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You've done the heat loss, and chosen a furnace or air handler. Now you have to design a system to distribute the conditioned air to each room. This system will be based on the cfm output of the blower, and the total cfm will have to be distributed proportionally to the rooms according to their needs. The btu and cfm output will seldom match exactly the house's requirements, so the extra will have to be rationed out. The furnace will have a specification sheet which will list the various blower speeds and outputs.

There are numerous methods of designing a ducted heating or cooling system. And if we sat around thinking hard enough, I'm sure we could come up with a couple more. We could engineer the heck out of the situation if we wanted to, but most of us don't get paid for creativity or unusual design techniques, so I'm going to review one proven method, and leave it at that.

In technical terms, the system will be a low velocity, reducing extended plenum perimeter system. It is more work saying it than installing it. In simple terms, it means that the trunk line tapers as it goes, and that the supply outlets will be near the exterior walls, in this case the floors, and the returns will be located on the inside walls. The ductwork size ,as always , is based on the friction component of the moving air versus the duct itself, and the blowers ability to counter this friction. Again, what this really means is that the air doesn't really want to move, but the blower will move it anyways. It is always noted in units of inches of water, or In. Wg., and the velocity, or the speed of the air will be in FPM or feet per minute. These concepts and abreviations are useful and helpful in their own right, but rapidly lose their value when you are crawling around on your belly measuring a trunkline through a crawl space, or dripping sweat in a two hundred degree attic. For residential applications with limited duct lengths, get one of those rotating duct calculators from a salesman, set it at point 1, and go; the chart below, approximates the cfm while the fpm remains under 700 for branches and 1000 for trunklines (Supply branches should be limited to output maximiums of 8000 btu for heating, and 4000 btu of cooling unless construction methods dictate otherwise, and should always contain a manual damper for air flow adjustment).

Here's a chart of the relationships you should end up with (all measurements in inches):

cfm round rectangular supply
register (min)
return grille
60 5 2 1/4 x 10 4 x 10
2 1/4 x 12
12 x 4
100 6 2 1/4 x 12
3 1/4 x 10
4 x 12
4 x 10
6 x 12
150 7 3 1/4 x 14 4 x 14 8 x 12
200 8 4 x 14 6 x 14 8 x 14
300 9 8 x 8 8 x 14 10 x 14
400 10 8 x 10   14 x 14
500 12 8 x 12   20 x 14
600   8 x 14    
700 14 8 x 16   24 x 14
800   8 x 18    
900 16 8 x 20   30 x 12
1000   8 x 22   30 x 16
1200   8 x 24    
1400   10 x 22    
1600   10 x 24    
2000   10 x 30    
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