Heating and Cooling Info from Warmair.net
Ductwork and Distribution
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 output 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 blower's 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 anyway. 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
abbreviations 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 200
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 (min) |
| 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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