My small (800sf finished, 1600 if you include unfinished basement), low load house has really bad, poorly designed broken ductwork. I don't forsee the need for new mechanical system for a while, because the existing one was installed in 2010. My question is how critical is it to install return ducts in every room except kitchen & bath? Should I insist on return ducts installed up high on the wall? It was pitched to me that I should consider turning my existing supply registers (interior walls, on the wall) and just put in new supplies on exterior walls near windows. I have insulation in my 2x4 walls, but I still have hot and cold spots. I'm thinking it's probably to do with the LOUSY ductwork. Any tips or cautions when redesigning a duct system?

You should have a rescheck done...i think that is what it is called (HVAC load) for your home. If you have a few cold/hot spots, maybe just address those? Are there supply vents in those areas already? or is there no supply vent? A vent doesn't have to be near a window anymore (with good building techniques). The days of crappy drafty windows are long gone (unless your home/windows are very old) and the supply can be placed in the room just about anywhere to "throw" the proper conditioned air across the room. After all, it's better to have a well-insulated and sealed house than pump conditioned air into drafty rooms that will leak your expensive air out into the exterior. It would probably be easier to remove some drywall and add 1/2 or 3/4 in of EPS foam board to the inside of the wall (taping the seams) then drywall over that. That would substantially increase the room's ability to hold it's temperature longer. I couldn't imagine tearing out drywall around the house to get to ductwork to replace or repair or retape it all. Sounds like a far more difficult and time consuming project. What do you feel is your main issue for hot/cold spots? Badly insulated/sealed home? Poorly located ducts? Incorrectly sized ducts?

If you leave the doors open, the return ducts aren't so critical. If you have hot and cold spots within a single room, consider adding more insulation. If you have temperature variations between rooms, then duct dampers will allow you to adjust that.

On an older house, especially one with single pane windows, it is easy to get drafts and cold spots by the windows due to convection currents. This is why heat registers are usually directly below the windows.
It is also a bit hard to diagnose these things from the keyboard
Really the first step is to figure out how the existing ductwork is doing.
What I would want to know is the total cfm that the air handler is producing, total external static pressure, air temp before and after heat exchanger, total duct leakage, duct leakage to the outside of the house, size and type of filter, then at each supply register, measure air temp and cfm delivered. Lastly with the doors closed, measure the pressure differences in the rooms with the air handler on and off.
Next up is blower door testing of the house, and a heat loss analysis of the building itself, and the relative loss of each room.
Now you have everything you need to solve the puzzle. It is not that quick to do all that, It is about 75% of my normal energy audit, and would take me most of the day to do. On the other hand, if someone is going to redo all the ducts, that would take knowing a lot of that info anyway to design it correctly so you might as well get it now.
In general you can undercut all the doors to provide a return air pathway from closed off rooms. How big the undercut is depends on the amount of air delivered by the supply ducts and the pressures measured in the rooms.
Cheers,
Eric

Posted By Surfsup on 14 Nov 2014 07:23 AM
What do you feel is your main issue for hot/cold spots? Badly insulated/sealed home? Poorly located ducts? Incorrectly sized ducts?

I'd say poor airflow is the main problem. I have two retun vents upstairs on the south exterior wall of the house. The coldest room in the house has a north wall and it shares a register with another room. The design of the duct system is poor and so is the condition of the ducts. They are broken, leaky and have bad airflow. My insulation is good. Air sealing pretty good too.

You can add a booster fan to fix a single duct issue.