Originally Posted by HalfHand
I have no doubt I can install my duct work myself. I've looked and found some general over views on designing a duct work system however they are pretty generic and since a well designed system can be the gift that keeps on giving. My question is:
I've found some places on the internet that will design my duct work if I send them room dimensions and some other generic information. They want $250-$300 for doing this, seems pretty reasonable. However, I don't think they are asking enough questions. I'm worried I'd end up with something that would work, but not work as well as it could.
The house is completely gutted and I've done a lot of preliminary work to make it energy efficient, now is not the time to cut corners. So the question is should I find someone local who will do the design for me (which I know is going to be more expensive if I can even find someone to do it without getting the job of installing it). Or will they end up just taking room measurements, asking some generic info and giving me a design?
It sounds like what you need is someone to design your trunk line, and give you an idea of where to put the branches.
Round duct comes is relatively generic sizes, so the question is how much of each size.
If you're installing in the attic (easiest) you essentially have one long run down the length of the house (trunk) with branches going to each room. These branches are going to be 6" pipe or flex (most likely) so the starting point is figuring out how many branches are coming off. You'll want the runs to be as perpendicular to the trunk as possible to minimize material costs as well as velocity loss.
You can get away with just one register per room if you size it (and the duct diameter properly) though sometimes it's just easier to put in two registers (usually near windows) on 6" dia runs.
Feel free to PM me if you have any specific sizing questions - there are also a few nuances to installing ductwork/running flex that make the job more complete.
EDIT: I should add that while it is an exact science, the real world rarely behaves in an exact manner and even perfectly designed systems need balancing at the register level (think of closing a couple vents in the car and the AC blowing harder out of the open ones.)