Installing ductwork for heating and air conditioning is a bit like putting together a huge three-dimensional jigsaw puzzle.
The basic rules of the game, however, are the same, whether you're retrofitting an older house, working with new construction, or adding heating and air conditioning to a new room, attic or basement.
Basic Rule No. 1: Hot air rises, cold air falls.
Ducts should be installed to take maximum advantage of natural air movement. For instance, for air conditioning to work properly, air returns, the large ducts that carry air back to the central unit, need to be installed high up on the wall of each upper floor, to capture warmer air and return it for cooling. Efficient heating means installing a return at a low point on the first floor, to capture cooled air and return it for heating.
Registers that deliver warm air should be in the floor or low on the wall; registers to deliver cool air should be installed high up on the wall. (The best systems may require a mix.)
Basic Rule No. 2:
Ducts should be installed so they run as straight as possible from the basement to each register. That's because the more turns you make, the slower the air will move and the less air will be delivered to the register.
Basic Rule No. 3:
Every register should be fed directly from the central unit (or basement trunk line) by its own duct. If you stacked registers on a single duct on second and third floors, for instance, all the heat would go to the second floor.
Basic Rule No. 4:
Map out your duct path to upper floors before you start cutting any holes. Ducts should be able to run between studs in the wall and then turn into a joist bay (the space between two joists). Make sure the studs and bays line up well enough to leave a clear path.
Basic Rule No. 5:
When you make a turn, use a fitting designed for the purpose. That will keep the air moving efficiently. (An old-fashioned shortcut was to make square turns, but they cause eddies that reduce air movement.)
Basic Rule No. 6:
Plan heating ducts to emerge at external walls - those are the coldest walls. Registers should be placed in the middle of the external wall (in the floor or low on the wall); if there are two external walls, both need registers in the middle. (If the room is tiny, one duct may work, even if there are two external walls.)
Basic Rule No. 7:
Ducts that run through unheated spaces should be insulated. An uninsulated duct will lose a lot of its heat on the way to the register. (This rule is especially important if you're putting an addition over an unheated crawl space.)
Even if you're not installing the system yourself, knowing the rules will make you a more informed heating and air-conditioning systems consumer.
In our case, we hired an installer to put in the furnace, the air-conditioning equipment, basement trunk lines, exhaust flue through the roof and the largest air return, which goes to the very top of the house, a cathedral ceiling on the third floor.
The rest of the ductwork we're doing ourselves - partly to save money, but partly because we are so fussy about where it goes.
We always add a couple of rules to the basics. For instance, because it's an old house and we're retrofitting with the first real heating and air conditioning the structure has ever had, we don't want it to show. So all the ducts will be concealed in the walls and ceilings. There'll be no chases (drywall boxes), except for the largest cold-air return and the exhaust flue, which run on either side of an existing fireplace flue. We were able to use one of these chases for some plumbing pipes as well, and to conceal it behind a closet on the second floor.
Our other extra rule is that air conditioning has to be just as effective on upper floors as it is on the first floor. That means running high air returns for every major room on the second and third floors.
Fortunately, we discovered something called "button-lock oval pipe, " which is easy to work with and fits neatly into the space between 2-by-4 studs. It comes in 5-inch, 6-inch and 7-inch widths, so it delivers a lot of air. There are dozens of fittings to connect it around corners, to registers, and to round pipe (used for floor registers).