locations for them to go!!!!! There must be someone out there who can help me?

quote:

But, my system functions just fine!

No it doesn't!

You just don't UNDERSTAND that it doesn't!

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You not only need 'supplies' to every room, you also need 'returns' from every room.

If you do not have a supply and rreyurn from every room (with some exceptions) your system is NOT 'working fine' and you are wasting your time as an amateur in trying to tell us otherwise..!

Someone who might suggest ways to add significant ductwork to what is a less-than-perfectly designed system, would NOT be helping you. Someone who suggests retaining a good HVAC contractor, or even better, a balancing contractor, to DESIGH your ULTIMATE system properly, so that hwen you turn it on, it works well, IS HELPING YOU!

You cannot willy-nilly add and subtract ductwork, and mess around with diffusers, without unbalancing a system that must be hanging on by a thread now, as far as balancing goes.